8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P6.57Free energy of water droplet on rough hydrophobicsurface from Wenzel to Cassie state: a moleculardynamic studySandip Khan 1 and Jayant K. Singh 11 Indian Institute of technology kanpur, Department of chemical Engineering, UttarPradesh 208016, Kanpur, IndiaWetting on rough surfaces has been studied since long back due to it’s potential application inmicro-fluidics devices, micro-electronics, coating etc. It is well understood that roughness andsurface energy have significant role in wetting phenomena and explained by two main hypothesisattributed to Cassie and Wenzel states. During last couple of years, an extensive effort has beenmade to understand these two states experimentally [1-3] as well as computationally [4-5]. It isrealized that the both states are distinguished by free energy barrier. Transition between Cassie andWenzel states can be observed by different approaches such as applying external field [1], verticalvibration [2], heating [3], external force or pressure [4], addition of some solvent [5] etc. In thisstudy, the transition from the Wenzel state to the Cassie state for water droplets of different sizeson a rough graphite surface has been investigated with the help of molecular dynamic simulationsby creating artificial path along a surface-fluid interaction scale. We accurately calculate the freeenergy barrier between these two states by means of thermodynamic integration for differentroughness factor. Free energy barrier is found to be strong function of pillar height as well aspillar gap. We also observe that the transition from Wenzel to Cassie undergoes various localphase transitions along the thermodynamic path.[1] Manukyan, G. ; Oh, J. M. ; Ende, D. V. ; Lammertink, R. G. H. ; Mugele, F. Phys.Rev. Lett. , 106, 014501, 2011[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, 2010.57