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

Thu 811:10-14:00P7.33Morphological transition of water droplets confined onrectangular postsDavide Ferraro, 1 Tamara Tòth, 1 Matteo Pierno, 1 Giampaolo Mistura, 1 CiroSemprebon, 1 and Martin Brinkmann 21 Dipartimento di Fisica - Università di Padova, Via Marzolo, 8, 35131, Padova,Italy2 Max Plank Istitute for Dynamic and Self-Organization, Gottingen, GermanyWe present the results of a comprehensive study of the bulging transition undergone by waterdrops confined on the hydrophilic top face of rectangular posts. An analogous transition has beenobserved for a similar system of growing water droplets condensing on hydrophilic stripes [1],however the impact of the stripe geometry was not considered. By controlling the liquid volumewe were able to detect the onset of the discontinuous and hysteretic transition as the length of thepost is increased. Numerical calculations show either a continuous or discontinuous growth ofthe height as the drop volume is increased which is controlled by the length to width ratio of thepost and the absolute size of the post. This behavior has been confirmed by direct measurementsof water drop profiles on submillimetric rectangular posts. To facilitate the spreading of the dropand enhance its confinement on the top face, we have realized rectangular posts that present alarge wettability contrast between the highly hydrophilic top surface and the highly hydrophobicvertical walls. The various posts have all the same width but different length. They have beenrealized from a master in SU8 produced by photolithographic techniques. Through a doublereplica molding process, a positive copy of the master has been obtained in PDMS, an hydrophobicpolymer. The top face has been chemically functionalized to acquire an hydrophilic character.Before this treatment, the vertical walls of the microstructures have been suitably masked. Knownvolumes of pure water have been pumped from a central hole made in the post and the shape ofthe resulting drops have been imagined along three different orthogonal directions using a custommade optical set-up [2,3]. The experimental profiles are found to be in very good agreement withthe simulated ones.[1] Gau et al. Science, 283, 46-49 (1999)[2] C. Semprebon et al. Langmuir 25, 5619-5625 (2009)[3] T. Toth et al. Langmuir 27, 4742-4748 (2011)33