Position-dependent rates of film growth in drying colloidal suspensions on tilted air-water interfaces

Suspended particles in a solvent form a packed film when the solvent evaporates. We investigated film growth rates in a narrow channel on a tilted drying interface and observed clear differences in the rates of film growth. Films grew faster at one end and slower at the other; thus, the slope of the packing front, i.e., the boundary between the packed film and the drying suspension, changed as drying proceeded. However, the difference in film growth rates became smaller as the slope of the packing front changed and the rates of film growth at the either end ultimately became identical. We found that the differences in the rates of film growth were proportional to cosθ, where θ is the angle defined by the slope of the packing front. We constructed a mathematical description to successfully express the time evolution of both the difference in the growth rates and the packing front angle θ. Relationships between drying-induced flow of bulk suspensions and transport of suspended particles to the tilted packing front are discussed.