An experimental test of the scaling prediction for the spatial distribution of water during the drying of colloidal films

A Peclet number, Pe, for the drying of colloidal. films can be used as a predictor of the uniformity of water concentration in the direction normal to the. film. Uniform drying is predicted to occur when Pe < 1, whereas with Pe > 1, a layer of packed particles is expected to develop above a more dilute layer. Routh and Zimmerman have more recently proposed that the particle concentration gradient between the packed and dilute layers, d phi(p)/dz, will scale as Pe(1/2). Here, this scaling relation is tested experimentally with magnetic resonance pro. ling data obtained from waterborne colloidal. films dried under conditions to yield a range of Pe. It is found that d phi(p)/dz increases with Pe but scales as Pe (0.8). This disagreement with the prediction can be attributed to an underestimate of Pe when there are greater non-uniformities of drying, because of an unquantified slowing down of the evaporation rate.