Effect of Surface Wettability on Crack Dynamics ... - ACS Publications

May 14, 2015 - coffee ring effect1 as well as the patterns formed by dried layers of paint/coating. .... Deionized water (Milli-Q, 99% pure) was used ...
0 downloads 0 Views 9MB Size
Download PDF
Article pubs.acs.org/Langmuir

Effect of Surface Wettability on Crack Dynamics and Morphology of Colloidal Films Udita Uday Ghosh,† Monojit Chakraborty,† Aditya Bikram Bhandari,† Suman Chakraborty,‡ and Sunando DasGupta*,† †

Department of Chemical Engineering and ‡Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721302, India S Supporting Information *

ABSTRACT: The effect of surface wettability on the dynamics of crack formation and their characteristics are examined during the drying of aqueous colloidal droplets (1 μL volume) containing nanoparticles (53 nm mean particle diameter, 1 w/w %). Thin colloidal films, formed during drying, rupture as a result of the evaporation-induced capillary pressure and exhibit microscopic cracks. The crack initiation and propagation velocity as well as the number of cracks are experimentally evaluated for substrates of varying wettability and correlated to their wetting nature. Atomic force and scanning electron microscopy are used to examine the region in the proximity of the crack including the particle arrangements near the fracture zone. The altered substrate−particle Derjaguin−Landau−Verwey− Overbeek (DLVO) interactions, as a consequence of the changed wettability, are theoretically evaluated and found to be consistent with the experimental observations. The resistance of the film to cracking is found to depend significantly on the substrate surface energy and quantified by the critical stress intensity factor, evaluated by analyzing images obtained from confocal microscopy. The results indicate the possibility of controlling crack dynamics and morphology by tuning the substrate wettability. Fracture or cracking7 of thin films and the associated instabilities like dynamic delamination8 reduce the longevity and utility of coatings.9,10 A prerequisite in the domains of photonics,11microelectronics,12 microfluidic devices,13 disease detection,14 and drug delivery15 is the directed self-assembled layers of micro- and nanoparticles16 devoid of physical imperfections or faults. Several attempts have been made towards curbing these instabilities, which include usage of soft particles,10 coating hard particles with soft shells,17 addition of polymeric plasticizers,18 or emulsions and sequential deposition of multiple layers.19 The methods proposed so far have primarily focused on altering the suspension composition or mechanical properties of the gel,20 the drying modes,21−23 as well as the drying kinetics.24 External perturbations in the form of applied low-voltage electric field during drying of laponite gels have shown promising results. A distinct reduction in the number of microcracks apart from dependence on polarity, strength, and duration of applied field is observed for DC25 and AC potentials.26,27 The crack formation mechanism is a complex function of several parameters such as particle size,28 charge,29 shear modulus,30 and process parameters such as evaporation rate.31 The manipulation of substrate properties32 to control the crack

1. INTRODUCTION Drying of suspensions is a ubiquitous phenomenon involving solvent evaporation and results in a film made up of the selfassembled layers of the particles. Typical examples include the coffee ring effect1 as well as the patterns formed by dried layers of paint/coating. Particles devoid of solvent gradually accumulate in close-packed arrays at the three-phase contact line of the colloidal droplet. Simultaneously, the capillary pressure builds up and the particles undergo aggregation to form a consolidated solid region,2 known as the compaction front.3 It is postulated that crack growth is directly dependent on the movement of the compaction front, governed by the rates of fluid evaporation and subsequent fluid refilling at the air−fluid interface. Thereby, the length scale of the compaction front demarcates the two observable regimes, namely, evaporation limited and flow limited.3 However, the increase in the inherent film capillary pressure4 takes a toll on the thin film and leads to its collapse. A pre-existing flaw loop in the film may be also held responsible for the origin of concentric, circular cracks as per the hypotheses of the Xia−Hutchinson model.5 Drying of pastes6 differs from that of conventional colloidal suspensions for it involves more of plastic deformation compared to elastic deformation. Spring-network models are used to study crack formation in pastes, and attempts are being made to incorporate the dynamic changes in the rheological properties of pastes during drying in the existing models.6 © XXXX American Chemical Society

Received: February 21, 2015 Revised: May 14, 2015

A

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

2.1.2. Substrate Preparation and Its Characterization. Glass slides (BLUESTAR, Polar Industrial Corp., INDIA) were used as substrates. The cleaning protocol of the slides was comprised of ultrasonication in acetone followed by ultrasonication in deionized water for 10 min each to remove organic contaminants and dust particles, which can adversely affect the drying of colloidal film. The glass slides are oven dried to ensure removal of traces of moisture. The wetting properties of the glass slides were altered using the protocols described below. The substrates have been classified into three major categories, namely, hydrophilic, intermediate, and hydrophobic. 2.1.2.1. Hydrophilic. Cleaned glass slides were plasma treated using a Harrick Plasma Cleaner (PDC32G) for 60 s immediately prior to the experiments. It is to be noted that the time scale for the experiments (maximum 30 min) was significantly smaller than the time scale of loss of the original hydrophilicity (approximately 4 h). 2.1.2.2. Intermediate. Cleaned glass (using the protocol as mentioned earlier) slides were used without any pretreatment. 2.1.2.3. Hydrophobic. The substrates were coated with a layer of PDMS to render them hydrophobic. The base and cross-linker (SYLGARD 184, obtained from Dow Corning) were mixed in a weight ratio of 10:1. The mixture was placed in a vacuum desiccator to remove air bubbles formed during mixing. The glass slide was coated with the mixture using a spin coater with a slow spreading step at 500 rpm for 30 s followed by 5000 rpm for 70 s. The PDMS layer thus formed was cured by placing it overnight in a hot air oven at 95 °C. The thickness of the layer was measured using a surface profilometer (Veeco-Dektak) and found to be ∼13 μm. Sessile droplets of 1 μL of DI water and 1 μL of colloidal suspension 1 w/w % were separately placed on each of the three types of substrates. The mean equilibrium contact angles were measured using a Goniometer (Ramèhart, Germany) by the sessile drop method. DROPImage Advanced software, attached to the goniometer, was used for measurement of the contact angles, and the results are listed in Table 1. The droplets of colloidal suspensions of 1 μL volume with a

