Controlling breath figure patterns on PDMS by concentration variation ofethanol-methanol binary vapors

K Nilavarasia and V Madhurimab

Department of PhysicsSchool of Basic and Applied SciencesCentral University of Tamil Nadu

Thiruvarur - 610005, Tamil Nadu, India.anilavarasi@students.cutn.ac.in; bmadhurima@cutn.ac.in

(Dated: November 9, 2018)

In this paper, the self-assembly of condensed droplets on smooth and constrained surfaces undersaturated vapor atmosphere of ethanol and methanol binary system is reported. Hexagonally orderedarray of pores are obtained on smooth surfaces with saturated vapors of binary liquids without theassistance of any additives. The results show that the addition of small amount of ethanol tomethanol plays a role very similar to that of surface active agents in inducing the formation ofregular droplet array. The effect of constraints on self-assembled droplet pattern such as movementof contact line and depinning of contact line is also investigated. It is observed that the pore size,pore shape, pore depth and ring diameter are influenced by the atmosphere of binary vapors inaddition to the commonly held attribution to the surface tension of the solvent. Contact anglestudies of the patterned substrates showed hydrophobicity with very high adhesiveness to water andWenzel’s state of wetting.

Keywords: Breath figures, constrained and smooth surface, ethanol, methanol, binary vapors, contact line,pinning/ depinning

I. INTRODUCTION

Formation of self-assembled droplet array on sub-strates (breath figure technique), an efficient bottom-upapproach of templating, uses liquid droplets as sacrificialtemplates for the preparation of micro-structured porousfilm [1–4]. This technique involves self-assembly of con-densing water droplets into hexagonal arrays on surfacesdue to capillary and Marangoni forces. This process typ-ically involves four steps as follows. Step 1. Casting ofpolymer solution on the substrate surface in the presenceof humid air. Step 2. Condensation of water vapor intomicro droplets due to evaporative cooling of the solvent.Step 3. Growth and self-assembly of droplets into hexag-onal array due to capillary and Marangoni forces. Step4. Finally, complete evaporation of the solvent and wa-ter leads to ordered array of pores [2, 5–7]. The resultantdroplet patterns on the polymer films is significantly af-fected by subtle changes in the casting conditions suchas polymer and its structure, solvent, air flow, humidity,surface temperature and substrate [5].

The influence of polymer[8–11], solvent[9, 12–16],temperature[9, 17], substrates[9], and humidity[9, 18, 19]has been investigated in detail. However very few re-ports investigated the influence of vapor atmosphereon the self-assembly of droplets on surfaces. For in-stance, Ding et al., studied the influence of ethanoland methanol vapors on the formation of self-assembleddroplet patterns on the polystyrene-block-poly-dimethylsiloxane(PS-b-PDMS) surface. They observed that thesurface tension and the enthalpy of solvents in the at-mosphere are responsible for the formation of porousfilms[5]. Xiong et.al., reported the formation of micro-spheres on poly(styrene)-b-poly(butadiene) (PS-b-PB)

co polymers using solvent vapor atmosphere. They alsostated that surface tension is a key parameter in theformation of patterns [20]. Bai et al., fabricated poly-meric nano/microstructures on poly(9,9-dihexylfluorene)in mixed water and methanol vapor atmospheres [21].Zhang et al., reported a modified process for preparingporous films in methanol vapor with conventional poly-mers, by adding a small amount of surface active agentinto the casting solution, such as siloxane- and fluorine-containing block copolymers [22]. The previous studiesare performed for either pure methanol or ethanol. Itis observed from the above literature that the hexagonalarray of pores with methanol vapors are obtained onlywith the addition of either high surface tension water inthe environment or surface active agents to the castingsolution.

