(452i) Wetting Transition of Ethanol-Water Droplet On Smooth and Pillared Surfaces: A Molecular Dynamic Study

Wetting phenomena of a liquid to solid are ubiquitous in nature [1]. It has technological relevance, as large number of industrial, manufacturing, biological applications, require uniform and rapid wetting; whereas, others demand poor wetting. It is necessary to achieve such spreading, possibility by adding surface active agents such as surfactants or alcohols which modifies the interfacial tension [2]. Ethanol in water, as a mixture, is used in numerous applications for medical, paints, perfumes, and deodorants products. Though some works on nano-confined ethanol-water mixtures have been conducted, wetting transition of such mixture on on rough surfaces is limited in the literature.

In the present work, the effect of ethanol concentration and the spreading mechanism of ethanol-water droplets on smooth and textured surfaces are investigated using molecular dynamic simulations. The macroscopic property such as contact angle is found to decreases with increasing ethanol concentration in the droplet. We rigorously study the system size effect on the contact angle of water-ethanol drop on smooth graphite surface as a function of ethanol concentration. It has been found that the ethanol molecules accumulate in the three phase contact line and the vapor-liquid interface. However, wetting behavior of the droplet on rough surfaces is quite different from that on the smooth surface. We carefully examine the different wetting states as a function of surface fraction and pillar height. Ethanol molecules are found to accumulate in the grooves rather than on the top of the pillar. For pillared surfaces,  the ethanol-water drop is found in the partial Wenzel state at lower ethanol concentrations. On the other hand, at higher concentration it is in the complete Wenzel state which is in good agreement with the experimental results[3].  We also attempt to quantify the behavior of the mixture in the Cassie-Baxter and Wenzel states in terms of continuum relations seen recently [4].

References

[1] A. Otten and S. Herminghaus, Langmuir 20, 2405 (2004).

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

[3] J. Fidler and P.M. Rodger, J. Phys.Chem. 103, 7695 (1999).

[4] F. Leroy and F. M. Plathe, J. Chem. Theo. and Comp. 8, 3724-3732(2012).