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Contact line Dynamics during Evaporation of Extended Colloidal Thin Films: Influence of Liquid Polarity and Particle Size Udita Uday Ghosh, Monojit Chakraborty, Soham De, Suman Chakraborty, and Sunando DasGupta Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03267 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016
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Contact line Dynamics during Evaporation of Extended Colloidal Thin Films: Influence of Liquid Polarity and Particle Size
Udita Uday Ghosh1, Monojit Chakraborty1, Soham De2, Suman Chakraborty3 and Sunando DasGupta1* 1
Department of Chemical Engineering, Indian Institute of Technology Kharagpur, India.
2
3
Department of Chemical Engineering, Jadavpur University, Kolkata, India. Department of Mechanical Engineering, Indian Institute of Technology Kharagpur,
India.
*Corresponding author Email:
1
[email protected]; Ph: +91 - 3222 - 283922
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Abstract Exercising control over the evaporation of colloidal suspensions is pivotal to modulate the coating characteristics for specific uses wherein the interactions between the liquid, the particles and the substrate control the process. In the present study, the contact line dynamics of a receding colloidal liquid film comprising of particles of distinctly different sizes (nominal diameters 0.055 µm and 1µm and surface unmodified), during evaporation is analyzed. The role of the liquid polarity is also investigated by replacing the polar (water) liquid with a relatively non-polar liquid (Isopropyl alcohol) in the colloidal suspension. The characteristics of the evaporating receding meniscus, namely the film thickness and the curvature are experimentally evaluated using an image analyzing interferometry technique. The experimental results are assessed in conjunction with the augmented Young-Laplace equation highlighting the roles of the relevant components of the disjoining pressure and the polarity of the liquid involved in the colloidal suspension.
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1. INTRODUCTION Manipulation of the particle deposition caused by evaporation induced self-assembly of colloidal particles in a sessile colloidal droplet is important in diverse industrial processes like drop-casting, spin-coating, inkjet printing,1,2 separation of biological units and size based particle sorting3,4. It is essential to probe and optimize the parameters affecting the fundamental process of evaporation from the sessile colloidal droplets to achieve control over the deposition patterns.5 The nature of the evaporation process is complex as the system consists of three components (particle, substrate, liquid) and minor alteration in the property of a single component, perturbs the process significantly. Such perturbations have been reported in a number of experimental investigations, prominently the effect of substrate wetting state,6,7 elasticity,8 shape9,10 and dimension6 of the colloidal particle and the volatility of the liquid11. These perturbations directly affect the internal (capillary) flows within the droplet during drying. To compensate for the loss of liquid, the contact line (droplet edge) physically recedes. However, the movement of the contact line is opposed by the constant accumulation of particles resulting in a phenomenon commonly termed as ‘self-pinning’.12,13 The subtle balance between the rate of the contact line recession and the rate of particle deposition determines the final deposition pattern.14 In majority of the colloidal droplet evaporation studies, a characteristic stick-slip movement of the contact line is observed and is postulated to be driven by the internal modulations in the meniscus shape.15 In situ investigation of the underlying colloidal self-assembly process during deposition is so far restricted to the tracking of particle motion in horizontal and vertical configurations of sessile droplet drying.16,17 However, to obtain critical insights into this phenomenon, a
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comprehensive study, correlating the evolution in the meniscus shape with the movement of the contact line, is necessary. Real time tracking of the meniscus shape is experimentally challenging since the region of interest is of the order of few microns only.18 Typically, the shape of the extended menisci of partially wetting liquids is governed by the strength of the intermolecular and capillary forces between the liquid film and the solid substrate. The extended meniscus can be divided by into three regions based on the magnitudes of these forces, its thickness and curvature as - adsorbed, transition and capillary (Figure 1). The adsorbed region is the flat, non-evaporating ultra-thin liquid film, strongly attached to the solid substrate by the intermolecular forces and is only few nanometers in thickness, requiring specific measurement techniques for its visualization such as ellipsometry. This is followed by the transition region, wherein the film thickness increases leading to the weakening of the intermolecular interactions at the solid-liquid interface with the possibility of significant evaporation/condensation. The sharp change in curvature at the transition region becomes almost constant in the adjacent region characterized by a film thickness of the order of microns where both the forces are present.19