Ethanol-methanol binary liquids are interesting be-cause of their complex anomalous behavior that arisesdue to the molecular structuring of the moieties aroundeach other in solution [23]. The inter-facial structuringand hence the complex properties of the binary liquidsare significant in affecting the formation of self-assembleddroplet formation. In this paper, we present a strategyof obtaining ordered droplet arrays with a binary vaporof ethanol-methanol and a comprehensive description ofthe role of ethanol in stabilizing methanol droplets dur-ing the formation of self-assembled droplet arrays. Theinfluence of constraints of the underlying substrate onthe self-assembly process is also investigated.

The presence of constraints on the underlying sub-strate also influences the resultant droplet patterns. Itis observed from the literature that if the scale of con-straints is of the order of 50µm, then the constraintshave no influence on the self-assembly process[9, 24]. If

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the scale is of the order of capillary length or less, thenthe constraints makes the difference in the pattern ofdroplets self-assembly (Figure 1). This is due to the ef-fect of forces exerted on the contact line of the condens-ing liquid droplet[25]. Understanding the dynamics ofcontact line of condensing liquid droplets is of criticallyimportant in the surface science and in many biomedical[26–28]and industrial applications [28].

FIG. 1. Cartoon shows the effect of constraints with respectto the scale. a. there is no influence of the constraint whenthe constraint is of the order of 50µm b. Movement of contactline as shown in blue line when the constraint is of the orderof capillary length or less.

On an ideal smooth surface, the contact line remainspinned which is not true in reality [29]. On a realsurface, pinning and depinning of contact line occursdue to the liquid-liquid interaction force (L-L force)and solid-liquid interaction force (S-L force). L-L forcepulls the contact line towards the interior of the dropletand the S-L force will make the contact line to getpinned on the substrate [25, 30, 31]. This pinning anddepinning transition also leads to different evaporationmodes viz., constant contact angle mode and con-stant contact radius mode [25, 32–34]. The origin of themovement of contact line is still not well understood [25].

The present work experimentally investigates the in-fluence of saturated vapors of ethanol-methanol binaryvapors (over entire concentration regions) on the self-assembled droplet patterns on PDMS surfaces over bothsmooth and constrained substrates. An attempt is alsomade to study the influence of constraints on the un-derlying surface and tried to explain the self-assemblyof droplets in terms of intermolecular interactions of thebinary liquid vapors.

II. EXPERIMENTAL DETAILS

A. Materials

Polydimethyl siloxane (PDMS) from SIGMA Aldrichand chloroform (purity 99.9%) purchased from EM-PLURA are used as polymer and solvent respectively.Methanol and ethanol are purchased from Sigma Aldrichand Merck Emplura respectively. The purities ofmethanol and ethanol are 99%. Commercially availableSONY recordable DVDs are used as substrates. TheDVD disc is cut into pieces of 1cm×1cm. The two poly-mer layers are carefully peeled off. The polycarbonatesupport and the smooth polymer layer (the other one)are used as constrained and smooth surface respectively.

B. Preparation and characterization of binaryliquids

Binary mixtures of ethanol and methanol are preparedby mixing appropriate volume of each liquid in anairtight stopper glass bottle. The accuracy of the binarysystem is ±0.1mg and they are measured using a weigh-ing balance. Surface tension of the binary systems forall concentration range are measured using Rame-Hartcontact angle goniometer. Pendant drop method is usedin calculating the polar and dispersive parts of surfacetension.

The infra-red spectra of the binary system in thespectral range from 400 to 4000cm−1 are measured byFTIR Spectrometer of Perkin Elmer. The dielectricstudies are carried out using 40 GHz Vector NetworkAnalyzer with Dielectric Assessment Kit of Rhode andScharwz. The refractive index measurements of thebinary system are carried out using Abbe’s refrac-tometer. Calibration is performed by measuring therefractive indices of doubly distilled water using Nalight. All the experiments are done at room temperature.

C. Self-assembly of binary liquid droplets

In the current experiments, mixture of ethanol andmethanol at various volume ratios are chosen to producevapor atmospheres. 3ml of mixture of binary liquidsis added in a petri-dish and kept inside a sealed glasschamber to form a saturated vapor atmosphere. Thesmooth and the constrained surfaces are positioned atleast 1cm above the liquid. About 300 micro liter ofPDMS-chloroform solution is casted on the surface of thesubstrates using a micro-syringe under the binary liquidsaturated vapor atmosphere at 260C. After the completeevaporation of the solvent, a thin layer of polymer is lefton the substrate surface.

