Lubricant Impregnated Nanotextured Surfaces

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1 Supporting Information: Enhanced Condensation on Lubricant Impregnated Nanotextured Surfaces Sushant Anand, Adam T. Paxson, Rajeev Dhiman, J. David Smith, Kripa K. Varanasi* Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA *Professor Kripa K. Varanasi, KEYWORDS dropwise condensation, slippery surfaces, droplet mobility, liquid impregnated surfaces, nanotextured surfaces 1

2 Spreading Coefficients of Krytox and Ionic Liquid. The equilibrium spreading coefficient, for Krytox and BMIm on water is given by S ow(v) = γ ov γ wv γ ow and the equilibrium spreading coefficient for water on the lubricant is given by S wo(v) = γ ov γ wv γ ow. In these definitions, γ ov, γ wo and γ wv are the surface and interfacial tensions between phases at equilibrium, that is, after water and the lubricant become mutually saturated. These surface and interfacial tensions can be calculated from the profile of a pendant drop. 1 The details on spreading coefficient calculations are given below, and values of all the surfaces and interfacial tensions and the resulting spreading coefficients are provided in Table S1. Spreading Coefficient of Ionic Liquid. The ionic liquid used in the current study ([BMIm + ][Tf 2 N - ]) is hydrophobic and considered to be immiscible with water. 2 However, it is slightly hygroscopic and has a very low solubility in water (~0.77% wt/wt). 3 The properties of BMIm upon saturation with water are available for surface tension (γ ov ), 4 density 5 and viscosity. 5 In absence of information on surface tension of water saturated with ionic liquid, this value was measured. Solutions comprising different concentrations of ionic liquid in water were prepared. Solutions were sonicated to completely dissolve the ionic liquid in water and a 5 µl drop taken from the resulting solution was used to calculate the interfacial tension (γ wv ). The results of surface tension at different concentrations of ionic liquid in water are given below: 2

3 γ w(v) (mn / m) Ionic liquid/water (%wt/wt) Figure S1. Change of Surface Tension of Water due to partial miscibility of ionic liquid. From Figure S1, saturated value of surface tension of water in presence of ionic liquid was found as 42 mn/m calculated when ionic liquid/water weight% was ~1.0%. The spreading coefficients of water on BMIm (S wo ) and of BMIm on water (S ow ) are given in Table S1. Table S1. Spreading coefficients of water-krytox and water-[bmim + ][Tf 2 N - ] systems. γ wv γ ov γ ow S wo(v) S ow(v) Krytox Ionic liquid As shown in the videos of microscopic growth, water droplets exist as micro-lenses on the surface of the ionic liquid during the growth process (Video S2). This suggests that the change of surface tension of water occurs at times scales much smaller than condensation time scales. It should be noted that similar behavior has been noted with benzene and 1,1-diphenyl-ethane. 6 3

4 Camphor has also been observed to dissolve slightly in water, and this partial miscibility results in a decrease in the surface tension of water. 6 Effect of vapor concentration on contact angle of oil on solid. Since condensation experiments are performed under saturated vapor conditions, it is important that the spreading behavior of oil on the solid does not change under these conditions. Experiments were performed to verify that the contact angle of oil on solid is not affected by the vapor concentration. A droplet of lubricant (BMIm) was placed on OTS treated smooth silicon surface and exposed to the sample under different saturated vapor conditions inside the condensation rig. Subsequently, images acquired at different vapor pressure conditions were analyzed for contact angles using ImageJ software. Contact angle of the droplet (θ os(v) ) was also measured in presence of air and was found to be 65±4 degrees. The results (Table S2) indicate that the contact angle remained statistically unchanged at different vapor pressures. Table S2. Contact Angle of BMIm on OTS treated silicon at various saturated vapor conditions Vapor Pressure (kpa) θ os(v) (deg) ± ± ± ± ±4 ~1.0 66±4 4

5 Droplet mobility analysis. From the videos, the frames were extracted for analysis of droplet mobility that was performed using ImageJ. To obtain scaling for converting the pixel information into actual sizes, a micrometer reticule (10mm scale / 100 div) was photographed at the same focal distance as during the experiment. The change in droplet location ( s) in time t was used to calculate droplet speed as V = s / t. This droplet speed is associated with a droplet of size D avg = (D 1 + D 2 ) / 2 where D 1 and D 2 are diameter of droplet at time t and t+ t respectively. Since droplet coalescence is an extremely rapid process, performing such droplet mobility measurements over a longer period of time and having large sample space can provide a timeaveraged value of mobility for a given droplet size drops of different sizes were chosen randomly and analyzed for determining droplet mobility on lubricant-impregnated and unimpregnated nano-textured superhydrophobic surface (Figure 4c). The resolution of droplet displacement measurements was 10 µm. To obtain a representative value of mobility, droplet diameter was rounded to the nearest multiple of 100 µm, and a median for droplet speed of all the droplets with the same size was calculated. Factors influencing lubricant lifetime. As noted previously 7, lubricants impregnated into textured surfaces can resist drainage due to external forces (eg., gravity) as they are stabilized by capillary forces. For a vertical surface, the force balance between the capillary forces and the gravitational force can be written as γ ov /b ~ ρ o gl, where ρ o is the density of lubricant, g is acceleration due to gravity and l is the height of the lubricant column. The capillary force scales inversely with the feature spacing and can be significantly increased by using textures with finer spacing to resist external forces. In absence of condensation, the surfaces used in this study were stable for many days even when held vertically in direction along gravity. 5

