Evaporation Dynamics of Mixed-Nanocolloidal Sessile Droplets

Subscriber access provided by READING UNIV

Article

Evaporation dynamics of mixed-nanocolloidal sessile droplets Binita Pathak, Sandeep Hatte, and Saptarshi Basu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03578 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17 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

Langmuir

Graphical Abstract

1 ACS Paragon Plus Environment

Langmuir 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

Page 2 of 17

Evaporation dynamics of mixed-nanocolloidal sessile droplets Binita Pathak, Sandeep Hatte and Saptarshi Basu* Department of Mechanical Engineering, Indian Institute of Science, Bangalore-560012, India *Corresponding author email: [email protected]

Abstract Evaporation dynamics of a particle laden droplet has been a topic of interest in recent times due to its widespread applications, ranging from surface patterning to drug delivery systems. The interplay of evaporation induced internal flow dynamics, contact line dynamics and nanoparticle self-assembly govern the morphologies of the residual structures. Fine tuning of these residual structures is thus possible by controlling the governing parameters. A nanoparticle laden sessile droplet placed on a hydrophobic substrate undergoes buckling phenomenon which results in a dome-like structure with cavity on the surface. In the present work, it is shown that the addition of SDS (sodium dodecyl sulphate) surfactant in minute concentrations (0.005 % wt. to 0.02 % wt.) can affect the contact line dynamics and subsequent buckling dynamics of a nanoparticle laden droplet evaporating on a hydrophobic substrate. With increase in the initial SDS concentration the morphologies of the residual structures show transition from a buckled dome structure to a flat flower like shape. Moreover, a critical SDS concentration (> 0.0075 % wt. in 20 % wt. silica) is identified for the complete suppression of buckling instabilities. Lastly, the effects of droplet spreading on the surface crack dynamics are discussed.

Introduction Drying droplets are fundamental building blocks in many industrial processes such as sprays in pharmaceutics, food processing, ink-jet printing and drug delivery, among others.1-5 Droplets of colloidal suspension are especially crucial in these practical applications. Evaporation of liquid causes self-assembly of particles in the colloidal suspension and leads to the formation of final structures with unique topological features (like coffee rings). Such structures with tailored morphologies can be obtained by controlling the governing parameters which is central to many industrial applications.


Page 3 of 17 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

Langmuir

Deposition of particles at the periphery of a drying droplet famously known as the coffee ring effect was first explained by Deegan.6 Numerous investigations have been conducted in the past decade related to physiochemical processes involved in the drying of droplets in sessile, pendant or levitated modes as well as in sprays.7-13 Picknett et al.14 investigated the evaporation dynamics of sessile droplets and theoretically predicted the evaporation rate and lifetime of the droplets. Hu and Larson numerically developed a model for the evaporation rate of such droplets which was validated with the experimental results.15 They also investigated the effects of thermal Marangoni stresses on the internal flow field of an evaporating droplet.16

Internal fluid flow field aggravates the self-assembly of particles inside a droplet (colloidal suspension). Agglomeration of colloidal particles leads to the formation of a porous visco-elastic shell at the surface of a drying droplet. The shell undergoes sol-gel transition which causes buckling instabilities.17 Buckling is identified as surface indentations that develop into cavities as the evaporation progresses. Typically, such cavities undergo subsequent growth and lead to the formation of bowl-shaped residual structures.

