Evaporation of Nanosuspensions on Substrates with Different

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Evaporation of nanosuspensions on substrates with different hydrophobicity Lionel Perrin, Anna Pajor-#wierzy, Shlomo Magdassi, Alexander Kamyshny, Francisco Ortega, and Ramon Gonzalez Rubio ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15743 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Evaporation of nanosuspensions on substrates with different hydrophobicity Lionel Perrin1*, Anna Pajor-Swierzy2, Shlomo Magdassi3, Alexander Kamyshny3, Francisco Ortega1,4, Ramón G. Rubio1,4 1

Departamento de Química Física I, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain 2 Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30239 Cracow, Poland 3 Casali Center for Applied Chemistry, Institute of Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel 4 Instituto Pluridisciplinar, Universidad Complutense, 28040 Madrid, Spain

E-mail*: [email protected]

Keywords: Evaporation, Suspension, Nanomaterial, Sessile droplet, Contact angle

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Abstract Liquid drop evaporation on surfaces is present in many industrial and medical applications, e.g. printed electronics, spraying of pesticides, DNA mapping, etc. Despite this strong interest, a theoretical description of the dynamic of the evaporation of complex liquid mixtures and nanosuspensions is still lacking. Indeed, one of the aspects that have not been included in the current theoretical descriptions is the competition between the kinetics of evaporation and the adsorption of surfactants and/or particles at the liquid/vapor and liquid/solid interfaces. Materials formed by an electrically isolating solid on which a patterned conducting layer formed by the deposits left after drop evaporation have been considered as very promising for building electrical circuits on flexible plastic substrates. In this work an exhaustive study of the evaporation of nanosuspensions of latex and hydrophobized silver nanoparticles on four substrates of different hydrophobicity. The advancing and receding contact angles as well as the time dependence of the volume of the droplets have been measured over a broad range of particle concentration. Also, mixtures of silver particles and a surfactant, commonly used in industrial printing have been examined. Furthermore, the adsorption kinetics at both the air/liquid and solid/liquid interfaces have been measured. Whereas the latex particles do not adsorb at the solid/liquid and only slightly reduce the surface tension, the silver particles strongly adsorb at both interfaces. The experimental results of the evaporation process were compared with the predictions of the theory of Semenov et al. (Evaporation of Sessile Water Droplets: Universal Behaviour in Presence of Contact Angle Hysteresis. Colloids Surf. Physicochem. Eng. Asp. 2011, 391 (1– 3), 135–144) and showed surprisingly good agreement despite the theory was developed for pure liquids. The morphology of the deposits left by the droplets after total evaporation was studied by scanning electronic microscopy, and the effect of the substrate, the particles nature and their concentrations on these patterns are discussed.


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1. Introduction The control and understanding of the evaporation of sessile suspensions as well as the deposition of particles during this process is fundamental in numerous applications: ink-jet printing, medical tests, DNA mapping, painting, agriculture, heat exchangers, food industry, biology, crime scene investigation.1–6 Materials formed by an electrically isolating solid on which a patterned conducting layer formed by the deposits left after drop evaporation, e.g. coffee rings, have been considered as very promising for building electrical circuits on flexible plastic substrates.7 Despite of this strong interest there still is a lack of understanding for correlating the morphology of the deposits after drop evaporation and the properties of metallic nanosuspensions. The design of this type of materials requires to be able to predict the kinetics of their formation and the morphology of the deposits. A detailed theoretical approach remains challenging because of limited computational capacities and the complexity of understanding the interactions between the substrate, gas phase and liquid phase, as well as the multicomponent aspect of the different phases in the case of solutions and suspensions. Indeed, the modelization of sessile droplet is still strongly limited by the number of components inside the droplet and their adsorption at the liquid-air and solid-liquid interfaces.8–10 Most of the studies consider symmetrical configuration with stationary conditions, which is rather the exception in nature or industrial processes.11 Furthermore, some authors have shown that the droplet may no longer remain spherical when the contact radius of the droplet starts to recede.11–13 The evaporation of a sessile droplet generates an outward capillary flow inside the droplet.14 Also, the evaporation flux, that is not homogeneous over the surface of the droplet, cools the liquid-air interface that creates temperature gradient through the bulk of the drop.15 The temperature gradient induces a surface tension gradient that creates a Marangoni flow. These different phenomena might strongly affect the heat transfer between the substrate and the liquid droplet, and the gas phase. Therefore, the evaporation process and the time dependence of the contact angle are correlated to the cooling of the surface.16 It has been shown that for wetting surfaces the influence of the thermal properties of the substrate and its thickness increased with the cooling effect 17–19 and that a higher thermal conductivity of the substrate increased the evaporation rate of the droplet preventing the evaporation-induced cooling that appears when the substrate is thermally isolating.20 The temperature of a water sessile droplet can be assumed homogeneous and equal to the temperature of the substrate for wetting highly thermally conductive substrates at ambient temperature.21 A strong dependence of the magnitude and direction of the Marangoni flow on the contact angle and on the ratio between liquid and substrate thermal conductivity was also observed.16,22 On nonwetting substrates, Sobac and Brutin21 found no thermal effects on highly thermally conductive substrate for water at ambient conditions. However, by increasing the temperature of the substrate, they observed that it was necessary to take into account the buoyancy effect in the gas phase as in the wetting case. They also found that the cooling of the liquid-air 3 ACS Paragon Plus Environment


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interface was not negligible anymore, and thus temperature gradient between the substrate and the liquid-air interface of the droplet has to be accounted for. Meanwhile, Gelderblom et al.23 showed that thermal effects and convection in the gas phase are negligible for water regardless of the contact angle at ambient conditions on highly conductive substrate. By studying methanol on nonwetting surface, Chandramohan et al.24 showed that Marangoni flow dominated over capillary and buoyancy flows for highly conductive substrates. Most of the studies on pure liquids are limited to water, alcohol and alkanes.24–26 Janssen et al.27 studied the wettability of a broad range of technologically relevant solvents like nheptane, chlorobenzene, acetonitrile, Fluorinert FC-3283, etc. on various modified silicon dioxide substrates, although they did not study the evaporation process. In the case of pure water droplets, Hu and Larson28,29 predicted theoretically that Marangoni flow should be present, although they could not observe its effect in their experiments as previous experimental studies did.14,30,31 They suggested that this can be due to the fact that the liquid-air interface of a water droplet is easily contaminated, which is not the case of liquids with smaller surface tension where the Marangoni flow can be visualized. They calculated that a surface concentration of 300 surface active molecules/µm2 could be sufficient to suppress the Marangoni flow in water. For organic liquids Marangoni flow was shown to impact strongly the flow field inside the droplet.24,29 Regarding the evaporation process, buoyancy effects for pure water was shown to be negligible both in the liquid and gas phases at ambient conditions.22,32 For evaporation controlled by diffusion of the vapour into the ambient gas, studies showed that the time dependence of V2/3, V being the volume, was linear for hydrophilic, hydrophobic and superhydrophobic substrates regardless the contact line was pinned or not.27,29,33 However, due to smaller surface to volume ratio, the slope of such a linear plot was smaller when the triple phase contact line (TPCL) moved. The use of surfactants usually increases the wettability of a sessile droplet by reducing its surface tension even at small concentration34, which is important in many industrial applications, e.g. ink-jet printing. Furthermore, the outward capillary flow due to evaporation will induce an inhomogeneous distribution of the surfactant at the surface thus inducing a Marangoni flow.35 It has been reported that surfactants have little effect on the evaporation rate of its aqueous solutions.35,36 In the case of suspensions, particles can deposit on the substrate during the evaporation time, and form aggregates on the solid/liquid interface with a heterogeneous local concentration. This phenomena leads to specific self-structured assemblies after complete drying as the socalled “coffee ring” effect.14,30,37 During the whole process, the pinning of the droplet and the evaporative flux that takes place mainly at the TPCL are responsible of the deposition of particles at the TPCL by capillary flow, whereas an opposite Marangoni flow could counteract this effect by acting more in favour of a homogenous deposition on the solid/liquid surface.38,39 The temporal evolution of distribution of particles inside the drop and on the substrate during evaporation is strongly influenced by the nature, the size, the shape, the concentration of particles and their affinities with the solvent, the substrate and other non-


