Effects of the rate of evaporation and film thickness on nonuniform drying of film-forming concentrated colloidal suspensions.

Narita, T.; Hébraud, P.; Lequeux, F.

In this paper, we report on nonuniform distribution of film-forming waterborne colloidal suspensions above the critical concentration fc of the colloidal glass transition during drying. We found that colloidal suspension films dry nonuniformly when the initial rate of evaporationEand/or the initial thickness l0 are high. We found that a Peclet numberPe, defined asPe=El0/D, whereDis the diffusion coefficient of the colloids in the diluted suspensions, does not predict uniformity of drying of the concentrated suspensions, contrary to the reported work on drying of diluted suspensions. Since the colloidal particles are crowded and their diffusive motion is restricted in concentrated suspensions, we assumed that above fc water is transported to the drying surface by hydrodynamic flow along the osmotic pressure gradient. The permeability of water through channels between deforming particles is estimated by adapting the theory of foam drainage. We defined a new Peclet numberPe’by substituting the transport coefficient of flow (defined as the permeability divided by the viscosity, multiplied by the osmotic pressure gradient) for the diffusion coefficient. This extended Peclet number predicted the nonuniform drying with a criterion ofPe’>1. These results indicate that the mechanism of water transport to the drying surface in concentrated suspensions is water permeation by osmotic pressure, which is faster than mutual diffusion between water and particles --that has been observed in diluted suspensions and discussed by Routh and Russel. The theory fits well the experimental drying curves for various thicknesses and rates of evaporation. The particle distribution in the drying films is also estimated and it is indicated that the latex distribution is nonuniform whenPe’>1.

EVAPORATION (Chemistry); SUSPENSIONS (Chemistry); GLASS transition temperature; COLLOIDS; PROPERTIES of matter

European Physical Journal E -- Soft Matter, 2005, Vol 17, Issue 1, p69

1292-8941

Academic Journal

10.1140/epje/i2004-10109-x