Therefore, morphologies of the final structures are determined by a complex interplay of several governing parameters which include the rate of evaporation, internal flow dynamics, contact line dynamics and particle agglomeration. Hu et al.18, among others19-20, showed that the morphologies of the nano-aggregates and their deposition pattern can be manipulated by controlling temperature profile of the substrate using patterned heating. Such residual structures can also be controlled by inducing vapour mediated interactions as in a droplet array.21-22 In addition, initial solution composition and the substrate hydrophobicity (initial apparent contact angle) also play a critical role in determining the deposit morphologies.23-24 Das et al.25 have demonstrated the effect on initial contact angle of an electrolytic drop on a charged surface. Variations in particle size, type and distribution also cause significant changes in the final structures. Yunker et al.26 have demonstrated that the coffee-ring effect can be eliminated to produce residual patterns of uniform deposition by controlling the shape of the suspended particles. In the current study, a more subtle and active method namely mixed colloidal suspension is chosen to control the evaporation dynamics and residual morphologies in the context of sessile droplet architecture. Studies based on the methodology of adding multi-colloidal components are fundamental in spray drying,27-28 but very few studies have explored the dynamics at a fundamental level in a single droplet.29 We have previously examined the evaporation dynamics and associated caving phenomena in a contact-free (levitated; constant contact angle mode) multi-colloidal droplet system.8 Mixed colloids drastically alter the buckling dynamics and fine tunes the final topologies. In the present work, the dynamics of mixed colloidal droplet is investigated in sessile mode to provide additional insights into the contact line dynamics i.e. stickslide behaviour (completely absent in levitated droplets). 3 ACS Paragon Plus Environment

Langmuir 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

Page 4 of 17

We incorporated sodium dodecyl sulphate (SDS) surfactants in aqueous suspension of nano sized silica particles and explored the drying characteristics of the droplets on a hydrophobic substrate. Addition of surfactant in nano-colloidal droplet alters the spreading behaviour thereby changing its morphology as the evaporation progresses. Additionally, the dynamics of buckling of a nano-colloidal droplet is also affected by increasing the concentration of SDS (0.005 % by wt. - 0.02% by wt.) and is completely suppressed above a critical SDS concentration (> 0.0075 % by wt.). The internal flow pattern provides insights into the excessive spreading of droplet with increasing concentration of SDS. Inclusion of mixed colloids in droplets demonstrates drastic changes in the final residual structures as elucidated by the scanning electron microscopy (SEM) images.

Experimental Methodology We used an initial aqueous colloidal silica suspension (LUDOX TM40, diameter 22±2 nm, pH~9, Sigma Aldrich, India) diluted to 20% (by wt.) in de-ionised water as the base case. Sodium dodecyl sulphate (SDS, BioReagent, ≥ 98.5% (GC), Sigma Aldrich, India) was added in various concentrations (0.005 % by wt. to 0.02% by wt.) and sonicated in an ultrasonicator for 30 minutes. The Critical Molar Concentration (CMC) of SDS in water is around 2.3 % by wt. The experimentally chosen value of SDS concentration is in the range of 2.1 − 8.6 × 10 times the CMC. SDS was

included in such low concentrations so that the thermo-physical properties of the working fluid are not affected, except at the interface. Droplets of 3 µl volume were deployed on a MPL (Microporous Layer; initial contact angle ~ 135˚±3˚) substrate using a syringe pump assembly (Holmarc). Temporal variation in the droplet shape and subsequent buckling dynamics was recorded at 1 fps (side view imaging) using Nikon D7200 SLR camera fitted with a Navitar zoom lens assembly. The ambient was maintained at 25±2˚C and 45±3 % relative humidity during the entire droplet lifetime. The setup was enclosed in order to reduce the effects of convection on droplet evaporation. A top mounted microscope assembly was also used to locate the position of shell buckling (by identifying the indentation mark on surface) and cavity formation. For the characterisation of internal flow dynamics, the aqueous TM+SDS solution was seeded with 0.008% (by vol.) solution of rhodamine coated

polystyrene particles diameter ≈ 860 nm and side view images obtained at 1 fps were overlapped

to obtain the streaklines of flow inside the droplet. At the end of evaporation process, the residual structures of samples were characterized by the images obtained from scanning electron microscope. The set-up is schematically shown in Fig. 1. Results and Discussion Dynamics of a nano-colloidal sessile droplet