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volatile component.37,40–42 The complexity of these processes lies on the coupling between evaporation and particle circulation/deposition. From the theoretical point of view, it has been shown that the evaporation is highly dependent on the droplet base radius, the thermal conductivity of the substrate and the average temperature of the liquid-air interface10,14,20,30 in the case of diffusion controlled evaporation of a sessile droplet of pure liquid with a spherical cap shape. Wang et al.43 studied numerically the effect of the evaporation cooling effect and showed that, for a small temperature decrease of the liquid/air interface, the rate of evaporation increases with the contact angle, but when a strong cooling effect takes place, the rate of evaporation decreased with increasing contact angle. Their results agreed with the experiments reported in the literature. Maatar et al.44 showed that the volatility of the liquid influences the droplet lifetime but not the quasi-steadiness of the process for thin substrates, regardless of its conductivity and the temperature of the substrate. However, in the case of thick heated substrates with small conductivities, transient effects can appear. Semenov et al.45 proposed a model based on computer simulations46 for describing the evaporation of a sessile droplet of pure water in the presence of a contact angle hysteresis. They found universal laws to describe the first and second stage of evaporation independently of the nature of the liquid, in good agreement experimental results on different substrates.45 The theory was also compared with the experimental results of surfactant solutions. Doganci et al.36 studied sodium dodecylsulfate (SDS) and Semenov et al.47 studied Silwet-77 over a broad range of concentrations, below and above the critical aggregation concentration (CAC). The experimental results were found to show good agreement with the theoretical predictions for the first stage of evaporation for different relative humidity and temperatures in the case of Silwet-77. Also, they found that the nature of the aggregates (micelles for SDS and vesicles for Silwet-77) did not influence the first stage of evaporation. Regarding the second stage, differences were obtained for the two surfactants at very low concentrations only whereas above the CAC the agreement was good. To explain this behaviour, the authors suggested that at high concentrations both the solid/liquid and liquid/vapour interfaces were saturated of surfactants and therefore were not affected by the increase of the surfactant concentration in the droplet due to evaporation. However, at very low concentrations there was a competition between evaporation kinetics and the kinetics of adsorption at both interfaces. The authors proposed a modified theory for the second stage which showed a reasonably good fitting of the experimental data for concentrations well below the CAC. In the case of suspensions, some experiments were performed using suspensions of spherical stabilized latex particles with a radius of 0.02, 0.2 and 1µm at 22 and 35°C on PTFE, and found good agreement for the first stage but that the volume fraction and size of particles influenced the kinetics of evaporation for the second stage of evaporation48. Trybala et al.49 compared theoretical predictions and experimental results on the evaporation of nanosuspensions of silicone dioxide (SiO2), titanium dioxide (TiO2) and carbon powder of two different particle sizes on Teflon, Polyethylene and silicon wafer. In some cases, only the first stage of evaporation was observed while the receding contact angle could not be measured experimentally. Nonetheless, they found good agreement between experimental and 5 ACS Paragon Plus Environment


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theoretical results for the first and second stage of evaporation whenever both stages were measured. They pointed out that the particles that were used did not adsorb on the solid nor at the liquid/vapour interface. This can explain why the presence of particles in the droplet did not affect the evaporation in comparison to a pure liquid. It has to be noted as well, that in this case, the concentration of nanoparticles was relatively small and equal to 0.01 in terms of volume fraction. Only a change of wettability was observed in some cases with different advancing and receding contact angle. To the best of our knowledge no difference has been found between experimental results and the universal laws derived by Semenov et al.45 for the first stage of evaporation for pure liquids, suspensions of hydrophilic nano/microparticles and surfactants. Although a lot of different configurations might still be studied. In the present work, we propose to study the role of the adsorption at the liquid/vapour and solid/liquid interfaces for two types of nanoparticles on the wettability and the evaporation of a sessile droplet of aqueous suspensions using different substrates. We used slightly hydrophobic Latex particles which are shown to slightly adsorb at the liquid/vapour interface but not at the solid/liquid interface. Also we are using hydrophobic Silver particles which present high adsorption at both liquid/vapour and solid/liquid interfaces. The later suspension presents a strong interest in ink-jet printing because of its property of high electrical conductivity. A wide range of concentration for both types of particles is studied on several substrates, mainly hydrophilic. Metallic nanoparticle suspensions constitute the base of inks used for electronic circuits. Because of the very different properties between these two suspensions, it is expected to observe very different behaviour regarding wettability, evaporation process and pattern formation. Comparison with universal laws of Semenov et al.45 is achieved for the first stage of evaporation as well as qualitative discussion regarding the pattern formation in dried drops of the remaining particles on the different substrates.

2. Theory 2.1 Theoretical description of the evaporation of a sessile droplet For a sessile droplet with a spherical cap shape, the temperature in the bulk depends on the thermal properties of the substrate and on the rate of evaporation. Semenov et al.45 assumed that the temperature in the bulk and the average surface temperature are constant over the whole time of evaporation process. As a consequence and considering mass conservation, the rate of evaporation can be written as follows:

dV DM = -2π [csat (Tav ) - Hcsat (T∞ )] F(θ)L dt ρ

(1)

D is the diffusion coefficient of vapour in the air. M and ρ are the molar mass and the density of the liquid respectively. Csat is the molar concentration of saturated vapour at the surface of the droplet. H is the relative humidity of the ambient air, Tav is the average surface


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temperature and T∞ is the temperature at infinite distance from the droplet. The function F(θ) used is :

F(θ)= (0.6366θ +0.09591θ 2 - 0.06144θ 3 ) / sinθ, 2

θ < 10º 3

4

(0.00008957 +0.6333θ +0.116θ - 0.08878θ +0.01033θ ) / sinθ,

θ > 10º

(2)

as described by Picknett and Bexon50 for isothermal evaporation and the contact angle θ given in radians. The importance of F(θ) was experimentally demonstrated by Erbil et al..51 The evaporation rate described in Eq.(1) can be rewritten as:

With β = 2π

DM (csat (Tav )- c∞ ) , and: c∞ = Hcsat (T∞ ) ρ

dV = -βF(θ)L dt

(3)

(4)

C∞ is the molar concentration of vapour in the ambient air at infinite distance. In the case of pure liquid with a constant average surface temperature, the coefficient β remains constant during the evaporation process. This parameter cannot be calculated from the Eq.(4) since Tav is not measured.

2.2 Stages of the evaporation process of a sessile droplet In the case of partial wetting with a hysteresis, three stages are present during the whole evaporation of a droplet45: after a droplet is carefully deposited on a solid surface, a spreading is observed which correspond to an increase of the contact line between the droplet and the solid. In all our experiments, the solutions used present a relatively short stage of spreading in comparison to the different stages of evaporation that will be described. Thus, it is assumed that there is no evaporation during the stage of spreading of our sessile droplets. Then, the contact line reaches a maximum value and remains constant during the first stage of evaporation also called “constant contact radius” regime or CCR. The contact angle reaches the value of a so-called advancing contact angle. During the first stage of evaporation, the droplet evaporates and the contact angle decreases until the value of a receding contact angle. Then starts the second stage of evaporation, where this time the contact angle is constant but the contact line decreases. This stage is also called the “constant contact angle” regime or CCA. Finally a third stage appears where both the contact line and contact angle decrease until the complete evaporation of the droplet. All these stages are represented in Figure 1 through the temporal evolution of the contact angle and the contact line. In the literature were also reported different evolution for more complex systems with four stages of evaporation.52



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Figure 1 : Evolution of the contact angle and the contact line of a sessile droplet with the spreading stage in 1), the 1st stage of evaporation in 2) or CCR mode, the 2nd stage of evaporation in 3) or CCA mode and finally the 3rd stage of evaporation in 4). For practical reasons that are explained in the experimental measurements section, we will only focus in the present work on the first stage of evaporation. 2.3 First stage of evaporation Here is described the model of a diffusion controlled evaporation for a pure liquid sessile droplet with spherical cap shape by Semenov et al.45 regarding the first stage of evaporation. During the first stage of evaporation, we consider a constant contact radius, L=L0, through the time of evaporation with a contact angle θ that decreases from an advancing contact angle θadv to a receding contact angle θrec. Thus we can write the initial condition:

