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

P6.8Fri 911:10-14:00Roughness-enhanced acceleration of spreading ofcompletely wetting fluidsJolet de Ruiter, 1 Dirk van den Ende, 1 and Frieder Mugele 11 Twente University, PO Box 217, 7500 AE, Enschede, NetherlandsWe study the spreading dynamics of simple fluids on surfaces of variable complexity. Thecomplexity is tuned by preparing substrates with a variable density of silica microspheres inthe range of 2 to 42% surface coverage. Spreading of the completely wetting, picoliter-volume,droplet follows a time-dependent power law, R ∼ t n(t) . The spreading exponent n changesdramatically with the changing morphology of the spreading drop: the spherical cap drop (I)develops a foot ahead of the spreading reservoir (II). Subsequently the reservoir starts retracting(III) until a spreading film remains (IV) with thickness in the order of the substrate roughness.The initial spreading in the spherical cap regime is enhanced by substrate roughness similar tothe spreading of a droplet in the so-called Wenzel state on a micropost array [1]. The spreadingexponent in this regime increases with roughness from n = 1/10 (Tanner’s law) to a maximum of= 0.21 . In the film-spreading regime, we do not observe the imbibition law (n = 1/2) expectedfor ordered micropost arrays [2,3]. It is visually confirmed that the fluid does not imbibe the openspaces, but rather wets the complex surface structure in two steps: from the macroscopic wettingfront a thin film extents that nucleates the formation of a meniscus at a nearby particle. Theparticle is subsequently engulfed by the front. We proceed to study the microscopic wetting details.[1] G. McHale, M. I. Newton, N. J. Shirtcliffe, J Phys Condens Matter 21, 464122 (2009).[2] J. Bico, C. Tordeux, D. Quere, Europhys. Lett. 55, 214-220 (2001).[3] L. Courbin, E. Denieul, E. Dressaire, M. Roper, A. Ajdari, H. A. Stone, Nature Materials 6,661-664 (2007).8