Page 5 of 17 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

Langmuir

A colloidal droplet resting on a substrate shows dramatic physico-chemical changes during its entire drying lifetime (Fig. 2a). The internal flow induces transport of uniformly dispersed nanoparticles which is followed by collision and agglomeration of the particles. The agglomeration (perikinetic and orthokinetic) results in the formation of a thin shell of non-uniform thickness at the droplet surface (phase II in Fig. 3a). Loss of water due to continuous evaporation causes sol-gel transition of the shell, resulting in visco-elastic characteristics. Non-uniformity in thickness of the shell is attributed to the dynamic process of particle self-assembly which predominantly occurs in two stages: firstly, shear induced interactions and agglomeration kinetics near the substrate forms base of the shell. Subsequently the recirculating flow induced perikinetic agglomeration near the liquid-air interface results into the formation of hollow shell (Fig. 3a). This preferential direction of porous shell formation (from base towards top) results in the weakest spot (minimum shell thickness) near the apex region. Continuous evaporation from the menisci of the randomly packed aggregates of nanoparticles develops an interfacial compressive capillary pressure given by Darcy’s law as: Pcap =

µJτ k

(1)

where, µ is the dynamic viscosity, J is the evaporation flux, τ is the thickness of the shell, and k is the permeability. The shell undergoes an inversion of curvature (buckling) to relieve the strain developed by the capillary pressure (Pcap) (phase III in Fig. 3a). Buckling is initiated when the capillary pressure exceeds the critical value Pcritical ≈

10Y α

(Y: Young’s modulus; α: Foppl–von Karman number). 30

Onset of buckling is marked by the formation of a cavity from the apex region (minimum shell thickness) which propagates both radially and vertically as the solvent continues to evaporate through the porous shell. Basu et. al.24 have successfully related the tendency of nano-colloidal droplet undergoing buckling to the formation of dome structure with sufficiently higher values of RB ; (where, h

B

hB and RB are the droplet height and radius at the onset of buckling). Hydrophobicity and initial nanoparticle concentration are the two important parameters characterizing the formation of dome structure and subsequent buckling phenomenon. Nanoparticle laden droplet is observed to undergo buckling only if the initial nanoparticle concentration is greater than the minimum threshold particle loading rate,24 which is around 3% by wt. for a droplet evaporating on a MPL substrate. In the present experiments, it is observed that the addition of SDS in minute concentrations to the base case leads to physico-chemical changes in the droplet during evaporation which affects the subsequent buckling dynamics and the final structure (Fig. 3b).

Dynamics of SDS added nano-colloidal sessile droplet Effects of SDS on Evaporation 5 ACS Paragon Plus Environment

Langmuir 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

Page 6 of 17

Sodium Dodecyl Sulphate (SDS) surfactant is amphiphilic in nature, consisting of hydrophilic sulphate ion head and hydrophobic hydrocarbon tail. The surfactant molecules predominantly occupy the region of liquid-air interface, and effectively reduces the surface tension (by lowering the energy at the interface). The concentration of SDS molecules along the interface is mainly governed by the internal flow patterns in an evaporating droplet. For example, for a droplet evaporating on a hydrophilic substrate, the radially outward capillary driven flow accumulates the surfactant molecules predominantly near the three phase contact line thereby generating Marangoni eddies as explained by Still et al.31 However, in the present study, as the buoyancy driven internal flow is directed towards the top region in the droplet bulk (explained later), it is justified to assume that there is no SDS concentration gradient along the interface. The strength of Marangoni flow can be interpreted through the Peclet number comparison. The Peclet number due to Marangoni flow =

∆ !"

(where, #$ :

characteristic length scale of droplet, %: viscosity of the solvent, and D: colloidal particle diffusivity

obtained from Stokes-Einstein equation)

even for aminimal change in surface tension ( ∆& =

1 '(/' due to SDS concentration gradient) scales as ~ O (6). However, the Peclet number due to evaporation; * =

+ "

; calculated from the experimentally observed velocity values ,$ ≈ 30 %'/.