θ t=0 = θad

(5)

In the work, only droplets with a spherical cap shape are studied. Thus, the volume of the droplet is only a function of the contact angle and can be written as: 3

V = L f(θ),

(1- cosθ)2 (2+cos(θ)) f(θ)= sin3θ

(6)

It is possible to rewrite the rate of evaporation Eq.(3) using the Eq.(6) and by introducing a reduced time τ = t / tch, with tch = L02 / β :


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f ′(θ)

dθ = − F(θ) dτ

(7)

By integration of Eq.(7), with the boundary condition Eq.(5), the following result is obtained:

A(θ , θad )= τ

That can be rewritten as:

θad

′ / F(θ)) dθ With A(θ , θad ) = ∫θ ( f (θ)

∫

π/2

θ

π / 2 f (θ) ′ f ′(θ) dθ dθ = τ + ∫ θ ad F(θ) F(θ)

Leading to: B(θ) = τ%

(8)

(9) (10)

f ′(θ) dθ and τ% = τ + B(θ ad ) is a new dimensionless time. The latter θ F(θ) equation is universal and only requires the values of the advancing and receding contact angles that have to be determined experimentally, since little is known on their modelization.49 Where B(θ)= ∫

π/2

3. Materials and Methods 3.1 Materials Milli-Q water is used as the solvent and is produced in our laboratory. A cosolvent DB (Diethylene glycol monobutyl ether) is provided by Sigma-Aldrich (Germany). Two types of nanoparticle suspensions were studied: a suspension of 8 wt % of sulphate modified Latex beads that is provided by Sigma-Aldrich (Germany). The size of the particles was 25 nm according to dynamic light scattering measurements. Two solutions of 26 wt % of highly hydrophobized Silver-nanoparticles are also studied. They were produced as described by Magdassi et al.53. The size of the particles was about 10 nm. These two Silver suspensions were made according to the same process, but they present different interfacial properties as it will be shown later. Measurements with dynamic light scattering show important aggregation in the case of Silver suspensions which might explain this behaviour. Nevertheless, good reproducibility was observed for each suspension up to eight months after their synthesis. Four different solid substrates of different wetting properties were used: glass (microscope slide from Knittel Glass), PEN (Polyethylene naphthalate) purchased to DuPont, PET (Polyethylene terephthalate) purchased to Jolybar, and PTFE (polytetrafluoroethylene) purchased to Sigma-Aldrich. 3.2 Experimental methods In order to remove any contamination of the surface of the glass and, as a consequence, to improve the flatness of the surface, it was soaked in Pyranha solution. All the substrates were 9 ACS Paragon Plus Environment


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cleaned with the following process before doing any experiments: strong cleaning with milliQ water, then with pure ethanol and final drying with an air jet. Milli-Q water was used as the solvent to prepare different concentrations of the suspensions. Sulfate-Latex suspension was diluted at 0.08 and 0.8 wt %. The first Silver-polymer suspension, herein after referred to as Ag_1, was diluted at a concentration of 15 wt % with 10 wt % of the cosolvent DB and without DB. The second Silver-polymer suspension, Ag_2, was diluted at 0.15, 1.5 and 15 wt % in milli-Q water only. Before every experiment, the suspensions were placed in an ultrasonic bath for 20 min to ensure that the particles are not aggregated. Care was taken to avoid any heating of the suspension during this process. Then a droplet of the solution is deposited on a substrate using a Hamilton micro syringe of 10 µl precise to ±0.1 µl. The droplet shape analyzer was running before putting the liquid on the substrate to allow following the spreading process and images are taken from the side and from the top at the same time, which allowed us to check the circular shape of the contact line corresponding to a homogeneous wettability of the liquid over the substrate. To ensure reproducibility, each experiment was repeated several times in the exact same conditions and the results are averaged. Because the liquid is added manually, measurements were performed on droplet with different volumes in the 2 to 4.5 µl range which showed no influence on the values of the advancing and receding contact angle. 3.3 Experimental set-up for the study of evaporation A homemade droplet shape analyzer is used to measure the droplet base radius L, the height h and the radius of curvature R of the droplet through the entire time of evaporation. The contact angle θ and the volume V were calculated from to the previously measured parameters and considering a spherical cap shape of the droplet. A typical time for total evaporation of the drops was around 15 minutes so that the droplet totally evaporates. The experiments were performed at room temperature 23ºC, and ambient humidity with 30% ± 5% of relative humidity in a closed chamber that allows us to prevent circulation of ambient air around the droplet for avoiding convection. Also, the chamber of 630 cm3 is not hermetic in order to prevent saturation of water-vapour due to evaporation of the droplet. Images of the droplet were taken at 1.875 images per second which corresponds to the maximum rate of the camera. The surface tension of the suspensions was measured with a platinum plate tensiometer Kruus K10 (Germany). Complementary measurements were done with a home-made drop tensiometer to discard any effect from the adsorption of the particles on the platinum plate. The adsorption at the solid/liquid interface was measured using a quart-crystal microbalance with dissipation KSV-500 (Finland) using sensors coated with a very thin film of the substrates studied in this work. 3.4 Drop shape 10 ACS Paragon Plus Environment

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In this present work, only droplets small enough to be considered as of a spherical shape are studied. The Bond number, Bo, has been used to ensure that the drops had the appropriate size. Bo is the ratio between surface tension forces and body forces, allows to define a critical length lc = (γ/ρg)0.5 where ρ is the density of the liquid, γ is the surface tension, g is the acceleration of gravity. For droplet of water it gives a value of lc = 2,7 mm. As a consequence, we studied droplet with a maximum radius smaller than this value in order to neglect the effect of gravity on the shape of the droplet. For suspensions, the Latex particles have about the same density as water and present a relatively small influence on the surface tension as discussed in interfacial activity section, thus se have neglected the influence on the critical length in comparison to pure water. However, for Silver particles, with a relative density of about 10, and strongly reduce the surface tension, the most different case is considered with a concentration of 15 wt % of Silver particles and 10 wt % of DB in water. A critical length of lc = 1,1 mm is obtained for a lowest surface tension of γ = 28 mN·m-1 measured. The axisymmetric aspect of each droplet has been checked experimentally since the capillary length might evolve during the time of evaporation through a change of surface tension and/or density of the liquid in the case of suspensions.

4. Experimental results 4.1 Interfacial activity 4.1.1 Adsorption at liquid-air interface: effect on surface tension Figure 2 shows the surface tension of the solutions used in this work. The slightly hydrophobic Latex particles produce a relatively small change of the surface tension except for the highest concentration (8 wt %) in which it decreases until 60 mN.m-1. For the first Silver nanosuspension, Ag_1, the highly hydrophobized particles affect strongly the surface tension at the concentration of 15 wt % down to a value of 35 mN.m-1, similar to that of many surfactant solutions close to the critical micellar concentration. This means that the particles adsorb strongly at the liquid-air interface. When 10 wt % of the cosolvent DB is added to 15 wt % of Ag_1, the resulting surface tension is slightly under the case without DB and close to pure DB (below 30 mN.m-1).



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Figure 2: Surface tension of solutions used in this study obtained by pendant drop method. 4.1.2 Adsorption at solid-liquid interface The experiments show that the Latex particles have a negligible adsorption on the substrates studied for the concentrations of 0.08, 0.8 and 8 wt %. Figure 3 shows that the silver particles Ag_1 adsorb strongly onto glass substrate. The adsorption is much stronger when the cosolvent DB is added. It is worth mentioning that the adsorption kinetics clearly competes with the evaporation kinetics since in many cases the full adsorption time is of the same order as the typical evaporation time. It has to be remarked that these experiments were performed in a close cell, so no evaporation exists. The adsorption on the solid substrates will change the solid/liquid interfacial tension which appears in the Young equation, thus it will affect the behaviour of the contact angle. This will also have a very relevant effect for the second stage of evaporation where the contact line retracts. Therefore, in this work we will focus only on the first stage of evaporation.


