Drying-induced back flow of colloidal suspensions confined in thin unidirectional drying cells

A clear back flow was observed in the thin unidirectional drying cell of a colloidal suspension. Flow around the colloidal-particle packing front was more complex than expected, even though a colloidal suspension was confined in a narrow space with a submillimeter-scale or shorter gap height. We propose that an increase in particle concentration around the packing front induces downward flow, which is the origin for back flow inside the cell. A mathematical model, which considered both a drying induced horizontal flow and a circulation flow caused by a concentration gradient of particles, showed a reasonable agreement with experimental data for the width of the back-flow region. The concentration gradient of particles was not negligible and it generated a rather complicated flow even in a thin drying liquid film.