is ~ O (1). This value of experimental Peclet number proves that Marangoni flow is non-existent in our study. This further strengthens our assumption that there is no SDS concentration gradient along the interface. In the droplet bulk, the physiochemical properties (viscosity, conductivity) of the fluid show abrupt changes beyond the CMC due to the formation of aggregates of SDS molecules (Micelles). In the present work, the maximum SDS concentration chosen is significantly less than the CMC so that the micelle formation does not occur and the effects of SDS are restricted only to the liquid-air interfacial region. Therefore, the properties of the entire droplet remain unaltered due the presence of SDS. Droplets of SDS+TM suspension demonstrate different dynamics as compared to the pure TM droplets. Fig. 2(b-e) shows the different stages of evaporation for the selected range of SDS concentrations. The lowest SDS concentration chosen in experiment (0.005 % by wt.) do not show any significant alterations in the evaporation dynamics compared to the base case of pure TM suspension (Fig 2b). However, with an increase in SDS concentration, the droplet shows higher tendency of flattening by virtue of contact line spreading (phase III in Fig. 3b). Flattening of droplet has a direct correlation with the phenomenon of buckling (explained later). Fig. 4d clearly shows that the SDS concentrations considered in the current experiments do not affect the rate of droplet evaporation which is obtained within an experimental error of 3-8 % (standard deviation). This is also validated by theoretically comparing the droplet volume regression rates of pure TM suspension and TM with the highest SDS concentration (0.02 % by wt.). For a droplet evaporating on a substrate, the rate of volume decay is given by Fick’s law as: 6 ACS Paragon Plus Environment

Page 7 of 17 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

Langmuir

dVt -2πDMRc tf(θt)(cs -c∞ / = dt ρ

(2)

where, V is the volume of droplet of contact radius RC. “θ” is the droplet contact angle, 0 is the density of the working fluid. CS and C∞ are the saturation and ambient vapour concentrations. For θ>10°, fθ= (0.00008957+0.633θ+0.116θ2 -0.08878θ3 +0.01033θ4 )/sinθ.32 fθ and Rc takes into account the contribution of the changes in droplet shape. The average percentage change in droplet decay rate dt between maximum SDS concentration (0.02 % by wt.) covered in the experiment and dV

pure TM droplet 12

dV dV - dt TM dt SDS dV dt TM

34 is estimated to be around 2.1%, which is insignificant (subscripts

“TM” and “SDS” represents pure TM and TM+0.02 % SDS droplets respectively). Therefore, it is justified to neglect the minimal change in the rate of evaporation of the droplets across various concentrations of added SDS (0-0.02 % wt.) into the TM solution. However, the individual profiles of the parameters defining the droplet geometry, like contact radius, contact angle and height show significant deviation from the base case (pure TM suspension) with an increase in the initial SDS concentration. This deviation is credited to the spreading of droplet as a result of lowering of surface energy with the addition of SDS surfactant. The spreading behaviour can

be easily understood by quantifying the instantaneous droplet 3-phase contact line as shown in 5

Fig. 4a, which is accompanied with simultaneous changes in contact angle and droplet height as evident from Fig. 4b and 4c respectively. For the range of SDS concentration covered in the experiments (0.005% - 0.02% by wt.), contact line spreads to about 1.02 to 1.75 times the initial value respectively. The corresponding decrement in the contact angle and droplet height is observed to be about 0.68-0.27 and 0.52-0.16 times the initial values respectively. Consequent to spreading, the droplet evaporation modes (like constant contact angle, constant contact radius, stick slip) show significant differences with SDS concentration. With increase in droplet spreading, time spent in

constant contact radius mode is reduced from 1 × 67 (20 % wt. TM) to 0.78 × 67 (20 % wt. TM + 0.02 % wt. SDS). Zone 2, characterised by droplet spreading demonstrates the mixed mode of evaporation (simultaneous change in contact radius and contact angle). Therefore, time spent in mixed mode is equivalent to the duration of spreading as quantified in Table 1. TM 20 %

TM 20 %

TM 20 %

TM 20 %

+

+

+

+

0.005% SDS

0.0075% SDS

0.01% SDS

0.02% SDS

NA

NA

0.38

0.28

0.09

0

0

0.15

0.19

0.22

TM 20 %

Onset of Spreading 9

9 :

Duration of


Langmuir 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

Page 8 of 17

∆9

spreading 9 :