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Figure 3 : Adsorption of Silver suspensions Ag_1 with DB (full line) and without DB (dashed line) and on glass measured with the quartz-crystal microbalance with dissipation. After reaching a plateau, the substrate was washed with milli-Q water. Figure 3 shows that after rinsing the particles were completely removed from the substrate when DB is added, whereas some adsorbed material remained in the absence of DB. This means that the particles do not stick on the substrate as strongly as when DB is absent. This observation is important for practical applications such as ink-jet printing where spreaders are often included in the ink formulation. 4.2 Contact angle hysteresis We have a specific behaviour of the highly concentrated dispersions where the deposition of the particles at the TPCL during the first stage of evaporation might affect the measurement of the transition between the first and the second stage of evaporation. Indeed, in some cases, the ring of particles deposited at the TPCL might be thick and dark enough to prevent us from studying the receding of the TPCL during the second evaporation stage, especially at the highest concentration of particles. The images from the top show a spreading shadow disk at the beginning, afterwards the first stage of evaporation occurs with a constant contact line during which particles start to deposit mainly at the TPCL. Finally, when the contact line starts to recede, which corresponds to the second stage of evaporation, this change of regime cannot be followed from the top because of the particles deposited at the TPCL when the liquid recedes. In a similar way, from the images of the side, the height of the ring makes impossible to follow the receding contact line. In order to obtain in these cases the value of the receding contact angle, the starting point of the receding stage had to be checked by eye and compared, as a confirmation, to the decrease of the height of the droplet which is lower in the second stage than during the first stage of evaporation. Despite these difficulties, good reproducibility was found in these cases as well. The values of the advancing and receding contact angles and their uncertainties are reported in Table 1. Table 1 : Values of the advancing and receding contact angles for all cases studied. Liquid

Substrate

Advancing contact angle θa

Receding contact angle θr 13

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Water

0.08wt% Latex

0.8wt% Latex

8wt% Latex

15wt% Ag_1

15wt% Ag_1 + 10%DB

0.15wt% Ag_2

1.5wt% Ag_2

15wt% Ag_2

Teflon PET PEN Glass PTFE PET Glass PTFE PET Glass PTFE PET Glass PET PEN Glass PET PEN Glass PTFE PET Glass PTFE PET Glass PTFE PET Glass

106.8±1.0 75.2±1.0 73.3±1.0 45.2±1.0 105.7±1.0 75.6±1.0 42.8±1.0 100.3±1.0 71.6±1.0 32.0±1.0 92.2±1.0 67.3±1.0 24.1±1.8 24.6±1.8 19.2±1.8 15.8±1.8 24.7±1.8 19.2±1.8 16.4±1.8 100.7±1.0 94.6±1.0 70.1±1.0 83.0±1.0 55.9±1.0 -

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96.7±1.0 62.9±1.0 44.9±1.0 29.8±1.8 94.4±1.0 54.9±1.0 92.1±1.0 61.1±1.0 81.7±1.0 52.5±1.0 17.9±1.8 13.0±1.8 12.9±1.8 17.9±1.8 13.9±1.8 11.1±1.8 88.3±1.0 83.7±1.0 56.7±1.0 74.1±1.0 38.4±1.0 -

4.2.1 Latex Particles Figure 4 shows the values of advancing and receding contact angle found experimentally on three different substrates: glass, PET and PTFE for pure water and concentrations of 0.08, 0.8 and 8 wt % of Latex particles. For every suspension, the advancing contact angle is equal or smaller than the one of water on the same substrate. The time of spreading is about the same order than for pure water regardless of the composition, which means that the rearrangement of the particles is fast after deposition on the substrate. However, a noticeable change of the advancing and receding contact angles for each concentration of particles is observed, even for the lowest concentration of 0.08 wt %.


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Figure 4 : Experimental values of the advancing and receding contact angle of suspensions of Latex at 0.08, 0.8 and 8 wt % compared to pure water on PTFE, PET and Glass substrates. The advancing contact angle, θa, decreases as the concentration of particles increases. However, for the receding contact angle, θr, no general trend was observed. In the case of PTFE, θr was smaller for higher concentrations as in the case of θa. On PET, θa is only similar to that of pure water for the lowest concentration of particles, which means that a relatively small quantity of particles, and their rearrangement at the TPCL, do not affect the wettability of the droplet (recall that in this case the surface tension is very close to that of water). Nevertheless, the receding contact angle is smaller than that of water and of concentration higher than 0.8 wt % but smaller than 8 wt % of particles. This difference is higher than the uncertainties. In the case of glass it was impossible to observe the receding contact angle at all the concentrations. A similar behaviour was described by Trybala et al.49, where the authors did not observe a receding stage for droplet of pure water or suspensions of four different types on silicon wafer. These authors suggested that the receding contact angle for all these cases were smaller than the final contact angle observed, or that the droplet was not of a spherical shape anymore. However, in our cases, the camera from the top did not show any recession of the TPCL which leads us to conclude that probably in these cases only the first stage of evaporation occurs. It seems that the deposition of particles at the TPCL pins the liquid front and prevents the recession of the liquid. During the first stage of evaporation, the height of the drop decreases, and so the contact angle, until the droplet becomes a thin film covering the entire initial surface contact between the liquid and the solid. The images from the top show only a shadow disk at the beginning of the first stage of evaporation, when the light beam cannot get through the thickness of the droplet. Then, the height of the droplet becomes small enough that it is not possible to observe correctly the shape of the droplet with the side camera. However, from the top it is possible to see the position of the front line as the thickness of the remaining liquid decreases thus becoming transparent. This is what can be expected when the particles form a relatively thick layer, sign of a coffee ring effect. At this stage of the evaporation, we can consider a film of liquid instead of a droplet. Finally, the 15 ACS Paragon Plus Environment


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contact line starts to recede until complete evaporation of the remaining film. This specific behaviour was not observed on the other substrates. Corkidi et al.54 observed the same evaporation process with similar configuration. They studied suspensions of microparticles of polystyrene on borosilicate glass substrate and observed that the droplets evaporate with constant contact line until the apparition of a film for each of their case before observing the receding stage. 4.2.2 Silver particles (Ag_1) The values of the advancing and receding contact angle for this suspension are much smaller than for pure liquid as it can be seen in Figure 5 a). In this case, the interfacial activity of Ag_1, presented in interfacial activity section, shows a much stronger adsorption of the particles on both interfaces than with the Latex nanoparticles and confirm the increased wettability. The difference of wettability for Ag_1 with or without the cosolvent DB is negligible although Ag_1 show stronger adsorption with DB but similar surface tension. We can assume, as the spreading stage is relatively fast, that the increase of particle adsorption on the substrate with the use of DB might have a slower kinetics, this might explain the similar values of advancing contact angle independently of the initial concentration. Regarding the receding contact angle, the particles might adsorb more on the TPCL with DB, but they do not stick so strongly as shown in Figure 3. As a consequence, one can imagine that adsorbed particles might easily come back into the bulk because of the Marangoni flow or the pinning of the droplet that might induce some local stress at the TPCL were the particles mainly adsorb and could explain the low impact of the cosolvent DB on the receding contact angles. As for the Latex particles on glass, it was not possible to see a receding contact angle for Ag_1 either on this substrate and the value that is represented is the smallest observed.

Figure 5 : Experimental values of the advancing and receding contact angle compared to pure water of suspensions of a) Ag_1 at 15 wt % on PET, PEN and glass substrates and b) Ag_2 at 15, 1.5 and 0.15 wt % on PTFE, PET and glass substrates.