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To check the dissolutivity of chloroform with DVD, two

FIG. 2. Photograph of DVD dipped ethanol solution after 30minutes which shows no precipitates

blank experiments are performed. The substrates aretreated with ethanol and checked for precipitates. It isfound that with the given time of 1 minute, there is noformation of precipitate. To further confirm the dissolu-tivity, the DVD discs are treated with chloroform and thetreated discs are used for patterning under saturated va-pors of water. It is observed that the patterns formed onthe chloroform treated smooth and constrained surfacesis similar to untreated patterned surfaces. Therefore it isconfirmed that within the time span of the experiment,the chloroform doesn’t dissolve the upper layer of DVDdiscs. The supporting images are shown in (Figure 2 and3)

FIG. 3. 3D CLSM images of chloroform treated a. smoothand b. constrained substrates patterned under water-vapor.

D. Characterization of patterned surfaces

The patterns formed on the substrate surface are char-acterized using the Leica Confocal Scanning Electron Mi-croscope TCS SP8 model. The transmission mode with

the excitation wavelength of 561 nm is used to capturethe images of formed patterns. Rame Hart contact anglegoniometer equipped with a charge coupled device cam-era along with an advanced drop image program is usedto measure the contact angle of water with the surfaces.3µl drop of distilled water is placed on the substratesthrough a microsyringe which is placed onto the stage.The measurement is recorded after the drop reaches equi-librium. The substrate is moved to allow another dropto be placed on the substrate. Atleast 10 measurementsare taken and all are reproducible up to ±20.

III. RESULTS AND DISCUSSION

Precise experimental conditions are required forself-assembly of droplet patterns since evaporation andfilm formation occur in few seconds. To avoid thepossibility of uncertainties with dynamic method ofself-assembly of droplets, the static method explainedin the experimental section is employed to fabricatethe structures on smooth and constrained substrates.This method is quite robust under saturated vapor ofwater. Few works have systematically examined theself-assembly of droplet patterns on smooth surfaces inethanol and methanol environment [5]. In the presentcase the binary system of ethanol and methanol areemployed as vapor atmosphere during the process ofself-assembly of droplets.

Most alcohols interact via strong repulsive forces andweak attractive forces. But an important part of the in-teraction is the strong, directional hydrogen bonds theyform which are important in determining its structure.To understand the intermolecular interactions betweenmethanol-ethanol, the properties like surface tension,refractive index and static dielectric permittivity of thebinary system over the entire concentration range arestudied and tabulated in Table 1. From our recent workon methanol-ethanol binary system, it is observed thatall the properties show deviation from ideal behaviorat 10% to 30% of methanol. The deviation in theproperties of binary system is attributed to the increaseof hydrogen bond strength with increase in methanolconcentration. At higher concentration of ethanol, theLondon dispersion forces dominate over the hydrogenbonds [23]. With these deviations, it is also expecteda change in self assembly pattern of droplets over thesubstrates.

An attempt at explaining self assembly of liquiddroplets in terms of intermolecular interactions and theeffect of constraints on the pattern formation, a dropof PDMS-chloroform solution is casted over smoothand constrained substrates in the vapor environment ofethanol-methanol binary system. The solvent is allowedto evaporate. When the solvent evaporates completely,a porous film is left on the substrate. The experiment is

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TABLE I. Variation of properties of ethanol-methanol binary system with the concentration of methanol

