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

Thu 811:10-14:00P7.59Computing pressure tensor profile of an impingingdroplet by molecular dynamicsTakahiro Koishi, 1 Kenji Yasuoka, 2 Shgenori Fujikawa, 3 and Xiao C. Zeng 41 University of Fukui, Bunkyou 3-9-1, 910-8507, Fukui, Japan2 Keio University, Yokohama, Japan3 RIKEN, Saitama, Japan4 University of Nebraska, Nebraska, United States of AmericaSuperhydrophobic surfaces can be achieved by microtextured surface patterning [1]. A dropleton the textured surface can be in either the Wenzel state in which the droplet is in full contactwith the surface or in the Cassie state in which the droplet is in contact with top of the texturedsurface and the air pockets trapped between surface grooves. We recently estimated the freeenergy barrier separating the Wenzel and Cassie states by molecular dynamics (MD) simulationsof impinging water droplet on nano-pillared surfaces [2]. An experimental study of impingingdroplets on superhydrophobic textured surfaces has been recently reported by Deng et al. [3]. Theeffective water hammer pressure was introduced by the researchers to explain their observationof three droplet states: wetting, partial wetting, and nonwetting. The droplet impingingmentprocess can be divided into the contact stage and the spreading stage. In the contact stage, theinitial impact of the droplet onto the surface gives rise to a water hammer pressure. The effectivewater hammer pressure is due to the water hammer pressure at the spreading stage on the texturedsurface. Motivated by Deng et al. ’s experiment, we have performed MD simulations to compute amicroscopic property of impinging droplet on nano-pillared graphite surfaces. Three componentsof the local pressure, P x , P y and P z , are calculated to estimate the pressure profile of the impingingdroplet. We find a sharp peak in the time dependence of P z , the z-component of the pressuretensor at the lower part of the impinging droplet. It corresponds to the water hammer pressure inthe contact stage of the impingingment. The average of P x and P y becomes greater than P z at thespreading stage.[1] A. Lafuma and D. Quere, Nature Materials, 2, 457-460 (2003)[2] T. Koishi et al. , Proc. Natl. Acad. Sci. USA, 106, 8435-8440 (2009)[3] T. Deng et al. , App. Phys. Lett. , 94, 133109 1-3 (2009)59