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No further investigations were produced on this specific solution as the results were not reproducible anymore after several months. A new solution Ag_2 prepared in the same condition with the same components gave different results that are presented below. 4.2.3 Silver particles (Ag_2) For these suspensions, it was not possible to observe the advancing and receding contact angle at any concentration on glass. The droplet spreads on the substrate until it was not possible to measure the contact angle anymore. This might be explained by the higher flatness of glass in comparison to PET or PTFE. Here, the spreading lasts a long time relatively to all the others experiments (~ 2 min against ~ 30 s for other suspensions studied). Thus in this case, evaporation might not be negligible anymore during the spreading stage. The same behaviour was observed for 0.15 wt % of Ag_1 on PET. Here, the roughness is not sufficient to explain it. We might assume that chemical interaction associated to a low surface tension and the fact that a weak deposition of particles might not allow a TPCL to stick on the substrate easily. These two last hypotheses seem to be more important because a higher concentration of particles, thus lower surface tension, does not lead to such a spreading. In these cases, the contact angles are represented by a straight line (Figure 5 b)) at a value of 8° corresponding to the lowest contact angle that is possible to measure with our device. The Silver particles have more impact on the wettability than the Latex particles as it can be seen on every substrate for any concentration of particles used with a smaller advancing and receding contact angle. These results are in good agreement with the study on interfacial activity where the Silver particles show more adsorption on both solid/liquid and liquid/vapour interfaces. On PET, the advancing and receding contact angles are smaller for Ag_1 than for Ag_2 at 15% wt. Indeed, it has been shown that Ag_2 has greater surface tension than Ag_1 (Figure 2). Meanwhile on Glass, the advancing contact angle was small enough to not be measured by our equipment with Ag_2 although the value for Ag_1 was greater this time. This can be explained by a stronger adsorption of particles on the substrate with Ag_1, thus inducing a sticking of the TPCL during the spreading stage that wouldn't occur in the case of Ag_2. 4.3 Diffusion controlled evaporation The time dependence of the volume raised to the power 2/3 during CCR stage for water and all the suspensions studied on glass, PEN, PET and PTFE showed a linear decrease of V2/3 vs. t, which validates the hypothesis of diffusion limited evaporation, which confirms that no convection took place during the experiments and allows direct comparison with the theoretical model of Semenov et al.45. 4.4 Comparison of experimental results with theoretical predictions 4.4.1 Water



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Figure 6 shows the evolution of the contact angle as a function of the dimensionless time as expressed in Eq.(11) for pure water on three hydrophilic substrates (PET, PEN and glass) and one hydrophobic (PTFE). As expected and experimentally showed before by Semenov et al.45 for the case of pure water, our experimental results on different substrates show good agreement with the model describing the first stage of evaporation.

Figure 6 : Comparison between theoretical prediction (full line) and experimental data for water on PTFE, PET, PEN and Glass regarding the evolution of the contact angle θ as a function of a dimensionless time τ.

4.4.2 Nanoparticle aqueous dispersions The case of dispersions is sensitively different from that of a pure liquid. The particles do not evaporate, so only water is considered as the volatile component. During evaporation, the liquid/vapour area will continuously decrease and modify the equilibrium of adsorption of particles at this interface which could affect the surface tension of the droplet. The fact that only water evaporates does not necessarily mean that the concentration of particles will increase with time inside the droplet because this phenomenon is in competition with the adsorption of particles at the interfaces. However, except for very small droplets, the bulk concentration increases with time. During the first stage of evaporation, the deposition of particles on the solid surface appears mostly at the TPCL, but it can take place on the whole covered solid surface as well, even though this last process is probably marginal in our cases. Furthermore, the particles present different affinity with the liquid/vapour, solid/liquid interfaces and the bulk, so the composition of the droplet is probably not homogeneous since the beginning of the deposition of the droplet over the substrate and will continue evolving during the whole evaporation time. As a consequence, it is probable that the evolution of the rate of evaporation, as expressed in Eq. (12), might be affected by the presence of the particles 18 ACS Paragon Plus Environment

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during the first stage of evaporation in comparison to the case of pure water,

dV = -β F(θ)L . dt

In

this expression, F(θ) and L are purely geometrical. Nevertheless, the function F(θ) may not be valid anymore in the case of suspension as it was calculated assuming an average and constant surface temperature. The parameter β is strongly dependent on the properties of the liquid Eq.(2) and can be affected in different ways by the evaporation of a suspension β = 2π

DM [ csat (Tav ) - c∞ ] ρ

The change of concentration of the particles at the interface liquid/vapour during evaporation can change the surface tension which has been shown to be correlated to the latent heat of evaporation.55,56 This might change the molar concentration of the volatile component at the interface csat and affect as well the average surface temperature Tav. These parameters are present in the expression of β and assumed to be constant and homogeneous. Also, if the density ρ and the molar mass M of the particles are different from the ones of the solvent, their values will change during the evaporation thus affecting again the value of the parameter β. In summary, it would not be highly surprising to find discrepancies between the experimental results for the suspensions and the predictions of the model of Semenov et al.45, that, as already mentioned, was developed for pure liquids. Latex Particles Figure 7 shows the comparison between the model and our experimental results for the case of Latex nanoparticles at three different concentrations: 0.08, 0.8 and 8 wt % over PTFE, PET and glass. The evolution of the contact angle on glass was plotted until the minimum contact angle measured since it was not possible to observe a receding contact angle. The results of all the concentrations of Latex nanoparticles used on PTFE, PET and glass show a good agreement with the model which means that F(θ) and the coefficient β are not affected by the presence of particles. To explain such behaviour, we can advance that the density of the particles is similar to the one of the solvent, so the change of concentration will not affect significantly the average density of the suspension. Nevertheless, the molar mass is much more important for the Latex particles than for water (104.15 g/mole for styrene against 18.01 g/mole for water). To this point, we can discuss the competition between the evaporation process which tends to increase the concentration of particles in the droplet and the adsorption of particles on the substrate that tends to decrease it.



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Figure 7 : Comparison between theoretical prediction (full line) and experimental data for Latex nanoparticles at 0.081, 0.81 and 8.1 wt % on PTFE, PET and Glass regarding the evolution of the contact angle θ as a function of a dimensionless time τ. Measurements on micro-balance and tensiometer showed that the Latex particles only slightly adsorb on the solid/liquid and liquid/vapour interfaces. This explains the rather low impact on the wettability of these solutions in comparison to pure water on each studied substrate. However, at the relatively high concentration of 8 wt % of particles again no difference was observed with the model despite a higher impact on the wettability is observed (see Figure 4). It will be shown below that, after complete evaporation for all initial concentrations, the remaining structure of particles on the substrate shows a clear coffee ring effect proving that the particles adsorb somehow at the TPCL during the first stage of evaporation for all concentrations, and confirm that the concentration of particles might not necessarily increase during the evaporation process and as a consequence weakly impact the evaporation process. Silver particles (Ag_1) Figure 8 a) shows the comparison between the model and our experimental results for the case of Ag_1 at the concentrations of 15 wt % with and without 10 wt % of DB over glass, PEN and PET. The experimental data are very close to each other and superimpose on the theoretical prediction here as well, even though the contact angle hysteresis is large in comparison to the case of pure water.


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Figure 8 : Comparison of experimental data and theoretical prediction (solid line) with the evolution of the contact angle θ as a function of a dimensionless time τ for a) Ag_1 at 15 wt % with and without 10 wt % of DB on Teflon, PET and Glass and b) Ag_2 at 0.15, 1.5 and 15 wt % on PTFE, and PET. It is reasonable to assume that the interface liquid/vapour is already saturated in particles because of the high initial concentration of particles so that the surface tension will not change during the evaporation. Despite the difference of density of the particles and the solvent is very important, it can be assumed that the change of the average density of the suspension might be smaller than initially expected because the particles were seen to strongly adsorb at both interfaces, and the characteristic adsorption time on the solid is comparable to that of the total evaporation process, and thus higher than the one of the first stage. Furthermore, the particles might adsorb over the whole substrate and not only at the TPCL because of their strong affinity with the substrate and a Marangoni flow induced by surface action of the highly hydrophobized silver particles. Also, it has to be noted that it is difficult to obtain deviation from the model for small hysteresis (about 8° maximum), where a change of trend of evaporation would not appear for a relatively small change of volume. The adsorption at the interfaces liquid-vapour and liquid-solid of the particles at high concentration are fast in comparison to the evaporation. As a consequence, we can assume that the energy of the interface liquid-vapour γl-v and interface liquid-solid γl-s are constant during evaporation. As well, the energy at the interface solid-vapour γs-v is constant during CCR mode while it might change during CCA mode because, when the TPCL is receding, particles have adsorbed on the substrate modifying the γs-v. Thus, we can assume that at high concentration of particles, the energy of all interfaces γl-s, γl-v and γs-v is at equilibrium. Silver particles (Ag_2) Figure 8 b) shows the comparison between the experimental results for the suspensions of Ag_2 at 0.15, 1.5 and 15 wt % on Teflon, PET and the theoretical predictions. It was impossible to compare the cases of 0.15 wt % of Ag_2 on PET, nor on glass for all the



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concentrations used because the advancing contact angle was too small to be measured with our equipment. In spite of the huge difference of the values of the advancing and receding contact angle in comparison to pure water, good agreement with the model was found for the suspensions at all the concentrations studied. The same arguments discussed regarding the first silver suspension Ag_1 might explain this unexpected result, although here the values of the hysteresis are much larger. In the case of the smallest concentrations, the interface liquid/vapour might not be saturated with particles which might lead to a decrease of surface tension during evaporation. However this process can be compensated by the adsorption of particles on the substrate. Although for all concentrations of Ag_2 an eventual increase of the concentration of particles during evaporation would not have a strong effect on the coefficient β since it would lead to an increase of the average density and also of the average mass molar of the droplet, which can compensate each other in the M/ρ. The universal law of Semenov et al.45 seems sufficient to describe the first stage of evaporation for a lot of different configurations of complex solutions and, to our knowledge, no deviation have been found in the literature or in this present work. Even though good agreement for all suspensions was found with theoretical predictions, high differences in the pattern formation were observed between all the different solutions and the different substrates.