Perc. of Surface tension Refractive Dielectric Dielectricmethanol (mJ/m2)(±0.02) index (±0.001) permitivity (±2%) loss(±2%)0 22.00 1.360 24.80 8.5210 22.98 1.363 24.75 10.0012 22.14 1.362 24.65 6.4114 19.30 1.362 24.90 8.1616 19.46 1.360 25.33 9.0818 20.70 1.358 25.71 9.9819 19.88 1.358 25.58 3.3520 20.70 1.357 25.67 4.0721 19.01 1.355 26.72 4.5022 20.07 1.355 26.52 11.3324 19.76 1.355 26.85 11.4626 19.28 1.353 27.01 11.2028 19.94 1.352 27.18 11.7930 21.56 1.352 24.66 9.2640 21.47 1.347 24.79 9.0050 21.53 1.342 27.58 8.1560 21.45 1.337 29.20 7.5670 21.68 1.333 28.31 8.0680 21.53 1.331 29.82 7.3390 21.48 1.324 29.09 7.67100 22.10 1.327 34.38 8.88

FIG. 4. CLSM images of PDMS films prepared on smoothsubstrate under a) pure ethanol and b)Pure methanol .

repeated over entire concentration range of methanol inethanol in steps of 10% of volume.

A. Pattern formation on smooth and constrainedsurfaces

The Confocal Laser Scanning microscope (CLSM) im-ages of thus patterned smooth and constrained surfacesfor all molar fraction of methanol in the binary systemare shown in Figures 4, 5, 7, 9, 14 and 16 and the 3DCLSM images are also shown in Figures 6, 11, 10, 15 and17. The shape, size and overall pattern of the pores pre-pared under various concentration range of ethanol andmethanol are investigated. It is observed that there arevariations in the size and shape of the pores on PDMSfilm patterned under various volume fractions of binary

vapors of methanol and ethanol.

FIG. 5. CLSM images of PDMS films over smooth substrateprepared in various concentration of binary vapors of ethanol-methanol. a)10% of methanol b) 20% of methanol c) 30% ofmethanol d)40% of methanol e) 50% of methanol f) 90% ofmethanol.

Patterns on smooth surface:

The images of smooth surface patterned under pureand binary mixtures of ethanol and methanol are shownin Figure 4 and Figure 5 respectively. The size ofthe pores on the PDMS film prepared under variousconcentration of methanol vary according to the surfacetension of the liquids forming saturated vapor. Theshape of the pore openings also vary from polygon to

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FIG. 6. 3D CLSM images of PDMS films over smooth sub-strate prepared in various concentration of binary vapors ofethanol-methanol. a)40% of methanol b) 50% of methanol c)70% of methanol d)90% of methanol.

circular. Pores with irregular openings are formed onthe polymer film patterned under various concentrationsof methanol. 3D CLSM images are also shown in Figure 6

Figure 4a shows the pore formation on the smoothsurface patterned under vapors of pure ethanol. Itindicates that the ethanol vapors favor the hexagonalpore formation on PDMS films. The film is denselycovered with pores that are nearly 3.25µm in size. Thepores are separated from each other by a distance ofnearly 1µm. With the introduction of 10% of methanol,the patterns on the film slightly deviates from theirclose arrangement of pores. Figure 5a indicates that,the introduction of 10% of methanol decreased the poresize and nearest neighbor pore distance to 1.65µm and800nm respectively. Therefore, it is inferred from theobservations that the pore diameter can be controlledby the percentage of methanol in the binary systemforming saturated vapor atmosphere. This implies thatthe pore diameter is a function of surface tension of thebinary system.

Further increase in the concentration of methanolresults in increased pore size and pore density asshown in Figure 5c and d. It is also observed fromthe figure that at 30% of methanol, the pores areself-assembled into nearly hexagonal arrays. Around40% of methanol, the film is observed to have highlyordered hexagonal structures with a pore size of 4.4µm.This result indicates that intermolecular interactionbetween ethanol and methanol molecules increases withincrease in methanol concentration. Above 40% ofmethanol, the complex behavior of the binary systemsaturates indicating the stability in their intermolecular

forces which is also confirmed from the surface tensionmeasurements as shown in Table 1.