formation process has recently gained importance. The aim is to have higher critical cracking thicknesses, and several studies report innovative approaches to achieve the same. For example, replacing a rigid, solid substrate with a liquid surface such as mercury decreases the propensity toward cracking as has been reported in a study involving drying of submicrometer alumina particles.33 In a separate study involving compliant elastomer substrates, the characteristic length scales of the cracks is found to be inversely proportional to the substrate elasticity.32,34 This highlights the importance of in-plane constraints in the process of crack formation. A theoretical expression has also been provided relating the dependence of the crack spacing with the substrate modulus and film thickness. The crack shape is also found to be dependent on the surface functional group35 borne by the particle and its interaction with substrate. Despite the above advancements, the effect of substrate wettability on the crack formation process has not yet been explored. The focus has always been on the growth of the drying front36 and compaction front instead of the advancement of the crack tip.37 Previous studies38,39 report a single average velocity for a given event of ‘cracking’, although the capillary stress driving the crack propagation varies in a complex manner within the film with temporal variation. Therefore, a more realistic approach is to capture the physical phenomenon based on the instantaneous velocity and is measured experimentally herein. It is well established that the wetting state is closely linked to the interactions occurring at the interface, e.g., the interface formed by the substrate and the dried thin film. The DLVO theory40 is adopted to quantify the implicit role of the particle−substrate interactions arising due to the variation in substrate wettability.41 The goal is to effectively probe whether the manipulation of substrate wetting property can lead to control over the crack dynamics and morphology. In addition, the critical stress intensity factors are also evaluated using experimentally obtained characteristic parameters of a crack on wettability altered substrates. The aim is to connect the stress-bearing capacity of the film to the substrate wetting properties.

Table 1. Characterization of Substrate Wettability mean equilibrium contact angle (θ ± 2) (degrees)

2. MATERIALS AND METHODS

index

substrate

DI water

colloidal droplet

S1 S2 S3

hydrophilic intermediate hydrophobic

5 36 96

5 35 95

particle concentration of 1 w/w % were allowed to evaporate on substrates with specific wettability. The total evaporation times were noted along with that of pure DI water droplets of identical volumes. 2.2. Experimental Procedure. 2.2.1. Optical Microscopy. The colloidal droplets (1 μL volume and 1 w/w %) were placed on these substrates using a micropipette (Accupipet). An upright optical microscope (LEICA DM6000M), operated in the incident bright field mode, was focused at the periphery of the sessile colloidal droplet. The droplets were allowed to dry by natural evaporation at ambient conditions. The temperature and humidity conditions were carefully maintained at 25 °C and 40% relative humidity, respectively. The solvent gradually evaporated and triggered fluid flow toward the droplet periphery to compensate for the solvent loss, dragging the suspended particles along with it.1 The drying front gradually became prominent (easily observable under an optical microscope due to the enhanced contrast) and transformed into a thick region formed by particle compaction.3 This was followed by crack initiation wherein cracks started to appear simultaneously at several locations on the droplet edge and were monitored from its initiation. Time-lapse videos were recorded, featuring the different stages of crack formation with the 50× objective (1 pixel = 0.1079 μm) for the hydrophilic (S1) and intermediate (S2) substrates and 10× (1 pixel = 0.5396 μm) objective for the hydrophobic (S3) substrates. The videos of crack formation were analyzed using a frame-by-frame analysis. A sizable number of cracks having distinct stages, namely, crack initiation, propagation, and

2.1. Materials Preparation. 2.1.1. Preparation of Colloidal Suspension. Colloidal droplets with varying particle sizes and concentrations were subjected to natural drying. The images of the dried colloidal films for the substrates representing different wettability are presented in the Supporting Information. Crack formation is found to be most prominent for particle sizes in the nanoscale33 regime (see Figure S1, Supporting Information) irrespective of the substrate wetting state for the specified droplet volume and concentration. However, the nature of the crack dynamics and morphology is found to be a function of the substrate wettability and the primary focus of the present study. Hence, a particle having a size in the nanoscale regime (53 nm) is chosen and used for the experiments reported herein. The colloidal system was thus comprised of aqueous suspensions of polystyrene latex beads with nominal particle diameter of 53 nm obtained from Sigma-Aldrich. Dynamic light scattering (DLS) experiments were performed that confirmed the monodisperse nature of the nanoparticles used in this study (see Supporting Information for details). The uniformity of the suspension during the time of the experiments (maximum drying time for these experiments were 30 min) was confirmed by DLS of the suspension with a time interval of 5 min. Deionized water (Milli-Q, 99% pure) was used to dilute the colloidal suspension to the required concentration (1 w/w %). These nanosuspensions were sonicated for 10 min prior to each experiment to ensure homogeneity. B