6 The longevity of the lubricant on the surface during condensation depends upon several factors. These include drainage of lubricant, carried away either in the lubricant cloak or the wetting ridge, evaporation of lubricant, and solubility of the lubricant in the condensate. As discussed in our study, by selecting a lubricant (BMIm) that does not spread on water, we eliminate the lubricant lost due to cloaking. Similarly, by selecting lubricants with extremely low vapor pressure we eliminate losses due to evaporation. For condensation experiments conducted in ESEM and with this lubricant, we observed stable shedding ~2 hours. Beyond this time, the shedding behavior was gradually disrupted, as indicated by droplets pinning to the substrate. We attribute this primarily to the solubility of BMIm in water. BMIm has a small miscibility (<1% wt/wt) that affects its longevity when used as a lubricant. This factor may be mitigated by selecting a different lubricant that is completely immiscible with the condensate. As for the volume lost in the wetting ridge (Figure 1e, 1f and Figure 3a), although its detailed description and dependence on texture and interfacial energies is out of the scope of the current study, experimental evidence shows that the wetting ridge volume is a function of underlying micro-nano texture. Table S3 shows example of condensed droplets on BMIm impregnated silicon surfaces having same chemistry (treated with OTS) but with three different textures. The three surfaces are: a silicon surface with etched nanograss features; silicon surface having nanograss etched microposts (a = 10 µm, b = 7.5 µm, and h = 10 µm); and silicon surface having nanograss etched microposts (a = 10 µm, b = 75 µm, and h = 10 µm). From the images (Table S3) it is clear that making the underlying texture more dense, decreases the size of the wetting ridge and hence its volume. Using image analysis we estimated the wetting ridge volume surrounding the droplets. As shown in the table, by decreasing the texture spacing from 75 µm to 7.5 µm, we observe over 90% decrease in the volume of the wetting ridge for comparable sized 6

condensation. Furthermore, incorporating lubricant reservoirs could also help mitigate the longevity challenges. Table S3.

7 droplets on the two surfaces. On silicon surface with etched nanograss features, the wetting ridge appears non-existent even when we image the periphery of a condensed droplet under high magnifications. In light of above discussion, we believe that all of these factors can be addressed to optimize these surfaces for longevity under condensation. Furthermore, incorporating lubricant reservoirs could also help mitigate the longevity challenges. Table S3. Effect of surface texture on wetting-ridge shape and volume Micropost Spacing (µm) Image Wetting-Ridge (WR) Volume Estimation ( Droplet Diameter ) 7.5µm Droplet Diameter ( ) 75µm = ( WR Volume) 7.5µm WR Volume ( ) 75 µm = µm µm = µm µm 3 = 9.25% 0 (Bare Nanograss) Wetting Ridge volume cannot be determined due to negligible wetting ridge appearance 7

8 REFERENCES 1. Stauffer, C. E., The Measurement of Surface Tension by the Pendant Drop Technique. J. Phys. Chem. 1965, 69, Pfruender, H.; Jones, R.; Weuster-Botz, D., Water Immiscible Ionic Liquids as Solvents for Whole Cell Biocatalysis. J. Biotechnol. 2006, 124, Shvedene, N. V.; Borovskaya, S. V.; Sviridov, V. V.; Ismailova, E. R.; Pletnev, I. V., Measuring the Solubilities of Ionic Liquids in Water Using Ion-Selective Electrodes. Anal. Bioanal. Chem. 2005, 381, Freire, M. G.; Carvalho, P. J.; Fernandes, A. M.; Marrucho, I. M.; Queimada, A. J.; Coutinho, J. A. P., Surface Tensions of Imidazolium Based Ionic Liquids: Anion, Cation, Temperature and Water Effect. J. Colloid Interface Sci. 2007, 314, Jacquemin, J.; Husson, P.; Padua, A.; Majer, V., Density and Viscosity of Several Pure and Water-Saturated Ionic Liquids. Green Chem. 2006, 8, Harkins, W. D.; Feldman, A., Films. The Spreading of Liquids and the Spreading Coefficient. J. Am. Chem. Soc. 1922, 44, Lafuma, A.; Quéré, D., Slippery Pre-Suffused Surfaces. Europhys. Lett. 2011, 96(5),

Droplet mobility on lubricant-impregnated surfaces

Droplet mobility on lubricant-impregnated surfaces J. David Smith, a Rajeev Dhiman, a Sushant Anand, a Ernesto Reza-Garduno, a Robert E. Cohen, b Gareth H. McKinley, a and Kripa K. Varanasi* a Received