Self-assembly of droplet pattern is a surface tensiondriven phenomenon. Liquids of same surface tensionresult in similar pattern formation on unconstrainedsurfaces. Thus the patterns formed on smooth surfaceare similar for the concentration range of 40% to 90%of methanol as expected. A gradual change in the poly-meric pore morphology is observed when the saturatedvapor atmosphere is composed of pure methanol asshown in Figure 4b.

From the above results, it is observed that in smoothsurface, pure ethanol forms hexagonal array of pores.However, with the addition of methanol, pore patternsdeviates from the hexagonal array. On the other hand,i.e., with pure methanol highly disordered patterns areobserved. With the addition of small amount of ethanol,the droplets self-assemble themselves into hexagonalarray of pores. Thus the addition of small amount ofethanol to methanol plays a role very similar to thatof surface active agents in inducing the formation ofregular droplet array. Similar analysis is performedwith the constrained surfaces to investigate the roleof ethanol-methanol vapors. Interesting results areobserved as follows.

Patterns on constrained surface:

FIG. 7. CLSM images of PDMS films prepared on constrainedsubstrate under a) pure ethanol and b)Pure methanol.

Figure 7a shows the self assembled droplet patternformed on the constrained surface under saturated vaporof pure ethanol. It is observed that the film is coveredwith distorted unclosed rings of pores. The ring sizeand pore size are approximately 36.4µm and 3.8µm.Pores of size 0.5µm are found inside the distorted rings(size of the pores versus concentration of methanol isshown in Figure 8). On the addition of methanol, thepores self-assembles into patches of hexagons. Furtheraddition of methanol favors the formation of hexagonalrings. The ring shape attain its hexagonal alignmentat higher concentrations of methanol (Figure 9). The

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FIG. 8. Variation of pore diameter for smooth and con-strained surfaces patterned with various concentration ofmethanol in ethanol.(Top: entire concentration region; bot-tom: concentration region 10% to 30% of methanol).

pore density inside the ring increases with increase inmethanol concentration. 3D CLSM images for variousconcentration of methanol are shown in Figure 10

FIG. 9. CLSM images of PDMS films over constrained sub-strate prepared in various concentration of binary vapors ofethanol-methanol. a)10% of methanol b) 30% of methanol c)60% of methanol d)70% of methanol e) 80% of methanol f)90% of methanol.

With pure methanol as saturated vapor, the rings arefound to be highly ordered (3D image of pure methanolon constrained surface is shown in Figure 11) and denselypopulated with pores of size 1µm (Figure 7b) whereasethanol results in distorted pore array. This resultsare completely opposite to what is observed in smoothsurface. On constrained surface, despite the role ofsurface tension of ethanol-methanol binary system, the

FIG. 10. 3D CLSM images of PDMS films over constrainedsubstrate prepared in various concentration of binary vaporsof ethanol-methanol. a)50% of methanol b) 60% of methanolc) 70% of methanol d)90% of methanol.

FIG. 11. 3D CLSM image of PDMS films prepared in puremethanol vapor on constrained surface.

effect of constraints on the underlying surface dominatesin the formation of pore patterns.

B. Formation mechanism of self-assembled dropletpatterns under saturated binary vapors

To explain the differences in pattern formation pre-pared under the saturated vapor atmosphere of ethanol-methanol binary system, the physical properties of liq-uids forming the saturated vapor atmosphere, solvent

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and polymer are taken into account. The droplet placedon a surface spreads to a certain extent until the equi-librium is reached. At equilibrium, the total free energyis minimized and the droplet shape stops varying. It iswell known that the droplet shape is determined by theinter-facial tension between the polymer and the dropletand the intermolecular interactions within the droplet[35]. And the spreading co-efficient in terms of these twofactors is given by,

S = γp − (γd + γpd) (1)

where γp, γd and γpd are surface tension of the polymersolution, surface tension of droplet, and inter-facial ten-sion of polymer solution and droplet respectively. Thespreading co-efficient is used to characterize the spread-ing of droplets and it defines different regimes of wetting.The equilibrium contact angle, called the Young’s contactangle is given by [36],

