Velocity of lateral drying fronts in film formation by drying of colloidal dispersions. A 2D simulation.

Drying of colloids is always heterogeneous and proceeds by progression of drying fronts in various directions at various velocities. The fundamental mechanisms at the origin of appearance and motion of drying fronts are still not totally understood. This article addresses these questions in the case of lateral drying fronts by using the new simulation tool based on cellular automata we recently developed (Langmuir 2015 and 2017). For the first time, a 2D simulation is proposed. Silica dispersions were used as model colloids to test the simulation. Film profiles were measured during drying by optical profilometry as well as front velocities by image processing. In the cases of non-circular deposits (squares and rectangles), drying fronts in the plane of the film (x,y plane, x being the longest side in the case of a rectangle) do not move at the same speed along sides and diagonals, the velocity order being diagonal >x (longest side) >y (shortest side). The velocity contrast (difference between x and y sides) increases with the aspect ratio of the rectangle. This behavior is explained and accounted for by the 2D simulation presented in this article. Experimental results reasonably well validate the simulation.