Stratification in Drying Colloidal Films: A competition between diffusion, evaporation and diffusiophoresis*

Waterborne dispersions are an environmentally friendly formulation for coatings. There are a number of issues associated with drying such coatings such as reduced open time and crack formation. Blending dispersions with hard and soft components has been shown to allow better film formation; however, stratification of components can lead to regions of cracking. Alternatively stratification can be beneficial and used to place different components at desired regions within the film. This could be used for expensive anti-fungal agents in bathroom coatings or catalysts in the walls of tubular reactors.

To understand stratification in drying coatings, one dimensional drying of bi-disperse mixtures has recently been the subject of intense theoretical and experimental study. The experimental observation is of accumulation of smaller particles at the top surface of drying films. The physics driving such stratification is still being debated and the competing theories of cross interactions, diffusiophoresis and a surface interaction will be explored. We have written transport equations for an evaporating film containing a bi-disperse mixture of particles and examine the contribution from different terms.

The resulting film evolution equations allow for incorporation of arbitrary interactions between particles. We investigate the effect of different DLVO potentials and compare our predictions with experimental measurement