(179u) Wetting Transition of Water-Alcohol Mixture On Smooth and Pillared Hydrophobic Surfaces: A Molecular Dynamic Study

The mixture of ethanol and water plays a different role in wetting transition as the hydroxyl group of ethanol molecules can participate in the hydrogen bond network of water molecules and the ethyl group is comparatively small and hydrophobic in nature. It has been found that the ethanol molecules accumulate in the three phase contact line and the vapor-liquid interface on the smooth graphite surface and the contact angle of the droplet is found to decrease with increase in ethanol concentration[1]. Therefore, the interfacial tension across the contact line or line tension must play an important role in wetting of ethanol solution. In this study using molecular dynamics simulations, we rigorously calculate the line tension of water-ethanol solution on smooth graphite surface and system size effect on the contact angle of water-ethanol solution as a function of ethanol concentration.

The wetting behavior of ethanol solution on rough surfaces is quite different from that on smooth surface. Ethanol molecules are found to accumulate in the grooves rather than on the top of the pillar which is in contrast to the result of water-urea mixture on rough graphite surface[2]. Interestingly, we observe that as long as pillar spacing is comparable with the size of ethanol molecules, the ethanol molecules always prefer to stay in the grooves and hence enhance the wetting on the rough surfaces. We extensively study the wetting behavior of water-ethanol solution on rough graphite surfaces at different ethanol concentrations. We carefully examine the different wetting states as a function of surface fraction and pillar height. The transition from Cassie to Wenzel state is also found for certain surface fraction and pillar height with increase in ethanol concentration. For such pillared surfaces, the ethanol-water solution is found in the partial Wenzel state at lower ethanol concentration whereas in higher concentration it is in the fully Wenzel state which is good agreement with the results recently conducted in experimentally[3].  

[1] M. Lundgren, N.L. Allan, and T. Cosgrove, Langmuir 18, 10462 (2002).

[2] T. Koishi, K. Yasuoka, X.C. Zeng, and S. Fujikawa, Faraday Discuss. 146, 185 (2010).

[3] J.B. Boreyko, C.H. Baker, C.R. Poley, and C.-H. Chen, Langmuir 27, 7502 (2011).