Table 1. Characteristic parameters governing the spreading behaviour of TM 20 % by wt. droplet added with different minute concentrations of SDS surfactant. t: instantaneous time; te: total evaporation time. Effects of SDS on Spreading Dynamics Alignment of surfactant molecules at the liquid-air interface lowers the energy and as a result the droplet contact line spreads to a new equilibrium configuration. The rate of spreading is dependent on the rate of surface tension reduction. In a multicomponent droplet, the changes in the concentration of a component at the liquid-air interface determines the rate of change of surface tension given by the Gibbs adsorption isotherm equation. The spreading behaviour of a surfactant laden droplet plays a crucial role in the buckling dynamics and the final residual structures. Droplet spreading can be characterised by studying the contact line dynamics. The droplet contact line dynamics is divided in three different zones, as depicted in Fig. 4 (a). Zone 1 is the pure evaporation stage in which the droplet contact line is pinned to the substrate and both the contact angle and the droplet height decreases as the evaporation progresses (as depicted in Fig. 4b and 4c respectively). The streaklines of internal flow are directed towards the top region during the pure evaporation stage (zone 1) as shown in Fig. 5c and Movie S1 in supplementary. Such recirculating flow field generated within a droplet placed on a hydrophobic substrate is known to be buoyancy driven. Non-uniform spatial distribution of evaporation flux around the droplet dictates the internal flow dynamics which has a direct effect on self-assembly of the particles thereby determining morphology of the final structures.7 The flow pattern remains unaltered throughout the lifetime in case of a TM droplet (20% wt. in water). Affinity of SDS molecules towards the interface deviates the flow in radially outward direction. Radial dispersion of particles is evident in the flow streaklines imaging in zone 2 (Fig. 5d and Movie S1). Zone 2 is therefore characterised by the onset of droplet spreading. It is to be noted that the recirculation of flow occurs from the interfacial region and/or from the out-of-plane region of the droplet bulk, which is not straightforward from Fig 5. Both the onset and duration of zone 2 (spreading zone) are functions of the initial SDS concentration. Spreading is attributed solely to the presence of surfactant and hence is initiated in advance with higher initial SDS concentration (Fig. 4a and Table 1). Duration of spreading also increases with increase in SDS concentration (Table 1). Both contact angle (θ) and droplet height (h) show simultaneous decrease during zone 2 to compensate the increase in contact radius due to spreading. It should be noted that evaporation of solvent also contributes to the decrease of both θ and h. Both the


Page 9 of 17 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

Langmuir

factors (spreading and evaporation) lead to faster decay rates of the droplet parameters during zone 2 as compared to zone 1. The rate of contact line spreading can be quantified in terms of the temporal changes in contact radius dtc and contact angle dt . The rate of changes in Rc and θ is approximately the same for all cases dR

dθ

; dtc =6.5±0.27 µm⁄s and dR

dθ =(4.1±0.16) dt

×10-3 rad=s> (where spreading is evident) and is constant

throughout process. At the end of zone 2, the nanoparticles accumulated at the droplet base undergo agglomeration which provides resistance to the further spreading of contact line, and the remaining evaporation dynamics proceeds with pinned contact line, marked as zone 3 in Fig. 4a. The droplet behaves like a solid body with trapped solvent in the interstitial spaces of the particle aggregates. Therefore, Zone 3 is characterized by constant contact angle and contact radius. The height (h) shows minor reduction mostly due to evaporation of trapped liquid through pores and rearrangement of the particle aggregates. Effects of SDS on Buckling Dynamics The changes in the dynamics of droplet during evaporation have direct consequence on the subsequent buckling phenomenon. Aggregation and self-assembly of nanoparticles govern the formation of visco-elastic shell (as explained earlier). The particle aggregates forms a dome like structure which is a pre-requisite for buckling. Therefore, hydrophobicity which leads to dome-type structure is an important criterion for the initiation of buckling. In the present work, for the base case of nano-colloidal droplet (20% by wt. of TM), the sol-gel transition of the nanoparticle aggregates t

occurs at around, t ≈ 0.75 (te is the droplet lifetime). Therefore, hydrophobicity has to be maintained e

for a sufficiently longer time period prior to buckling to allow the process of sol-gel transition. Addition of SDS enhances the migration of particles to the interface (as stated in the previous section). Interfacial transport of particles leads to changes in droplet dynamics which tends to show more hydrophilic like behavior. The physical changes are apparent due to increase in contact line (spreading) during zone 2 in such droplets (Fig. 4a). Spreading inhibits the formation of dome-type structure. The transition from hydrophobic to hydrophilic behavior of the droplets is quantified in terms of τtr ?=