Several studies have focused on correlating the spatially varying
intermolecular force field and the capillary forces within the extended meniscus to its quantifiable characteristics (e.g., film thickness and curvature).20 The enhanced transport processes in the extended thin film is controlled by a delicate balance of the capillary and intermolecular forces. The effective pressure difference arising at the interface on account of these forces can be expressed as a function of the liquid-vapor surface tension, film thickness and interfacial radius of curvature in a compact form, known as the augmented
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Young Laplace equation, that was widely used for the isothermal and non-equilibrium systems as well as for predicting the underlying associated transport processes.18,21,22 However, these studies present the various facets of evaporating menisci of pure liquids only, devoid of colloidal particles.18,21,23–26 Introduction of colloidal particles has lead to interesting outcomes, like increase in the contact line velocity of the colloidal thin films in presence of electric field.27 An analogous macroscopic study of colloidal droplets subjected to electric field showed increased macroscopic contact angle and enhanced spreading.28 Recently, the thin film region of an evaporating menisci of colloidal films have been probed using confocal microscopy and a pronounced effect of the particle concentration on the thin film thickness is reported.29 However, the classical literature is mostly limited to theoretical investigations of apolar liquids and the distinction between evaporation characteristics of the colloidal suspensions in polar liquids and apolar liquids are not fully explored. A theoretical study postulated that a variation in the magnitude of attractive dispersion forces (induced dipole-induced dipole interactions) led to distinct alteration in the characteristic meniscus profile for a relatively invariant capillary pressure.30 The magnitude of the disjoining pressure for a polar fluid (water) was found to be greater by an order, as compared to the apolar liquid (carbon tetrachloride) in a subsequent theoretical report.31,32 However, a comprehensive study outlining the effect of inclusion of particles on the characteristics of the evaporating meniscus and the consequential effect on the contact line dynamics is yet to be done. The present study addresses the effects of the inclusion of colloidal particles in an extended, evaporating menisci of both polar (water) and apolar (iso-propyl alcohol)
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liquids. The experimental system comprises of naturally evaporating colloidal films on hydrophilized silicon substrates with evaporating menisci of pure liquids used as controls. A major focus of the present study is directed towards outlining the particle-liquid interactions stemming from the variation in liquid polarity for colloidal particles of distinctly different diameters (ϕ equal to 0.055 µm and 1µm). Real time evolution of the colloidal thin film during evaporation is characterized by the image-analyzing interferometry technique. The spatial variation in the film thickness and the corresponding meniscus curvature profiles are evaluated for the possible combinations of the particles and the liquid. These parameters influence the intricate particle-liquid interactions through the shape-dependent interfacial force field resulting in flow within the meniscus. The corresponding contact line movement of the film is tracked as well. The experimentally evaluated contact line dynamics as a function of the liquid polarity and particle characteristics (diameter) are examined in light of the augmented YoungLaplace equation.
2. MATERIALS Colloidal particles with two different nominal diameters, 0.055µm and 1µm, were obtained from Sigma Aldrich and were used to outline the effect of particle size on the evaporation process. The colloidal suspensions were ultrasonicated for ten minutes prior to each evaporation experiment. This has been done to ensure that the particles were homogeneously dispersed and the suspension was devoid of particle aggregates. These colloidal nanoparticles are known to be stable over months as per the product specifications. Thus, it can be safely assumed that the particles do not aggregate during the course of evaporation experiments with a maximum time period of ~200seconds. The