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

depicts the trend in the measured film thickness as a function of varying surface wettability. It is evident from Figure 1 that an increase in surface hydrophobicity results in thicker films. A similar trend has also been observed by other researchers during the formation of silica colloidal crystals.43 2.2.3. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) Imaging. A digital AGILENT (Nanonics model 5100) atomic force microscope equipped with PicoView software has been used to image the region in the proximity of the crack. The AFM used here has a silicon cantilever and has been operated in the intermittent contact imaging mode. Scanning electron microscopy (NovaNANO FESEM) has also been used to probe the particle arrangements close to the fracture zone. It has been operated in the high-pressure vacuum mode. The details are given in section 4.4.

arrest, were chosen and analyzed (details are given in section 4.3). Each crack tip was followed from its initiation (t = 0) till its observable growth ceased (see Supporting Information for real-time movies). The distance traversed by a specific crack was noted at suitable time intervals (see Supporting Information, Figure S3) to obtain its instantaneous velocity. Experiments for each substrate were repeated three times, and the associated deviations in the measurements were estimated to lie within ±10%. To evaluate the total number of cracks (N) on the periphery of each dried film it was necessary to view the entire dried colloidal film. This was achieved using an inbuilt function, ImageOverlay, embedded with the LeicaLAS software that allows realtime stitching and merging of the individual, successive, adjacent images. The numbers of cracks were then evaluated manually from these images. 2.2.2. Confocal Microscopy. The dried colloidal films were viewed under a confocal microscope (LEICA TCS SP5) to examine the morphology of each type of crack pattern. The crack opening, δ, and distance along the crack from its tip, r, were evaluated using magnified images of individual cracks (details of the images obtained using confocal microscopy are given in section 4.3). The thicknesses of the dried films (X in μm) were evaluated in the RT (reflection− transmission) mode by forming z stacks of each film.42 Figure 1

3. THEORY The DLVO theory has been invoked to account for the various interactions in the evaporating colloidal droplets.41 Colloidal particles, in the present analysis, are assumed to be hard spheres (glass transition temperature of polystyrene particles is 100 °C), neglecting deformation caused by particle compression during drying.44 The particles are assumed to interact with each other on a one to one basis. The total DLVO force for the colloidal system comprises of the electrostatic and van der Waals forces as FDLVO = FVdW + Fel (1) =(Fsubstrate − particle)VdW + (Fparticle − particle)VdW + (Fsubstrate − particle)el + (Fparticle − particle)el

(2)

The van der Waals forces acting between the particle−substrate and the particle−particle are expressed in eqs 3 and 4 FVdW,particle − substrate = FVdW,particle − particle =

2nA123r′3 3z 2(z + 2r′)2

(3)

nA131r′ 12z 2

(4)

where r′ is the particle radius, n has been assumed to be the maximum number of particles in hexagonal closed packed crystalline arrangement, which is six,43 formed as a result of the

Figure 1. Dried colloidal film thicknesses as a function of the substrate wettability (contact angle).

Figure 2. Schematic of the close-packing process of particles. The figure is not drawn to scale. C

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

forces for a similarly charged pair of particle−substrate combination (FDLVO) decreases from 6.83 × 10−09 N for hydrophilic substrate to 1.10 × 10−09 N for hydrophobic substrates. The same trend was observed for oppositely charged particle−substrate combinations as well (Table 3).

crack front opening and subsequent compression of the particles, and z is the minimum separation distance between the particles and the substrate. The Hamaker constant designated as A123 in eq 5a is for a system comprising of colloidal particles (A11) in an intervening fluid medium (A33) between the particles and the substrate (A22).40 Similarly, A131 is the Hamaker constant for the particle−particle interaction in fluid medium and expressed in eq 5b A123 = ( A11 − A131 = ( A11 −

A33 )( A 22 −

A33 )

4. RESULTS The entire process of crack formation can be divided into three distinct phases based on their sequence of occurrence. Evaporation of the solvent from the colloidal droplet marks the beginning of the process, as it transforms the droplet into a mass of particles entrained in a thin film. This leads to the creation of a region of accumulated particles. The lowermost layers of particles remain bound to the substrate, and the film is unable to relieve the stress generated due to the preceding events. The initiation of cracking is a process to relieve this stress. Additional information and insights are obtained through the dynamic analyses of the experiments. 4.1. Evaporation and Formation of a Nonhomogeneous Colloidal Film. In the course of the evaporation of a droplet of a pure liquid, the contact line retracts continually in the absence of surface heterogeneity.1 The rate of transformation of the colloidal droplet to a dried thin film is a function of the wetting nature of the substrate. The evaporation process in the current study however differs due to the presence of colloidal particles. The variation in the initial surface wetting condition also influences the overall evaporation rate. The overall evaporation rate is found to vary from 3.33 × 10−12 (for S1, hydrophilic substrate) to 1.11 × 10−12 m3/s (for S3, hydrophobic substrate) for a droplet containing colloidal particles, whereas that of a pure water droplet varied from 2.38 × 10−12 (for S1) to 0.666 × 10−12 m3/s (for S3). Typical inner coffee ring deposits (ICRDs)45 are found on the hydrophobic substrates (S3). This can be attributed to the presence of a characteristic late pinning stage reported in the case of hydrophobic substrates. The resulting ICRDs are found to be 22 ± 3% of the initial wetted diameter in the present case. The length of the contact line (perimeter of the drop footprint on the substrate) is found to be markedly smaller in the case of hydrophobic substrates (ICRDs). This is responsible for the slower evaporation rate on hydrophobic surfaces. The parameters affecting drying rate include colloidal suspension properties such as homogeneity, chemical and physical nature of the particle, particle concentration, solvent type, substrate properties, and ambient humidity. The solvent (water), suspension, and particle properties remain unchanged as the same colloidal suspension with constant particle concentration 1 w/w % and volume (1 μL) is used for all experiments and performed at constant temperature and relative humidity. Therefore, all pertinent parameters are kept constant except the nature of the substrate (in terms of surface

