Pigment Motion During Drying, 1991 Forest Products Symposium Proceedings

A modified Stokesian dynamics technique is used to describe the motion of spherical particles during the dewatering of suspension on a planar and sinusoidal substrate. Water leaves through evaporation and through one boundary of the flow domain. Particle-particle and particle-boundary hydrodynamic interactions, electrostatic forces, and surface tension forces are accounted for in the model. Mechanical interactions between particles are assumed when the sphere surfaces approach within a given particle “surface roughness”. The final microstructure of the pigments is obtained from the simulation.

The motion of a sphere past another sphere attached to a vertical boundary is described by the model. The sphere

trajectories predicted by the model are within 10% of experimental results. Mechanical interactions are needed to fully describe the sphere dynamics.

The final microstructure of the spherical particles is probed for different dewatering or evaporation rates. The model predicts the formation of a filtercake as liquid leaves through a boundary. Small drying rates produce a compact structure. The coverage of a sinusoidal boundary is described by the model. In-plane motion of the particles is small for a wide range of parameters. High solids volume fractions, low evaporation rates, and high electrostatic repulsive forces produce smooth coatings.