Berlin 2012 – scientific programme
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DY: Fachverband Dynamik und Statistische Physik
DY 29: Posters II
DY 29.31: Poster
Thursday, March 29, 2012, 17:00–19:00, Poster A
Drying fronts in colloidal films — Joaquim Li1, Bernard Cabane1, •Jan S. Vesaratchanon2, Michael Sztucki3, Jeremie Gummel3, and Lucas Goehring2 — 1PMMH, ESPCI, Paris, France — 2MPI for Dynamics and Self-Organization, Göttingen, Germany — 3ESRF, Grenoble, France
The drying of a colloidal film involves multiple transport processes, with the film properties changing dramatically as drying proceeds. Many such films are applied as a liquid dispersion, begin to dry from the edges inward (directional drying), and change into a porous solid as the result of evaporation. In two extreme situations the film may either dry into a flat, homogeneous layer, or may dry via the iconic 'coffee-ring' effect, where nearly all solid material is deposited at the edge of the film. However, the conditions that discriminate between these two limits are not understood, nor is the general physics controlling dispersant/particle transport during drying. We have studied this problem via Small Angle X-ray/Neutron Scattering, on directionally dried films of colloidal silica, whereby we measure the ordering of particles, their volume fraction, the film thickness and the water content simultaneously. We find that far from an edge, where a flat film develops, all material transport occurs in a thin transition region, of finite width, that propagates ahead of the drying front. In this region, like a polarization wave in filtration experiments, the gradient of osmotic pressure balances the drag force exerted on the particles by capillary flow toward the liquid-solid front. The growth or decay of such a region may lead to uneven deposits, such as ridges near the film edges.