(5a)

2

A33 )

(5b)

The interparticle and particle−substrate electrostatic forces are expressed in eqs 6 and 7 as Fel,particle − substrate = −2nπεrk

[ϕ12 + ϕ2 2 − 2ϕ1ϕ2 exp(kz)] [exp(2kz) − 1] (6)

Fel,particle − particle = −

εrϕ12 2k exp(kz) 4 exp(2kz) + 1

(7)

where ε is the permittivity of the fluid medium (water), k refers to the reciprocal of the Debye length, and Φ1 and Φ2 are the surface potentials of the particles and substrate, respectively. Surface potentials equivalent to zeta potentials have been used herein.41 Figure 2 schematically depicts the close-packing process of the nanoparticles and the relevant parameters. All constants involved in the evaluation of DLVO force have been listed in Table 2 and the individual force components in Table Table 2. Constants Used in the Evaluation of DLVO Force particle radius(r′) number of particles in closed packed arrangement (n) minimum separation distance between the particles and substrate (z) permittivity of the fluid medium (ε) reciprocal of the Debye length (k) Hamaker constant between polystyrene particles in water (A131) surface potential of polystyrene (Φ1)

26.5 × 10−9 m 643 0.4 × 10−9 m52,53 1 × 10−10 F/m (430 × 10−9) −1 −152 m 1 × 10−20 J52 15 × 10−3V52

3. It is evident from the order of magnitude of the forces listed in Table 3 that the electrostatic force between the particle− particle combinations may be neglected with respect to other force components. It can be inferred qualitatively that the particle−substrate attractive forces predominate in this situation and decrease with increasing surface hydrophobicity. The particle charge vis-à-vis the charge of the surface may play an important role during the self-assembly of nanoparticles and crack formation. Our calculations reveal that the DLVO

Table 3. Individual Force Component Variation with Surface Wettability parameter A123 Φ2 (mV) FVdW,particle−substrate FVdW,particle−particle Felectrostatic force,particle‑substrate Felectrostatic force,particle−particle Ftotal (FDLVO)

S1

S2

−2054

S3 −2052

3.00 × 10 −4052 4.89 × 10−9 N 8.28 × 10−10 N −3.77 × 10−10 N −3.46 × 10−16N 5.34 × 10−9N

3.70× 10 −3055 6.03 × 10−9 N 8.28 × 10−10 N −2.52 × 10−10 N −3.46 × 10−16N 6.61 × 10−9N D

0.58 × 10−20 40 −8956 0.96 × 10−9 N 8.28 × 10−10 N −1.34 × 10−9 N −3.46 × 10−16 N 4.40 × 10−10 N DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 3. Compaction front on (a) S1 (hydrophilic) and (c) S3 (hydrophobic) (Advent of Crack front is also shown).

EC = Γhdl

wettability), and it is reasonable to expect that the observed changes in the overall evaporation rates are solely due to the substrate surface property variations. Drying of sessile droplets of biological fluids like blood and Dextran have also revealed a strong dependence of evaporation rate on substrate wettability as well as humidity.14 4.2. Compaction Front and Crack Front Formation. A visibly dark front (as shown in Figure 3), commonly known as the compaction front,3 is formed due to the evaporation of the solvent and subsequent accumulation of the particles at the edge of the drop (coffee-ring effect).1 Local compaction fronts formed on the hydrophilic substrates (S1, shown in Figure 3) and intermediate (S2, not shown) are similar in nature. The thickness of the compaction front on the hydrophobic substrate is distinctly higher, as depicted in S3 of Figure 3. 4.3. Dynamical Evolution. Evaporation-induced drying in colloidal films generates capillary stresses (σ) and is given by46 σ = −(2γ/rM), where rM is the radius of curvature of the nanomenisci, formed at the surface of the drying film. rM is approximated to be the minimum pore radius,3 rPmin = 0.15r′, where r′is the particle radius (for the present case r′ ≈ 26.5 nm and γwater−air = 72 mN/m). This leads to a numerical value of the capillary pressure as high as −352 atm. This establishes the significant magnitude of the stress developed during the drying of thin colloidal films, which is principally responsible for the crack generation and propagation. The morphological changes observed during the drying of a colloidal film are due to the combined effects of the capillary stress and the presence of the lowermost layer of the particles near the substrate that oppose relaxation of the generated stress. This leads to the dissipation of the stress through the creation of cracks. The first sight of film rupture is denoted by the tearing of the compacted particle region and termed as crack initiation. This is closely followed by the advancement of the crack tip. However, this marching through a saturated fluid−solid network necessitates the expenditure of elastic energy. Assuming the film to be elastic and thin, the energy (E′) consumed for a single crack to progress by an infinitesimal length dl as proposed by Dufresne et al. in ref 46 is E′ = ασ 2h2

dl E

(9)

where Γ is the surface energy/unit crack surface area.46 Expressing the film thickness in terms of surface energy from eq 9 and substituting it in eq 8 results in the following expression of E′ as E′ =