tθ→π tsg

2

B (where, tθ→π represents the time instance when droplet loses its hydrophobicity 2

and tsg is the time instance of complete nanoparticle aggregation and sol-gel transition. “tsg ” is considered to be equivalent to that of the silica particles Ctsg ≈0.75 te /. A clear demarcation of τtr ≈ 0.6 is obtained which distinguishes the regions of buckling and non-buckling at the critical SDS concentration (> 0.0075 % wt. in 20 % wt. silica) (Fig. 6). The plot of

DE (ℎH FE

and IH are estimated 9

ACS Paragon Plus Environment

Langmuir 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

Page 10 of 17

prior to the sol-gel transition (tsg ≈0.75 te)) depicts a progressive decay which indicates higher tendency of droplet to deviate from the dome-shape with an increase in SDS concentration (Fig. 6). Higher values of

DE FE

(> 0.7) indicate buckling while for non-buckling cases, the values fall below 0.5.

Near the end of evaporation process, the stretching effect of liquid evaporating through the pores of nano-aggregates generates tensile stress resulting in wrinkling of surface followed by the formation of surface cracks.29 The radially inward orientation of the cracks (as seen from fig. 7) is a result of directional propagation of liquid front, from the pinned droplet contact line towards the centre. The wavelength (λ) of the surface wrinkling which defines the spacing between the cracks, depends on the radial distance (r) from the droplet center and thickness (H) of aggregates left behind by the radially inward propagating liquid front given as29: J

T

T U *L M ∝ NOP NRS V S QQ

(3)

where, E: Young modulus; υ: Poisson ratio of the nano-aggregates, and WXX : radial stress at position r from the droplet center. In the present study, the increasing hydrophilic nature of colloidal droplet with increase in initial SDS concentration, decreases the thickness of nanoparticle aggregates, from 0.48 mm (TM) to 0.17 mm (TM+0.02 % SDS) (resulting in increase in surface-to-volume ratio). As a result, the number of cracks generated increases (from 2 to 15), and consequently the angular crack spacing decreases (from π to 2π/15) as evident from Fig. 7. From Fig. 2d and 2e, it is noted that the excessive contact line spreading reduces the droplet to a more flat shape with no buckling. Final residual structures in these cases resemble that of a flat flower (Fig 7d and 7e). It is to be noted that for a SDS concentration of 0.005% by wt., the droplet spreading is not significant and hence the complete evaporation and buckling dynamics is similar to the base case (pure TM droplets) although the edges depicts slight flower-like appearance (Fig. 7b). Conclusion This paper provides insight into the control of physiochemical characteristics of a mixed colloidal sessile droplet. Evaporation and self-assembly of particles leads to the formation of dome-type buckled structures in aqueous silica dispersion droplets. Incorporation of SDS surfactant in silica dispersion alters the transient droplet dynamics leading to drastic change in the final structure. Transition of final dome-type structure to flower-shape morphology is attained at a critical concentration of SDS in the droplet. The phenomenon of buckling which is prominent in pure silica droplets can be controlled by changing the initial concentration of SDS and is completely suppressed beyond the critical value. Suppression of buckling is caused due to transport of SDS to the interfacial region leading to spreading of droplet. Initiation of spreading prior to sol-gel transition inhibits the dome-structure formation which is the key requirement for initiation of buckling. In addition, increase 10 ACS Paragon Plus Environment

Page 11 of 17 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

Langmuir

in the crack density and consequent decrease in crack spacing is attributed to the increment in surfaceto-volume ratio of nano-aggregate deposits with increase in initial SDS concentration. Dynamic control over the spreading and buckling of colloidal droplets leading to changes in final morphologies can play a significant role in application like surface patterning and coating.