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variance in the liquid polarity is introduced by the choice of two liquids with widely different dielectric constants (ε) namely, water (εP = 80) as the polar liquid and Isopropyl Alcohol (IPA) (εAP = 18), as the relatively non-polar liquid. Hildebrand parameter evaluated based on the corresponding Hansen parameters33 for the liquids used herein and are presented in Table I. Hansen parameters for the liquids signify the strength of solubility or miscibility of a solvent and are commonly used in industries like coatings to select a solvent for the dissolution of a pigment or another liquid, as may be the case. These parameters account for the total energy of vaporization of the liquid (solvent) and include the three major interactions that occur during this liquid to gas transformation. Broadly speaking, these interactions are the induced dipole-induced dipole interactions or the dispersion forces ( ∂ d ), permanent dipole–permanent dipole forces ( ∂ p ) and the hydrogen bonding ( ∂h ). Table I Characteristics of the liquids used in the present study
Total Hildebrand Liquid
parameter 0.5
(MPa )
Dispersion
Polar
Hydrogen
component
component
bond component
Surface Tension
(mN/m)
Water Isopropyl Alcohol
7
47.8
15.6
16.0
42.3
72
23.5
15.8
6.1
16.4
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It is evident from the numerical values of the polar component and the hydrogen bonding component that they are relatively higher for the polar liquid (Water). This can be attributed to the distinct differences in the liquid structures of water as compared to IPA. The lone pair over the oxygen atom in the water molecule implies it is more electronegative (δ-) and this causes it to exert a pull over the hydrogen atoms. This in turn gives rise to electron deficient charged positive nucleus of the hydrogen atom.34 Such positive nuclei attract electrons from the neighbouring atoms/molecules. In contrast, the hydrogen atoms in isopropyl alcohol are replaced by the relatively apolar methyl groups. Thus, the cohesive forces between the water molecules are stronger since it possesses hydrogen bonds and inherently hydrogen bond is stronger than the prevailing dispersion forces in isopropyl alcohol. The strong polar nature is also physically manifested in its high numerical value of surface tension. The procured colloidal stock suspensions were diluted using the respective liquid to obtain a solute concentration of 0.01(w/w) %. Silicon wafers were subjected to a rigorous cleaning procedure by dipping them in piranha solution (with Hydrogen peroxide and Sulphuric acid in the ratio of 1:3) to remove the organic contaminants. Multiple washings of the wafers in de-ionized water were carried out to ensure that no traces of the piranha solution remained, followed by air-drying in the oven to remove the moisture. The
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cleaned wafers were subjected to plasma treatment (1 minute in Harrick Plasma Cleaner, PDC-FMG) to render the substrate surface hydrophilic.35 To confirm that the substrates have attained uniform wettability, static contact angles were measured at five different locations using identical volume of colloidal droplets (1µl) and pure liquids. These contact angle measurements confirm that the contact angles are less than five degrees at all locations for both IPA and water, ensuring that the substrates have uniform wettability.
3. EXPERIMENTAL PROCEDURE 3.1. Optical microscopy Cleaned hydrophilic silicon wafers (Figure 1) were placed on the microscope stage and colloidal droplets (1µl, 0.01(w/w) %) of the diluted suspensions were dispensed on them using a micropipette.
Figure 1 Schematic of a thin wetting film formed by a colloidal droplet on a hydrophilic substrate depicting the (a) capillary, transition and adsorbed film regimes (b) Microscopic images of the interferometric fringes in the thin film formed at the edge of the droplet.
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The hydrophilic nature of the substrate ensured spontaneous wetting of the surface by the droplet leading to the formation of a thin extended liquid film. Colloidal droplets were allowed to evaporate freely and real time videos of the evaporating menisci at the edge of the droplet were captured at 20X magnification (1 pixel= 0.29µm). The experiments were performed at an ambient temperature of 25ºC and relative humidity of 40%. Images were extracted from the captured videos at suitable time intervals. Sequential images pertaining to the combinations of liquid and particle diameters are presented in Figure 2.
Figure 2(A) Evolution of a drying colloidal thin film of IPA and water depicting the three stages- (I) receding film meniscus, (II) rupture of thin film and (III) dewetted solid substrate with particle remnants. (B) Magnified view of the contact line at 0s, corresponding to d1 depicting the micro-particle deposits and depleted film.
The shape of the evaporating meniscus can be evaluated from the pattern of the interference fringes, formed due to the constructive (bright fringe) and destructive (dark fringe) interference of light reflected from the meniscus surface and the substrate. In case
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of colloidal liquids, the dispersed particles may cause scattering of the incident light. The parametric ratio (x)29 that is used to determine the specific regime of the elastic scattering
x= is given by
2π (φ / 2)
λ
where ɸ is the particle radius and λ is the incident wavelength
(~543nm). There are two possibilities in the present study based on the particle diameter, for nanoparticles (ɸ1=0.055µm, x1=0.318) x1