ασ 2EC2 Γ 2Edl

(10)

Γ is higher for a hydrophilic surface, thereby less energy will be required to initiate crack formation. Therefore, the number of cracks on such a surface is markedly higher than that on a hydrophobic surface (Figure 4). In the initial stages, the crack

Figure 4. Number of cracks in the dried film as a function of contact angle.

tip advances rapidly through the compaction region. The maximum stress dissipation is accompanied by viscous losses at the crack tip. Thus, a crack will continue to propagate as long as the energy (EC) consumed to create a crack is balanced by the bulk elastic energy stored in the film (E′). Thus, crack propagation is arrested when the stress falls below the critical value47 given by

(8)

where h is the film thickness, E is the elastic modulus of the film, and α represents the adhesion between the film and the substrate (α is assumed to be 1.25 for films attached only on one side47). This energy is consumed to drive the solvent flow as well as to create new surfaces. The instantaneous velocity (U) can be obtained as (ασ2h2/E)(dl/dt) ≈ (ασ2h2/E)U. The elastic energy (EC) used to create new surface during crack formation is expressed as

σc =

EΓ αh

(11)

The crack propagation and its nature are also governed by the consumption of the elastic energy, which has the film thickness explicitly incorporated in it. However, as supported by the experimental results (Figure 1), the film thickness is a strong function of wettability, and therefore, the thickness variation is implicitly taken into account while describing the E

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 5. Representative crack formation sequence depicting crack initiation, propagation, and final morphology on (a) hydrophilic, (b) intermediate, and (c) hydrophobic substrates.

crack propagation in terms of substrate surface property (wettability) variation. The entire sequence of crack formation thus comprises of initiation, propagation, and arrest. These are found to differ considerably based on the substrates wettability as depicted in Figure 5a−c. The time and location at which the first crack appears (crack initiation) is a function of the substrate wetting state and are found to be at different distances from the contact line. Crack initiation starts with a high velocity but decreases monotonically with time, before coming to a halt (Figure 6). The initiation velocities for S1 and S2 substrates are found to be about 300 μm/s. However, a significant reduction is observed for S3 substrates (25 μm/s). The peak initiation velocity and the rate of decrease of the propagation velocity decreases with wettability. The hydrophobic substrate in the present case is a soft polymer (elastomer), unlike the hydrophilic cases, wherein the substrate is glass. It is well known that substrate elasticity

influences the crack formation mechanism. A recent study has demonstrated that the quality of colloidal crystals (silica) formed is altered by the combined effect of substrate properties, namely, the substrate elasticity and surface energy, affecting the colloidal self-assembly and the particle−substrate interaction. This is manifested through the interfacial adhesion of the particles to the substrate. It follows that adhesion onto the soft polymers like PDMS is preferred as compared to the rigid glass substrates.43 The stress released through the ruptured film gets absorbed by the deformation of the soft substrate. This has resulted in the cracks opening up more (Figure 5) on soft hydrophobic substrates as compared to the rigid hydrophilic substrates. There has also been evidence wherein the interactions between the colloidal particles have been induced by the soft substrates (polyacrylamide (PAA)).48 However, such interactions are hitherto unheard of between polystyrene latex particles and PDMS. F

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

comparison to microsuspensions. Deep troughs can be observed on hydrophobic substrates that resemble trenches (S3 in Figure 5) and are completely different in size and shape from the previous two figures. To unravel the microstructure beneath such morphologically different cracks, AFM (Figure 7) and SEM imaging (Figure 8) in the vicinity of the cracks are performed. The details of the imaging technique have been provided in the Materials and Methods. The enhanced depth of the cracks on the hydrophobic substrate precludes the use of AFM,13 though islands of particle agglomerates (S3 in Figure 7) have been observed.49 The theoretical results presented in Table 3 demonstrate a significant decrease in particle−substrate attraction (with the particle−particle interaction remaining constant), thereby promoting particle agglomeration. Thus, substrate wettability manipulation may be one of the key prameters to control the final film morphology. 4.5. Influence of Substrate Wettability on Stress Dissipation. It is pertinent to mention here that even though crack dynamics of brittle solids is a well-researched area, the same is not fully explored in the context of colloidal thin films. The colloidal suspension on drying transforms into a thin film of brittle solids. Thereby, the laws of fracture mechanics for brittle solids are applicable to cracks in dried colloidal films.50 Crack initiation is closely linked to the local stress developed in the colloidal films, and its propagation is governed by these local stress fields. This field is characterized by a stress intensity factor K and its critical value, the critical stress intensity factor, Kc. These two parameters measure the rupture resistance of the dried film. This factor is dependent on the external “loading”condition and the “geometry” of the crack. Loading condition is the force that drives the crack formation process and thereby determines the resultant stress field in the vicinity of the crack. In the present scenario, the loading condition is equivalent to the capillary stress generated by the evaporation of the solvent. The geometry of the crack depends on the manner and direction of the displacement of crack surfaces with respect to one other. The morphologies of the different cracks obtained in these experiments closely resemble mode I (opening mode). It is possible to characterize the cracks on the basis of crack opening, δ, and the distance along the crack

Figure 6. Effect of surface wettability on crack propagation dynamics with regimes denoted as I, initiation, II, propagation, and III, arrest (d = 53 nm, φ = 0.01). Time at which the first crack appears (crack initiation) is different for the three surfaces and designated as t = 0. (Dotted lines are only guide for the reader’s eyes.)

