(757d) Estimation of the Free Energy of Water Droplet During Transition Between Wenzel to Cassie State: A Molecular Dynamic Study

Wetting on rough surfaces has been studied since long back due to it’s potential application in micro-fluidics devices, micro-electronics, coating etc. It is well understood that roughness and surface energy have significant role in wetting phenomena and explained by two main hypothesis attributed to Cassie and Wenzel states. During last couple of years, an extensive effort has been made to understand these two states experimentally [1-3] as well as computationally [4-5]. It is realized that the both states are distinguished by free energy barrier. Transition between Cassie and Wenzel states can be observed by different approaches such as applying external field [1], vertical vibration[2], heating[3], external force or pressure[4], addition of some solvent[5] etc.

In this study, the transition from the Wenzel state to the Cassie state for water droplets of different sizes on a rough graphite surface has been investigated with the help of molecular dynamic simulations by creating artificial path along a surface-fluid interaction scale. We accurately calculate the free energy barrier between these two states by means of thermodynamic integration for different roughness factor. Free energy barrier is found to be strong function of pillar height as well as pillar gap. We also observe that the transition from Wenzel to Cassie undergoes various local phase transitions along the thermodynamic path.        

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                [2]          Jonas, A.; Karadag, Y.; Tasaltin, N.; Kucukkara, I.; Kiraz, A. Langmuir, 27, 2150,2011.

                [3]          Liu, G.; Fu, L.; Rode, A. V.; Craig, V. S. J. langmuir, 27, 2595,2011.

                [4]          Koishi, T.; Yasuoka, K.; Fujikawa, S.; Ebisuzaki, T.; Zeng, X. C. Proc. Natl. Acad. Sci, 106, 8435,2009.

                [5]          Koishi, T.; Yasuoka, K.; Zeng, X. C.; Fujikawa, S. Faraday Discuss., 146, 185,2010.