5. Pattern formation The patterns formed after evaporation have been studied using scanning electronic microscopy (SEM). However, in some cases, it has not been possible to use this technique because the particle structures were destroyed during this process (8,1 wt % of latex particles on PTFE, 15 wt % Ag_2 on PET and on PTFE). 5.1

Latex particles

As mentioned in a previous section, the latex particles have little effect on the surface tension of the droplet. Furthermore, being soft material, their ability to deform might play a role on the packing during all the stages of evaporation as explained by Shao et al.3. Figure 9 shows the residuals formed by the Latex nanosuspensions on glass, PET and PTFE, with an initial concentration of 0.81 wt %. They show that increasing the hydrophobicity of the substrate (from A) to C)) leads to a more compact deposition because the Latex particles have to deposit on a smaller contact surface. Each case presents two different areas with cracks at the external area (Figure 9 A1) and aggregates at the centre (Figure 9 B2). Between the cracks and aggregates, the deposition of particles seems to be uniform in each area. Increasing the concentration of particles has a similar effect on the final pattern than increasing the hydrophobicity of the substrate (thus decreasing the contact surface), thus 22 ACS Paragon Plus Environment

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leading to an increase of the size and number of aggregates at the centre and cracks at the external area. Furthermore, the size of the external area increases with hydrophobicity or initial concentration, especially on PTFE where continuous structures, looking like petals with cracks parallel to the drying direction, start at the initial TPCL and reach the centre of the droplet.

Figure 9 : Deposition of Latex particles on A) glass, B) PET and C) PTFE with an initial concentration of 0.81 wt %. The subscript 1 and 2 correspond to different magnifications of the same sample. 5.2

Silver particles Ag_2

Figure 10 shows the impact of the concentration of silver nanoparticles Ag_2 on the final pattern on glass. As described for latex particles, the pattern formation of the silver particles Ag_2 can be divided in two areas with an external ring denser than the centre of the droplet (Figure 10 A2) which presents more dispersed aggregates (on the right side of the image). A higher concentration has the same effect as a decrease of wettability corresponding to a higher deposition of particles per surface area that was observed on PET and PTFE. The highest concentration in Figure 10 C) presents a homogeneous pattern. No cracks are visible for Ag_2 although they are present for latex particles at the external part for every concentration and as well for Ag_1 as seen in Figure 11 A) at 15 wt %.



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Figure 10 : Deposition of Silver particles Ag_2 on PET with an initial concentration of A) 0.15 wt %, B) 1.5 wt % and C) 15 wt %. The subscript 1 and 2 correspond to different magnifications of the same sample. 5.3

Effect of DB on Ag_1

Figure 11 presents the residues left by the 15 wt % suspension of silver particles Ag_1 on PET A) without DB and B) with DB. Both cases present two areas with a large external zone and a central one. Without DB, every zone is full of large cracks sign of a fast drying. When DB is added, the pattern looks homogeneous for each area. It seems reasonable to think that some DB remains with the deposited particles after the recession of the droplet. Indeed the cosolvent evaporates much slower than water. This is confirmed by the change of the pattern formation observed, at higher magnification, where the final pattern of dried drops is porous and homogeneous when DB is added. Meanwhile, without DB, the relatively fast drying of the solid surface leads to important mechanical stress on the structure and leads to numerous cracks divided in two zones, although, the aggregation seems more compact and homogeneous between the cracks. In any case, the high concentration of hydrophobic particles allows a strong deposition at the TPCL during the first stage of evaporation. After CCR more, the concentration of particles inside the droplet might still be high thus explaining the fully covered surface area.


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Figure 11 : Deposition of Silver particles Ag_1 on PET with an initial concentration of 15 wt % A) without DB and B) with DB. The subscript 1 and 2 correspond to different magnifications of the same sample.

6. Conclusions Experimental measurements of the contact angles and the evaporation of a sessile droplet have been performed on different substrates covering a wide range of contact angles. SulfateLatex and highly hydrophobized Silver nanoparticles with very different interfacial properties were suspended in pure water. The surface tension and solid adsorption of these suspensions were measured thanks to a tensiometer and a quart-crystal microbalance respectively. Droplets of these suspensions with a volume included between 1.5 and 4.5 µl were deposited on glass and various polymer substrates, hydrophilic (PEN, PET) and hydrophobic (PTFE). The evolution of the contact line, contact angle and volume were performed using a droplet shape analyser and assuming axisymmetry of the droplet. The impact on the wettability of a sessile droplet was observed for various concentrations regarding the value of the advancing and receding contact angles and shows a stronger influence for higher concentration of particles. The silver particles showed noticeable tendency to aggregation, however it did not impact the reproducibility of the experimental results up to eight months after the preparation of the solution. Nonetheless, the preparation of a new solution with the exact same procedure gives different behaviour regarding the wettability of the system. Once again, reproducibility was confirmed for this new solution regarding wettability and evaporation process. Regarding the evaporation, the evolution of the contact angle was compared to the theoretical predictions of Semenov et al.45,46,57 as a function of a dimensionless time for the first stage of 25 ACS Paragon Plus Environment


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evaporation. Stages with a receding contact radius couldn't have been measured by our equipment as the deposition of particles at the TPCL during the CCR mode prevent us from seeing clearly the recession of the TPCL. Nonetheless, good agreement is found for both types of suspension at any concentration between experiments and theory for the first stage of evaporation on all the different substrates. This conclusion is highly surprising as the particle concentration in the bulk and at the liquid/vapour and solid/liquid interfaces might change during the time of evaporation and modify the properties of the droplet such as the density, the molar mass and interfacial activities that affect the evaporation of the droplet. It has to be noted that the evolution of the average concentration of particles in the evaporating droplet cannot be predicted as adsorption of particles occurred at the TPCL and is not measured. As a consequence, the volume of liquid decreases with evaporation but the concentration of particles inside the bulk might not necessarily increase during the CCR mode, thus we have assumed that the average concentration could remain almost constant during this stage explaining the good agreement with the theoretical predictions. It is expected to observe differences for these dispersions regarding the second and the third stages of evaporation in comparison to the case of pure liquid. Differences were already observed between experimental results and a model developed for the second stage of evaporation for some cases in the literature36. This relatively simple model of Semenov et al.45 was able to predict the evaporation evolution during CCR mode for very complex situations although it is based on the evaporation of a sessile droplet with constant and homogeneous properties. A lot of different configurations might still be investigated using different shape of particles, solvents, solvent mixtures and superhydrophobic substrates in order to test the model and to have a better understanding of the evaporation of complex systems. Regarding the third stage of evaporation, a theory still is still lacking. Finally, the structure of the deposition of particles after complete drying on the solid surface was studied thanks to a scanning electronic microscope and was shown to be strongly influenced by the nature of the fluid, the concentration of particles and the nature of the substrate. The final deposit showed that particles adsorb during the CCR mode even for latex particles which showed poor adsorption on quartz-crystal microbalance. The deposited layer was shown to be thicker for higher initial concentration of particles and higher advancing contact angle. Aknowledgment This work has been supported by the E.U. through the Marie-Curie Initial Training Network “Complex Wetting” (CoWet), and by MINECO through grant CTQ2016-78895-R. References (1) (2)

Singh, M.; Haverinen, H. M.; Dhagat, P.; Jabbour, G. E. Inkjet Printing-Process and Its Applications. Adv. Mater. 2010, 22 (6), 673–685. Brutin, D.; Sobac, B.; Loquet, B.; Sampol, J. Pattern Formation in Drying Drops of Blood. J. Fluid Mech. 2011, 667, 85–95.