Supporting Information: Movie S1: Side view internal flow field visualization of a mixed-nanocolloidal sessile droplet References:

1. Broadhead, J.; Edmond Rouan, S.K.; Rhodes, C.T. The spray drying of pharmaceuticals. Drug Dev. and Indu. Pharm., 1992, 18(11-12), 1169-1206. 2. Chen, X.D.; Mujumdar, A.S. eds. Drying technologies in food processing. John Wiley & Sons, 2009. 3. de Gans, B.J.; Duineveld, P.C.; Schubert, U.S. Inkjet printing of polymers: state of the art and future developments. Adv. materials, 2004 16(3), 203-213. 4. Tang, K.; Gomez, A. Generation by Electrospray of Monodisperse Water Droplets for Targeted Drug Delivery by Inhalation. J. Aero. Sci., 1994, 25 (6), 1237–1249. 5. Sen, D.; Bahadur, J.; Mazumder, S.; Santoro, G.; Yu, S.; Roth, S.V. Probing evaporation induced assembly across a drying colloidal droplet using in situ smallangle X-ray scattering at the synchrotron source. Soft Matter, 2014, 10(10), 16211627. 6. Deegan, R.D.; Bakajin, O.; Dupont, T.F.; Huber, G. Capillary flow as the cause of ring stains from dried liquid drops. Nature, 1997, 389(6653), p.827. 7. Bansal, L.; Miglani, A.; Basu, S. Morphological transitions and buckling characteristics in a nanoparticle-laden sessile droplet resting on a heated hydrophobic substrate. Phys. Rev. E, 2016, 93(4), p.042605. 8. Pathak, B.; Basu, S. Phenomenology and control of buckling dynamics in multicomponent colloidal droplets. Journal of App. Phys., 2015, 117(24), p.244901. 9. Bhardwaj, R.; Fang, X.; Attinger, D. Pattern formation during the evaporation of a colloidal nanoliter drop: a numerical and experimental study. New Journal of Phys., 2009, 11(7), p.075020.


Langmuir 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

Page 12 of 17

10. Shaikeea, A.J.D.; Basu, S. Evaporating sessile droplet pair: Insights into contact line motion, flow transitions and emergence of universal vaporisation pattern. 2016, App. Phys. Lett., 108(24), p.244102. 11. Savino, R.; Paterna, D.; Favaloro, N. Buoyancy and Marangoni effects in an evaporating drop. Journal of thermophysics and heat transfer, 2002, 16(4), 562-574. 12. Sen, D.; Mazumder, S.; Melo, J.S.; Khan, A.; Bhattyacharya, S.; D'souza, S.F. Evaporation driven self-assembly of a colloidal dispersion during spray drying: volume fraction dependent morphological transition. Langmuir, 2009, 25(12), 66906695. 13. Zang, D.; Yu, Y.; Chen, Z.; Li, X.; Wu, H.; Geng, X. Acoustic levitation of liquid drops: Dynamics, manipulation and phase transitions. Adv. in Colloid and Interface Sci., 2017, 243, 77-85. 14. Picknett, R.G.; Bexon, R. The evaporation of sessile or pendant drops in still air. Journal of Coll. and Inter. Science, 1977, 61(2), 336-350. 15. Hu, H.; Larson, R.G. Evaporation of a sessile droplet on a substrate. The Journal of Phys. Chem. B, 2002, 106(6), 1334-1344. 16. 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. 17. Tsapis, N.; Dufresne, E.R.; Sinha, S.S.; Riera, C.S.; Hutchinson, J.W.; Mahadevan, L.; Weitz, D.A. Onset of buckling in drying droplets of colloidal suspensions. Phys. Rev. Lett., 2005, 94(1), p.018302. 18. Hu, H.; Larson, R.G. Marangoni effect reverses coffee-ring depositions. The Journal of Phys. Chem. B, 2006, 110(14), 7090-7094. 19. Li, Y.; Lv, C.; Li, Z. Quere, D.; Zheng, Q. From coffee rings to coffee eyes. Soft Matter, 2015, 11, 4669-4673. 20. Mehranfar, M.; Gaikwad, R.; Das, S.; Mitra, S. K.; Thundat, T. Effect of temperature on morphologies of evaporation-triggered Asphaltene nanoaggregates. Langmuir, 2014, 30(3), 800-804. 21. Chen, L.; Evans, J.R. Arched structures created by colloidal droplets as they dry. Langmuir, 2009, 25(19), 11299-11301. 22. Shaikeea, A.; Basu, S.; Hatte, S.; Bansal, L. Insights into Vapor-Mediated Interactions in a Nanocolloidal Droplet System: Evaporation Dynamics and Affects on SelfAssembly Topologies on Macro-to Microscales. Langmuir, 2016, 32(40), 1033410343. 12 ACS Paragon Plus Environment