γdcosθ = γp − γpd (2)

For a large number of droplets, θ is the inner angle ofthe droplet-vapor interface that makes with the polymerlayer. The relation connecting contact angle and spread-ing co-efficient is obtained by combining eqn.1 and eqn.2and is given as,

cosθ = 1 + (S/γd) (3)

The spreading parameter S > 0 indicates completewetting and S < 0 indicates partial wetting. In thepresent work, the surface tension of polymer solutionis kept constant. The surface tension of the dropletsis a varying quantity. With the variation in γd, theinter-facial tension of the polymer solution and dropletalso changes a little. Since ethanol and methanol aremiscible in chloroform, the inter-facial tension of poly-mer solution and droplet are assumed to be small. Theinter-facial tension of the polymer solution is measuredusing the pendant drop method and is measured tobe 17.11 × 10−3J/m2. The surface tension of binaryliquid of various concentrations of methanol are shownin Table 1

Therefore, the spreading parameter for the dropletof binary liquid for all concentrations can be expectedto be positive. The droplet impinging on the polymersurface will tend to spread. The formation of interveningpolymer layer due to condensation of the saturated vaporprevents the coalescence of thus formed droplets. As thedroplets grow, they are attracted each other through thecapillary forces and after complete evaporation of thesolvent, the self-assembled droplet arrays are formed.The surface tension of the template droplets determinesthe shape of the pores. The larger surface tension leadsto more spherical pores and deformation from sphericalshape occurs with the decrease in surface tension.

FIG. 12. Variation of pore depth for smooth and constrainedsurfaces patterned with various concentration of methanol inethanol. Uncertainty is found to be less than the symbol size.(Top: entire concentration region; bottom: concentration re-gion 10% to 30% of methanol)

Ethanol and methanol with surface tension22 × 10−3J/m2 and 22.1 × 10−3J/m2 leads to de-formed spherical pores. Figure 4 shows the formation ofdeformed pores with methanol droplets and shapes closeto sphere with ethanol droplets. With binary liquiddroplets, the deformation in spherical shape decreaseswith increase in methanol concentration and at 40%of methanol, the pores are almost spherical(Figure 5).Further increase in methanol concentration leads toincrease in deformation of spherical shape. Apart fromshape of the pores, the size and depth of the pores alsoshow variations with varying methanol concentration(Figure 8, 12). The variation in size arises from thedifference in evaporation enthalpy of the saturatedvapor. Evaporation enthalpy determines the amountof vapor to be condensed during the self-assemblyof droplets[5]. Ethanol with its greater evaporationenthalpy leads to smaller pores compared to the poresformed from methanol vapors. This is due to the factthat lower value of enthalpy leads to condensation oflarge amount of saturated vapor.

C. Effect of roughness on self-assembly process

In order to further probe the differences in poremorphology of constrained surfaces, the effect of depin-ning/pinning of contact line is investigated in detail.A bare/smooth surface uniformly coated with polymerlayer has no effect on the movement of contact line ofcondensed droplets. Whereas the spreading of liquidon surfaces is strongly influenced by chemical hetero-geneities and roughness of the surface [37, 38]. The

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contact angle of a drop on a heterogeneous surface isdetermined solely by the interactions occurring at thethree phase contact line. The wetting of heterogeneoussurfaces is controlled by the three phase structure at thecontact line and not by the inter-facial contact area [39].

The interaction between the fluid interface and theheterogeneities results in the movement of contactlines. This dynamics of contact line is determinedby the upper length scale of the droplets and thelower length scale set by the characteristic size of theconstraints/grooves[40, 41]. For a surface with infinitelylong grooves close to each other like that of numberof lines normal to the axis in a capillary, the contactline can lie either parallel to the grooves or at an angleφ to the grooves. The contact line gets pinned whenit is parallel to the grooves and it moves continuouslywithout any pinning when it lies at an angle φ [37].