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(3) (4) (5)

(6) (7)

(8)

(9) (10)

(11) (12)

(13)

(14)

(15)

(16)

(17)

(18)

(19) (20) (21)

Shao, F. F.; Neild, A.; Alan, T. Controlled Particle Self-Assembly in an Evaporating Droplet. Colloids Surf. Physicochem. Eng. Asp. 2012, 398, 64–68. Lin, W. Introduction: Nanoparticles in Medicine. Chem. Rev. 2015, 115 (19), 10407– 10409. Bou Zeid, W.; Vicente, J.; Brutin, D. Influence of Evaporation Rate on Cracks’ Formation of a Drying Drop of Whole Blood. Colloids Surf. Physicochem. Eng. Asp. 2013, 432, 139–146. Laan, N.; Smith, F.; Nicloux, C.; Brutin, D. Morphology of Drying Blood Pools. Forensic Sci. Int. 2016, 267, 104–109. Magdassi, S.; Grouchko, M.; Toker, D.; Kamyshny, A.; Balberg, I.; Millo, O. Ring Stain Effect at Room Temperature in Silver Nanoparticles Yields High Electrical Conductivity. Langmuir 2005, 21 (23), 10264–10267. Tarasevich, Y. Y.; Vodolazskaya, I. V.; Bondarenko, O. P. Modeling of Spatial– temporal Distribution of the Components in the Drying Sessile Droplet of Biological Fluid. Colloids Surf. Physicochem. Eng. Asp. 2013, 432, 99–103. Devlin, N. R.; Loehr, K.; Harris, M. T. The Separation of Two Different Sized Particles in an Evaporating Droplet. AIChE J. 2015, 61 (10), 3547–3556. Christian Diddens; Huanshu Tan; Pengyu Lv; Michel Versluis; J. G. M. Kuerten; Xuehua Zhang; Detlef Lohse. Evaporating Pure, Binary and Ternary Droplets Thermal Effects and Axial Symmetry Breaking. J. Fluid Mech. 2017, DOI: 10.1017/jfm.2017.312. Sáenz, P. J.; Sefiane, K.; Kim, J.; Matar, O. K.; Valluri, P. Evaporation of Sessile Drops: A Three-Dimensional Approach. J. Fluid Mech. 2015, 772, 705–739. Crafton, E. F.; Black, W. Z. Heat Transfer and Evaporation Rates of Small Liquid Droplets on Heated Horizontal Surfaces. Int. J. Heat Mass Transf. 2004, 47 (6–7), 1187–1200. Nguyen, T. A. H.; Nguyen, A. V.; Hampton, M. A.; Xu, Z. P.; Huang, L.; Rudolph, V. Theoretical and Experimental Analysis of Droplet Evaporation on Solid Surfaces. Chem. Eng. Sci. 2012, 69 (1), 522–529. Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389, 827–829. Dunn, G. J.; Wilson, S. K.; Duffy, B. R.; David, S.; Sefiane, K. A Mathematical Model for the Evaporation of a Thin Sessile Liquid Droplet: Comparison between Experiment and Theory. Colloids Surf. Physicochem. Eng. Asp. 2008, 323 (1–3), 50–55. Ristenpart, W. D.; Kim, P. G.; Domingues, C.; Wan, J.; Stone, H. A. Influence of Substrate Conductivity on Circulation Reversal in Evaporating Drops. Phys. Rev. Lett. 2007, 99 (23), 234502 (4). David, S.; Sefiane, K.; Tadrist, L. Experimental Investigation of the Effect of Thermal Properties of the Substrate in the Wetting and Evaporation of Sessile Drops. Colloids Surf. Physicochem. Eng. Asp. 2007, 298 (1–2), 108–114. Wang, Y.; Ma, L.; Xu, X.; Luo, J. Combined Effects of Underlying Substrate and Evaporative Cooling on the Evaporation of Sessile Liquid Droplets. Soft Matter 2015, 11 (28), 5632–5640. Erbil, H. Y. Evaporation of Pure Liquid Sessile and Spherical Suspended Drops: A Review. Adv. Colloid Interface Sci. 2012, 170 (1–2), 67–86. Dunn, G. J.; Wilson, S. K.; Duffy, B. R.; David, S.; Sefiane, K. The Strong Influence of Substrate Conductivity on Droplet Evaporation. J. Fluid Mech. 2009, 623, 329–351. Sobac, B.; Brutin, D. Thermal Effects of the Substrate on Water Droplet Evaporation. Phys. Rev. E 2012, 86 (2), 021602 (10). 27 ACS Paragon Plus Environment


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(22) Bouchenna, C.; ait saada, mebrouk; Chikh, S.; Tadrist, L. Flow inside Evaporating Water Sessile Drop: A Numerical Study. Proc. 15th Int. Heat Transf. Conf. Kyoto, Japan, August 10-15, 2014. (23) Gelderblom, H.; Marín, Á. G.; Nair, H.; van Houselt, A.; Lefferts, L.; Snoeijer, J. H.; Lohse, D. How Water Droplets Evaporate on a Superhydrophobic Substrate. Phys. Rev. E 2011, 83 (2), 026306 (6). (24) Chandramohan, A.; Dash, S.; Weibel, J. A.; Chen, X.; Garimella, S. V. Marangoni Convection in Evaporating Organic Liquid Droplets on a Nonwetting Substrate. Langmuir 2016, 32 (19), 4729–4735. (25) Shanahan, M. E. R.; Sefiane, K.; Moffat, J. R. Dependence of Volatile Droplet Lifetime on the Hydrophobicity of the Substrate. Langmuir 2011, 27 (8), 4572–4577. (26) Poulard, C.; Guéna, G.; Cazabat, A. M. Diffusion-Driven Evaporation of Sessile Drops. J. Phys. Condens. Matter 2005, 17 (49), S4213–S4227. (27) Janssen, D.; De Palma, R.; Verlaak, S.; Heremans, P.; Dehaen, W. Static Solvent Contact Angle Measurements, Surface Free Energy and Wettability Determination of Various Self-Assembled Monolayers on Silicon Dioxide. Thin Solid Films 2006, 515 (4), 1433–1438. (28) Hu, H.; Larson, R. G. Analysis of the Effects of Marangoni Stresses on the Microflow in an Evaporating Sessile Droplet. Langmuir 2005, 21 (9), 3972–3980. (29) Hu, H.; Larson, R. G. Marangoni Effect Reverses Coffee-Ring Depositions. J. Phys. Chem. B 2006, 110 (14), 7090–7094. (30) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Contact Line Deposits in an Evaporating Drop. Phys. Rev. E 2000, 62 (1), 756–766. (31) Boulogne, F.; Ingremeau, F.; Stone, H. A. Coffee-Stain Growth Dynamics on Dry and Wet Surfaces. J. Phys. Condens. Matter 2017, 29 (7), 074001 (12). (32) Guéna, G.; Poulard, C.; Cazabat, A. M. The Leading Edge of Evaporating Droplets. J. Colloid Interface Sci. 2007, 312 (1), 164–171. (33) Ramos, S. M. M.; Dias, J. F.; Canut, B. Drop Evaporation on Superhydrophobic PTFE Surfaces Driven by Contact Line Dynamics. J. Colloid Interface Sci. 2015, 440, 133– 139. (34) Kajiya, T.; Kobayashi, W.; Okuzono, T.; Doi, M. Controlling the Drying and Film Formation Processes of Polymer Solution Droplets with Addition of Small Amount of Surfactants. J. Phys. Chem. B 2009, 113 (47), 15460–15466. (35) Truskett, V. N.; Stebe, K. J. Influence of Surfactants on an Evaporating Drop: Fluorescence Images and Particle Deposition Patterns. Langmuir 2003, 19 (20), 8271– 8279. (36) Doganci, M. D.; Sesli, B. U.; Erbil, H. Y. Diffusion-Controlled Evaporation of Sodium Dodecyl Sulfate Solution Drops Placed on a Hydrophobic Substrate. J. Colloid Interface Sci. 2011, 362 (2), 524–531. (37) Askounis, A.; Sefiane, K.; Koutsos, V.; Shanahan, M. E. R. Effect of Particle Geometry on Triple Line Motion of Nano-Fluid Drops and Deposit Nano-Structuring. Adv. Colloid Interface Sci. 2015, 222, 44–57. (38) Still, T.; Yunker, P. J.; Yodh, A. G. Surfactant-Induced Marangoni Eddies Alter the Coffee-Rings of Evaporating Colloidal Drops. Langmuir 2012, 28 (11), 4984–4988. (39) Christy, J. R.; Sefiane, K.; Romain, B.; others. Velocity Measurement during Evaporation of Seeded, Sessile Drops on Heated Surfaces. 2nd Micro Nano Flows Conf. West London, UK, 1-2 September, 2009. (40) Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the Coffee-Ring Effect by Shape-Dependent Capillary Interactions. Nature 2011, 476 (7360), 308–311.