4.4. Morphology and Particle Arrangement. The final stage of crack formation is the cessation of crack propagation. These cracks have been classified on the basis of the repetitive structure with all characteristics of the pattern. Numerous fine hair-like cracks can be clearly seen on the hydrophilic substrate at the edge of the dried film in the images obtained using confocal microscopy (Figure 5, last row). Intermediate wettability substrates (S2) give rise to generic arch-shaped36 cracks that are usually found in dried nanolatex films and can also be observed in Figure 7 (S2), wherein secondary cracks propagate from previously formed primary cracks (S2 in Figure 7). The growth of the secondary cracks is limited by the distance between the primary cracks. Primary cracks readily interact with the secondary cracks to give rise to a network structure. It has been proposed that the difference in the instantaneous distance between the compaction front and the crack tip is responsible for the observed characteristic morphology.36 This characteristic pattern corroborates the claim of “difficulty in nucleation” in nanolatex gels in

Figure 7. AFM images of cracks on S1, S2, and S3 substrates. G

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 8. SEM images of crack morphology.

Figure 9. Evaluation of crack opening δ and distance along the crack from its tip r.

from its tip, r (as shown in Figure 9), for mode I (opening mode) cracks as50 δ=

8Kc E

r 2π

(12)

This equation relates the experimentally measurable parameters (δ and r) with the critical stress intensity factor. The Young’s modulus E of the colloidal film is a material property, independent of the substrate wetting state variation, and a constant for the all cases reported herein. It is assumed that during drying the rheological changes in these hard latex particles are negligible.44 The applicability of the equation has been discussed in the literature.50,51 It has been assumed that the film is elastic, although the regions very close to the crack tip bear permanent damage caused by the stress. Stress is inversely proportional to the square root of the distance along the crack from its tip (r). Therefore, the stress decreases progressively with the distance from the crack tip and diverges at the crack tip. The values of δ and r have been evaluated from several sets of cracks for each type of substrate from the images obtained using confocal microscopy (Figure 9), and a representative plot of variation in δ with distance from crack tip50 on hydrophobic substrates is given in Figure 10, validating the functionality between them as per eq 12. The crack opening (δ) increases whereas the distance from the crack tip (r)

Figure 10. Crack opening δ as a function of distance along the crack from its tip r for a representative wettability substrate (hydrophobic, S3 substrates.).

decreases with an increase in surface hydrophobicity (Figure 11). These experimentally estimated parameters, characterizing the cracks, are substituted in eq 12 to evaluate the ratio of the critical stress intensity factor (Kc) and the Young’s modulus (E) as a function of the wettability substrate. A film will undergo rupture only if K ≥ Kc and can be interpreted as the response of H

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

the crack propagation velocity (from 300 μm/s for a hydrophilic surface to 25 μm/s for a hydrophobic surface). The critical intensity factor, denoting the stress-bearing capacity of the film, increases significantly (by an order of magnitude) with increasing substrate hydrophobicity, resulting in higher threshold stress limit and better resistance to crack formation of a film on a hydrophobic surface.

■

ASSOCIATED CONTENT

S Supporting Information *

Details of the choice of colloidal particle size based on the images of the dried colloidal films, particle size Distribution as obtained from DLS experiments confirming the monodisperse nature of the nanoparticles, and dynamic evolution crack front propagation. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b00690.

Figure 11. Effect of wettability on crack characteristics. Lines are a guide to the reader’s eyes only.

■

the system (colloidal film) to the magnitude of the developed internal stresses. It is evident from Figure 12 that the critical

AUTHOR INFORMATION

Corresponding Author

*Phone: +91-3222-283922. E-mail: [email protected]. in; [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the Indian Institute of Technology Kharagpur (Sanction Letter no. IIT/SRIC/ATDC/CEM/2013-14/118, dated Dec 19, 2013).

■

REFERENCES

(1) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; et al. Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389, 827−829. (2) Goehring, L.; Clegg, W. J.; Routh, A. F. Solidification and Ordering during Directional Drying of a Colloidal Dispersion. Langmuir 2010, 26, 9269−9275. (3) Dufresne, E.; Corwin, E.; Greenblatt, N. A.; et al. Flow and Fracture in Drying Nanoparticle Suspensions. Phys. Rev. Lett. 2003, 91, 224501. (4) Lee, W. P.; Routh, A. F. Why Do Drying Films Crack? Langmuir 2004, 20, 9885−9888. (5) Jing, G.; Ma, J. Formation of Circular Crack Pattern in Deposition Self-Assembled by Drying Nanoparticle Suspension. J. Phys. Chem. B 2012, 116, 6225−6231. (6) Kitsunezaki, S. Crack Growth in Drying Paste. Adv. Powder Technol. 2011, 22, 311−318. (7) Griffith, A. A. The Phenomena of Rupture and Flow in Solids. Philos. Trans. R. Soc. London, Ser. A 1921, 221, 163−198. (8) Giorgiutti-Dauphiné, F.; Pauchard, L. Dynamic Delamination of Drying Colloidal Films: Warping and Creep Behavior. Colloids Surf., A 2015, 466, 203−209. (9) Lazarus, V.; Pauchard, L. From Craquelures to Spiral Crack Patterns: Influence of Layer Thickness on the Crack Patterns Induced by Desiccation. Soft Matter 2011, 7, 2552−2559. (10) Winnik, M. A. Latex Film Formation. Curr. Opin. Colloid Interface Sci. 1997, 2, 192−199. (11) Stein, A.; Schroden, R. C. Colloidal Crystal Templating of Three-Dimensionally Ordered Macroporous Solids: Materials for Photonics and beyond. Curr. Opin. Solid State Mater. Sci. 2001, 5, 553−564. (12) Jacobs, H. O.; Whitesides, G. M. Submicrometer Patterning of Charge in Thin-Film Electrets. Science 2001, 291, 1763−1767.