When the contact line moves over the surface there areoften points that remain pinned, inducing the contact lineto suddenly jump to a new position [42, 43]. Velocity ofthe contact line based on calculating the energy dissipa-tion per length of unit line is given by Brochart-Wyattand de Gennes as [44, 45],

V =θσ

6η ln(L/a)(cosθo − cosθ) (4)

where θ is the dynamic contact angle, σ is the liquid-vapor surface tension , θo is the equilibrium contactangle, L/a is the ratio of macroscopic to microscopiclength scales and η is the dynamic viscosity. From theabove equation it is clear that the contact line velocityis determined by the dynamic viscosity and surfacetension of the liquid. The contact line velocity is highfor liquids of high surface tension. So it is expected adifference in pattern formation on constrained surfacewith condensing droplets of various surface tension.

The dynamic viscosity of ethanol and methanol are0.001095Ns/m2 and 0.00056Ns/m2 respectively. Withlow dynamic viscosity, methanol droplets shows a highthe contact line velocity. This results in the formationof more aligned hexagonal rings compared with ethanolof comparatively low dynamic viscosity. (Supporting3D image of pure methanol on constrained surface isshown in Figure 11.) Thus, highly ordered hexagonalring patterns are obtained with saturated vapors ofhigher concentration of methanol. This also explainsthe complex variation in pore ring diameter withvarying concentration of methanol (plots are shown inFigure 13). It should also be noted that when the hori-zontal component of the force caused by the Marangonistress overcomes the Young’s force, the Marangoni ef-fect also has its influence on the depinning of contact line.

The energy barrier associated with the pinning anddepinning of contact lines in the constrained surface isdetermined by the size of the grooves. The competition

FIG. 13. Variation of ring diameter of pores on constrainedsurfaces patterned under various concentration of methanolin ethanol.(Top: entire concentration region; bottom: con-centration region 10% to 30% of methanol)

between the Young unbalanced force and the anchoringforces of the grooves is thought to dominate the pin-ning/depinning of contact lines [45]. Since the groovesize was maintained constant, the depinning/pinning ofcontact line is solely determined by the properties of con-densing droplets.

FIG. 14. CLSM images of PDMS films over smooth substrateprepared in various concentration of binary vapors of ethanol-methanol. a)12% of methanol b) 18% of methanol c) 19% ofmethanol d)21% of methanol e) 26% of methanol f) 28% ofmethanol.

D. Patterning under low concentration of methanol

To gain a better understanding of the role of surfacetension and other properties mentioned above, the

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FIG. 15. 3D CLSM images of PDMS films over smooth sub-strate prepared in various concentration of binary vapors ofethanol-methanol. a)12% of methanol b) 18% of methanol c)21% of methanol d)24% of methanol.

FIG. 16. CLSM images of PDMS films over constrained sub-strate prepared in various concentration of binary vapors ofethanol-methanol. a)12% of methanol b) 16% of methanol c)19% of methanol d)21% of methanol.

patterning is performed under lower concentration ofmethanol in ethanol-methanol binary system, wherecomplexity in their properties is reported [23]. Figure 14shows the polymer films over smooth substrate patternedunder saturated vapors of 10% to 30% of methanol(inethanol-methanol binary system)and Figure 15 showsthe 3D CLSM images of the same. It is evident thatconcentration increase of methanol beyond 10% favors

FIG. 17. 3D CLSM images of PDMS films over constrainedsubstrate prepared in various concentration of binary vaporsof ethanol-methanol. a)14% of methanol b) 16% of methanolc) 19% of methanol d)26% of methanol.

the formation of ordered hexagonal array of pores. Itis observed that the pore distribution shows complexvariation with varying concentration of methanol. Itis also observed from the figure that the pore size,pore depth and pore diameter varies significantly withvarying surface tension of the saturated vapors. Themaximum pore size and pore depth is observed forthe films patterned under saturated vapor of 22% ofmethanol. The pores are found to be highly deformedfrom circularity and at 19%, 21% and 28% of methanol,the circularity of the pores increases. This further provesthat the surface tension of binary vapors plays a majorrole in tuning the shape, size and distribution of thepores.