Page 29 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Carle, F.; Brutin, D. How Surface Functional Groups Influence Fracturation in Nanofluid Droplet Dry-Outs. Langmuir 2013, 29 (32), 9962–9966. (42) Crivoi, A.; Duan, F. Amplifying and Attenuating the Coffee-Ring Effect in Drying Sessile Nanofluid Droplets. Phys. Rev. E 2013, 87 (4), 042303 (8). (43) Wang, Y.; Ma, L.; Xu, X.; Luo, J. Expressions for the Evaporation of Sessile Liquid Droplets Incorporating the Evaporative Cooling Effect. J. Colloid Interface Sci. 2016, 484, 291–297. (44) Maatar, A.; Chikh, S.; Ait Saada, M.; Tadrist, L. Transient Effects on Sessile Droplet Evaporation of Volatile Liquids. Int. J. Heat Mass Transf. 2015, 86, 212–220. (45) Semenov, S.; Starov, V. M.; Rubio, R. G.; Agogo, H.; Velarde, M. G. Evaporation of Sessile Water Droplets: Universal Behaviour in Presence of Contact Angle Hysteresis. Colloids Surf. Physicochem. Eng. Asp. 2011, 391 (1–3), 135–144. (46) Semenov, S.; Starov, V. M.; Rubio, R. G.; Velarde, M. G. Instantaneous Distribution of Fluxes in the Course of Evaporation of Sessile Liquid Droplets: Computer Simulations. Colloids Surf. Physicochem. Eng. Asp. 2010, 372 (1–3), 127–134. (47) Semenov, S.; Trybala, A.; Agogo, H.; Kovalchuk, N.; Ortega, F.; Rubio, R. G.; Starov, V. M.; Velarde, M. G. Evaporation of Droplets of Surfactant Solutions. Langmuir 2013, 29 (32), 10028–10036. (48) Agogo, H. O. Mojado Y Evaporación de Disoluciones de Tensioactivos Y Nanoemulsiones, Ph.D thesis, Universidad Complutense de Madrid, Madrid, SP, 2013. (49) Trybala, A.; Okoye, A.; Semenov, S.; Agogo, H.; Rubio, R. G.; Ortega, F.; Starov, V. M. Evaporation Kinetics of Sessile Droplets of Aqueous Suspensions of Inorganic Nanoparticles. J. Colloid Interface Sci. 2013, 403, 49–57. (50) Picknett, R. G.; Bexon, R. The Evaporation of Sessile or Pendant Drops in Still Air. J. Colloid Interface Sci. 1977, 61 (2), 336–350. (51) Erbil, H. Y.; McHale, G.; Newton, M. I. Drop Evaporation on Solid Surfaces: Constant Contact Angle Mode. Langmuir 2002, 18 (7), 2636–2641. (52) Tan, H.; Diddens, C.; Lv, P.; Kuerten, J. G.; Zhang, X.; Lohse, D. EvaporationTriggered Microdroplet Nucleation and the Four Life Phases of an Evaporating Ouzo Drop. Proc. Natl. Acad. Sci. 2016, 113 (31), 8642–8647. (53) Magdassi, S.; Grouchko, M.; Berezin, O.; Kamyshny, A. Triggering the Sintering of Silver Nanoparticles at Room Temperature. ACS Nano 2010, 4 (4), 1943–1948. (54) Corkidi, G.; Montoya, F.; Hernández-Cruz, G.; Vargas, M.; Luviano-Ortíz, J. L.; Ramos, E. Evaporation Dynamics and Sedimentation Pattern of a Sessile Particle Laden Water Droplet. Exp. Fluids 2016, 57 (6), 99 (11). (55) Viswanath, D. S.; Kuloor, N. R. Latent Heat of Vaporization, Surface Tension, and Temperature. J. Chem. Eng. Data 1966, 11 (1), 69–72. (56) Strechan, A. A.; Kabo, G. J.; Paulechka, Y. U. The Correlations of the Enthalpy of Vaporization and the Surface Tension of Molecular Liquids. Fluid Phase Equilibria 2006, 250 (1–2), 125–130. (57) Semenov, S.; Starov, V. M.; Velarde, M. G.; Rubio, R. G. Droplets Evaporation: Problems and Solutions. Eur. Phys. J. Spec. Top. 2011, 197 (1), 265–278.



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Table of Contents Graphic


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Figure 1 : Evolution of the contact angle and the contact line of a sessile droplet with the spreading stage in 1), the 1st stage of evaporation in 2) or CCR mode, the 2nd stage of evaporation in 3) or CCA mode and finally the 3rd stage of evaporation in 4). 152x160mm (96 x 96 DPI)



Figure 2: Surface tension of solutions used in this study obtained by pendant drop method. 256x163mm (96 x 96 DPI)


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Figure 3 : Adsorption of Silver suspensions Ag_1 with DB (full line) and without DB (dashed line) and on glass measured with the quartz-crystal microbalance with dissipation. 182x108mm (96 x 96 DPI)



Figure 4 : Experimental values of the advancing and receding contact angle of suspensions of Latex at 0.08, 0.8 and 8 wt % compared to pure water on PTFE, PET and Glass substrates. 221x156mm (96 x 96 DPI)


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Figure 5 : Experimental values of the advancing and receding contact angle compared to pure water of suspensions of a) Ag_1 at 15 wt % on PET, PEN and glass substrates and b) Ag_2 at 15, 1.5 and 0.15 wt % on PTFE, PET and glass substrates. 445x159mm (96 x 96 DPI)



Figure 6 : Comparison between theoretical prediction (full line) and experimental data for water on PTFE, PET, PEN and Glass regarding the evolution of the contact angle θ as a function of a dimensionless time τ. 225x171mm (96 x 96 DPI)


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Figure 7 : Comparison between theoretical prediction (full line) and experimental data for Latex nanoparticles at 0.081, 0.81 and 8.1 wt % on PTFE, PET and Glass regarding the evolution of the contact angle θ as a function of a dimensionless time τ. 220x178mm (96 x 96 DPI)



Figure 8 : Comparison of experimental data and theoretical prediction (solid line) with the evolution of the contact angle θ as a function of a dimensionless time τ for a) Ag_1 at 15 wt % with and without 10 wt % of DB on Teflon, PET and Glass and b) Ag_2 at 0.15, 1.5 and 15 wt % on PTFE, and PET. 372x156mm (96 x 96 DPI)


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Figure 9 : Deposition of Latex particles on A) glass, B) PET and C) PTFE with an initial concentration of 0.81 wt %. The subscript 1 and 2 correspond to different magnifications of the same sample. 1014x513mm (96 x 96 DPI)



Figure 10 : Deposition of Silver particles Ag_2 on PET with an initial concentration of A) 0.15 wt %, B) 1.5 wt % and C) 15 wt %. The subscript 1 and 2 correspond to different magnifications of the same sample. 1020x510mm (96 x 96 DPI)


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Figure 11 : Deposition of Silver particles Ag_1 on PET with an initial concentration of 15 wt % A) without DB and B) with DB. The subscript 1 and 2 correspond to different magnifications of the same sample. 852x644mm (61 x 61 DPI)


Evaporation of Nanosuspensions on Substrates with Different

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