Figure 12. Effect of wettability on stress intensity factor.

stress intensity factor increases with substrate hydrophobicity. Thus, the films on hydrophobic substrates have a higher threshold stress limit and better resistance to crack formation as compared to hydrophilic substrates.

5. CONCLUSIONS The role of substrate wettability on the crack formation process in colloidal films has been investigated. Microdroplets of colloidal nanosuspensions have been subjected to natural drying on substrates with varying wettabilities (contact angles of 5°, 36°, and 96°). Time lapse optical microscopy has been used to characterize different regimes of crack dynamics, namely, the initiation, propagation, and arrest, as well as the velocity of crack propagation. Confocal, atomic force, and scanning electron microscopy techniques have been used to examine the crack morphology including the crack opening, the distance along the crack from its tip, and the thicknesses of the dried films. Theoretical analysis has established that the substrate−particle interaction is strongly influenced by the substrate wettability. The experimental results illustrate that the decrease in surface energy with increasing hydrophilicity enhances the tendency of the film to resist rupture and lowers I

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

(36) Pauchard, L.; Adda-Bedia, M.; Allain, C.; Couder, Y. Morphologies Resulting from the Directional Propagation of Fractures. Phys. Rev. E 2003, 67, 027103. (37) Xu, Y.; Engl, W. C.; Jerison, E. R.; Wallenstein, K. J.; Hyland, C. Imaging in-Plane and Normal Stresses near an Interface Crack Using Traction Force Microscopy. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 14964−14967. (38) Okubo, T.; Nozawa, M.; Tsuchida, A. Kinetic Aspects in the Drying Dissipative Crack Patterns of Colloidal Crystals. Colloid Polym. Sci. 2007, 285, 827−832. (39) Xu, P.; Mujumdar, A. S.; Yu, B. Drying-Induced Cracks in Thin Film Fabricated from Colloidal Dispersions. Drying Technol. 2009, 27, 636−652. (40) Israelachvili, N. J. Intermolecular and Surface Forces, 3rd ed.; Academic Press, Elsevier: USA, 2011. (41) Bhardwaj, R.; Fang, X.; Somasundaran, P.; Attinger, D. SelfAssembly of Colloidal Particles from Evaporating Droplets: Role of DLVO Interactions and Proposition of a Phase Diagram. Langmuir 2010, 26, 7833−7842. (42) Nikiforow, I.; Adams, J.; König, A. M.; Langhoff, A.; Pohl, K.; Turshatov, A.; D, J. Self-Stratification during Film Formation from Latex Blends Driven by Differences in Collective Diffusivity. Langmuir 2010, 26, 13162−13167. (43) Sun, S.; Huang, Y.; Zhao, B. Formation of Silica Colloidal Crystals on Soft Hydrophobic vs Rigid Hydrophilic Surfaces. Colloids Surf., A 2015, 467, 180−187. (44) Hunter, J. R.; Bensley, N. C. The Coagulation of Concentrated Colloidal Dispersions at Low Electrolyte Concentrations I.Rate of Coagulation and the Onset of Instability. J. Colloid Interface Sci. 1983, 92, 436−447. (45) Nguyen, T. A. H.; Hampton, M. A.; Nguyen, A. V. Evaporation of Nanoparticle Droplets on Smooth Hydrophobic Surfaces: The Inner Coffee Ring Deposits. J. Phys. Chem. C 2013, 117, 4707−4716. (46) Dufresne, E. R.; Stark, D. J.; Greenblatt, N. A.; Al, E. Dynamics of Fracture in Drying Suspensions. Langmuir 2006, 22, 7144−7147. (47) Hutchinson, J. W.; Suo, Z. Mixed Mode Cracking in Layered Materials. Adv. Appl. Mech. 1992, 29, 63−189. (48) Di Michele, L.; Yanagishima, T.; Brewer, A. R.; Kotar, J.; Eiser, E.; Fraden, S. Interactions between Colloids Induced by a Soft CrossLinked Polymer Substrate. Phys. Rev. Lett. 2011, 107, 136101. (49) Kralchevsky, P. A.; Denkov, N. D. Capillary Forces and Structuring in Layers of Colloid Particles. Curr. Opin. Colloid Interface Sci. 2001, 6, 383−401. (50) Goehring, L.; Clegg, W. J.; Routh, A. F. Plasticity and Fracture in Drying Colloidal Films. Phys. Rev. Lett. 2013, 110, 024301. (51) Lawn, B. Fracture of Brittle Solids, 2nd ed.; Cambridge University Press: New York, 1993. (52) Chhasatia, V. H.; Sun, Y. Interaction of Bi-Dispersed Particles with Contact Line in an Evaporating Colloidal Drop. Soft Matter 2011, 7, 10135. (53) Visser, J. On Hamaker Constants: A Comparison between Hamaker Constants and Lifshitz-Van Der Waals Constants. Adv. Colloid Interface Sci. 1972, 3, 331−363. (54) Drelich, J.; Mittal, K. L. Atomic Force Microscopy in Adhesion Studies, 1st ed.; CRC Press: USA, 2005. (55) Afshar, A.; Ghorbani, M.; Ehsani, N.; Saeri, M. R. A Study of Zeta Potential of Plasma Sprayed Hydroxyapatite Coating in Four Simulated Physiological Solutions. Int. J. Eng., Trans. B: Appl.. 2003, 16, 343−354. (56) Sze, A.; Erickson, D.; Ren, L.; Li, D. Zeta-Potential Measurement Using the Smoluchowski Equation and the Slope of the Current-Time Relationship in Electroosmotic Flow. J. Colloid Interface Sci. 2003, 261, 402−410.