Page 13 of 17 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

Langmuir

23. Park, J.; Moon, J. Control of colloidal particle deposit patterns within picoliter droplets ejected by ink-jet printing. Langmuir, 2006, 22(8), 3506-3513. 24. Basu, S.; Bansal, L.; Miglani, A. Towards universal buckling dynamics in nanocolloidal sessile droplets: the effect of hydrophilic to superhydrophobic substrates and evaporation modes. Soft matter, 2016, 12(22), 4896-4902. 25. Das, S.; Mitra, S. K. Electric double-layer interactions in a wedge geometry: change in contact angle for drops and bubbles. Phys. Rev. E, 2013, 88, 033021. 26. 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. 27. Sugiyama, Y.; Larsen, R.J.; Kim, J.W.; Weitz, D.A. Buckling and crumpling of drying droplets of colloid− polymer suspensions. Langmuir, 2006, 22(14), 6024-6030. 28. Sen, D.; Melo, J.S.; Bahadur, J.; Mazumder, S.; Bhattacharya, S.; Ghosh, G.; Dutta, D.; D’souza, S.F. Buckling-driven morphological transformation of droplets of a mixed colloidal suspension during evaporation-induced self-assembly by spray drying. The European Phys. Journal E: Soft Matter and Bio. Phys., 2010, 31(4), pp.393-402. 29. Zhang, Y.; Qian, Y.; Liu, Z.; Li, Z.; Zang, D. Surface wrinkling and cracking dynamics in the drying of colloidal droplets. Eur. Phys. J. E., 2014, 37, 84. 30. Landau, L. D.; Lifshitz, E. M. Theory of Elasticity; Pergamon Press: London, 1959. 31. Still, T.; Yunker, P. J.; Yogh A. G. Surfactant-induced Marangoni eddies alter the coffee-rings of evaporating colloidal drops. 2012, Langmuir, 28 (11), 4984-4988. 32. Y. O. Popov. Evaporative deposition patterns: spatial dimensions of the deposit, Phys. Rev. E, 2005 71(3), 036313.


Langmuir 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

Page 14 of 17

Figure 1. Schematic of the experimental setup.

Figure 2. Different stages of evaporation dynamics of (a) 20% TM (b) 20% TM + 0.005% SDS (c) 20% TM + 0.0075% SDS (d) 20% TM + 0.01% SDS and (e) 20% TM + 0.02% SDS droplet. Scale bar corresponds to 0.5 mm for all cases.


Page 15 of 17 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

Langmuir

Figure 3 Schematic to illustrate different stages of droplet evaporation in (a) pure aqueous silica suspension (20% by wt. TM) and (b) in mixed colloidal suspension (TM+SDS)


Langmuir 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

Page 16 of 17

Figure 4. Temporal variations in (a) contact radius (b) contact angle (c) droplet height, and (d) droplet volume across different SDS concentrations.

Figure 5. Depiction of changes in droplet morphologies during (a) zone 1; (b) zone2 and the corresponding internal flow streaklines during (c) zone 1 (d) zone 2 for TM+SDS droplets (0.02 % wt. SDS in 20 % wt. TM).

Figure 6. Primary axis: Variation of YZX ?=

tθ→π tsg

2

B; secondary axis:variation of the ratio of droplet

height to radius at the onset of sol-gel transition with the initial SDS concentration.


Page 17 of 17 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

Langmuir

Figure 7. Final residual structures showing the formation of dome structure and buckling location in (a), (b) and (c); while flower shaped flat residue with no buckling in (d) and (e). Scale bar corresponds to 0.2 mm.


Evaporation Dynamics of Mixed-Nanocolloidal Sessile Droplets

Recommend Documents