On examining the patterned constrained surface(Figure 16), the films patterned under 16%, 21%and 26% of methanol show ordered hexagonal ring ofpores. It is evident from these results that not onlythe Young’s unbalanced force but also the force due toconstraints has its role in determining the alignmentof pores. Figure 17 shows the 3D CLSM images ofpatterns on constrained surfaces. In summary, it isobserved that the pore diameter depends on the surfacetension of ethanol-methanol vapors and pore patternsare influenced by the pinning/depinning of contact linesin addition commonly attribution to the properties ofthe solvent.

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E. Pore aspect ratio

Pore aspect ratio (PD/d) is defined as the ratio be-tween the depth of the pores(D) and diameter(d) of thepores.i.e.,

PD/d = D/d (5)

PD/d for all patterned surfaces is calculated and is shownin Figure 18. From the figure it is observed that the max-imum depth-diameter aspect ratio obtained with smoothand constrained patterned surfaces are 7.1 and 7.3 re-spectively. For smooth surface, this maximum value isachieved under 60% of methanol and for constrained sur-face it is achieved under 12% of methanol. The poreaspect ratio is observed to be strongly influenced the in-termolecular interactions of binary vapors.

FIG. 18. Variation of depth-diameter aspect ratio forsmooth(top) and constrained(bottom) surfaces patternedwith various concentration of methanol in ethanol. Uncer-tainty is less than the symbol size

F. Contact angle studies

Contact angle studies are performed for the smoothand constrained patterned surface using Goniometer withdistilled water as the reference liquid. Surfaces exhibitedhydrophobicity with water contact angle of more than95o. A variation in contact angle is observed for filmspatterned under saturated vapors of binary system ofethanol-methanol as observed in Figure 19. Maximumcontact angle of 128o is observed for smooth patternedsurface under saturated vapor of 19% of methanol. Forconstrained surface, the maximum contact angle is ob-served at 22% of methanol. This variation of water con-tact angle confirms the variation in surface roughnesswhich in turn attributed to the formation of different

pore morphology on films patterned under various sur-face tension of vapor atmosphere. It is also observedfrom the results that the hydrophobicity/hydrophilicitycan be achieved by the proper choice of the concentrationof methanol.

FIG. 19. Variation of water contact angle for smooth(top) andconstrained(bottom) surfaces patterned with various concen-tration of methanol in ethanol.

IV. CONCLUSION

In conclusion, five main results are observed from thisstudy. 1.It has been demonstrated that it is possible toform ordered array of self-assembled droplets of methanolby the addition of small amount of ethanol without theuse of any surfactants/water. This is attributed to thehydrogen bond between the ethanol and methanol andthe ability of ethanol to bind strongly to water since itis more hydrophilic than methanol. 2. The differencein pattern formation over smooth and constrained sub-strates indicates a strong influence by the depinning ofthe three phase contact line formed between substrate,binary liquid vapor and PDMS solution. 3. Pore diame-ter is observed to be a function of surface tension of theethanol-methanol binary concentration. 4. Pore aspectratio is influenced by intermolecular interactions of bi-nary vapors. 4. The complex variation in the shape, sizeand distribution of the pores are observed in the sameconcentration range where the complex anomalous vari-ation in properties of ethanol-methanol binary system isreported earlier. 5. The contact angle studies indicatesthat the hydrophobicity/hydrophilicity can be achievedby the proper choice of the concentration of methanol.And hence it can find its applications as self-cleaningsurfaces, micro-fluidic devices, and water-repellant sur-faces.

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V. ACKNOWLEDGEMENT

The authors thank Naval Research Board of India forthe Contact Angle Goniometer.

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VI. AUTHORS CONTRIBUTIONS

All the authors were involved in the preparation of themanuscript. All the authors have read and approved thefinal manuscript.