(13) Wei Han, B. L.; Z, L. Drying-Mediated Assembly of Colloidal Nanoparticles into Large-Scale Microchannels. ACS Nano 2013, 7, 6079−6085. (14) Sobac, B.; Brutin, D. Desiccation of a Sessile Drop of Blood: Cracks, Folds Formation and Delamination. Colloids Surf., A 2014, 448, 34−44. (15) Velev, O. D.; Kaler, E. W. Structured Porous Materials via Colloidal Crystal Templating: From Inorganic Oxides to Metals. Adv. Mater. 2000, 12, 531−534. (16) Velev, O. D.; Gupta, S. Materials Fabricated by Micro- and Nanoparticle Assembly - The Challenging Path from Science to Engineering. Adv. Mater. 2009, 21, 1897−1905. (17) Wohlleben, W.; Bartels, F. W.; Altmann, S.; Leyrer, R. J. Mechano-Optical Octave-Tunable Elastic Colloidal Crystals Made from Core-Shell Polymer Beads with Self-Assembly Techniques. Langmuir 2007, 23, 2961−2969. (18) Lewis, J. A. Colloidal Processing of Ceramics. J. Am. Ceram. Soc. 2000, 83, 2341−2359. (19) Prosser, J. H.; Brugarolas, T.; Lee, S.; Nolte, A. J.; Lee, D. Avoiding Cracks in Nanoparticle Films. Nano Lett. 2012, 12, 5287− 5291. (20) Giorgiutti-Dauphiné, F.; Pauchard, L. Elapsed Time for Crack Formation during Drying. Eur. Phys. J. E 2014, 37:39, 1−7. (21) Shorlin, K.; de Bruyn, J.; Graham, M.; Morris, S. Development and Geometry of Isotropic and Directional Shrinkage-Crack Patterns. Phys. Rev. E 2000, 61, 6950−6957. (22) Salamanca, J. M.; Ciampi, E.; Faux, D. A.; Glover, P. M.; Mcdonald, P. J.; Routh, A. F.; Peters, A. C. I. A.; Satguru, R.; Keddie, J. L. Lateral Drying in Thick Films of Waterborne Colloidal Particles. Langmuir 2001, 17, 3202−3207. (23) Allain, C.; Limat, L. Regular Patterns of Cracks Formed by Directional Drying of a Colloidal Suspension. Phys. Rev. Lett. 1995, 74, 2981−2985. (24) Man, W.; Russel, W. Direct Measurements of Critical Stresses and Cracking in Thin Films of Colloid Dispersions. Phys. Rev. Lett. 2008, 100, 198302. (25) Khatun, T.; Dutta, T.; Tarafdar, S. Crack Formation under an Electric Field in Droplets of Laponite Gel: Memory Effect and Scaling Relations. Langmuir 2013, 29, 15535−15542. (26) Khatun, T.; Dutta, T.; Tarafdar, S. Crack Formation in Laponite Gel under AC Fields. Appl. Clay Sci. 2013, 86, 125−128. (27) Khatun, T.; Choudhury, M. D.; Dutta, T.; Tarafdar, S. ElectricField-Induced Crack Patterns: Experiments and Simulation. Phys. Rev. E 2012, 86, 1−8. (28) Chon, C. H.; Paik, S., Jr; T, J. B.; Kihm, K. D.; June, R. V. Effect of Nanoparticle Sizes and Number Densities on the Evaporation and Dryout Characteristics for Strongly Pinned Nanofluid Droplets. Langmuir 2007, 23, 2953−2960. (29) Sarkar, A.; Tirumkudulu, M. S. Consolidation of Charged Colloids during Drying. Langmuir 2009, 25, 4945−4953. (30) Singh, K.; Tirumkudulu, M. Cracking in Drying Colloidal Films. Phys. Rev. Lett. 2007, 98, 218302. (31) Kim, I.; Kihm, K. D. Hidden Cavity Formations by Nanocrystalline Self-Assembly on Various Substrates with Different Hydrophobicities. Langmuir 2012, 28, 9195−9200. (32) Smith, M. I.; Sharp, J. S. Effects of Substrate Constraint on Crack Pattern Formation in Thin Films of Colloidal Polystyrene Particles. Langmuir 2011, 27, 8009−8017. (33) Chiu, R. C.; Garino, T. J.; Cima, M. J. Drying of Granular Ceramic Films: I, Effect of Processing Variables on Cracking Behavior. J. Am. Ceram. Soc. 1993, 76, 2257−2264. (34) Holmes, D. M.; Vasant Kumar, R.; Clegg, W. J. Cracking During Lateral Drying of Alumina Suspensions. J. Am. Ceram. Soc. 2006, 89, 1908−1913. (35) Carle, F.; Brutin, D. How Surface Functional Groups Influence Fracturation in Nanofluid Droplet Dry-Outs. Langmuir 2013, 29, 9962−9966. J

DOI: 10.1021/acs.langmuir.5b00690 Langmuir XXXX, XXX, XXX−XXX