Uniform Coatings from Colloidal Suspensions

Wednesday, July 31, 2013 - 12:00pm - 12:50pm
Keller 3-180
Satish Kumar (University of Minnesota, Twin Cities)
Coatings made from colloidal suspensions play a central role
in a broad range of applications such as zeolite-based separation membranes, templates for photonic materials,
printed electronics, and biomedical engineering. Uniform coatings are often desired, but these can be difficult to achieve
due to various phenomena such as the well-known coffee-ring effect. We are interested in developing a fundamental understanding
of the factors that determine whether a coating is uniform, and in this talk I will discuss
our work on two problems concerning this issue. The first problem involves development of a mathematical model for the drying
of a droplet laden with colloidal particles. In contrast to prior work, our model accounts for depthwise gradients in particle
concentration, and can thus qualitatively capture the formation of colloidal skins that are observed experimentally. The
second problem involves an experimental study of the dip coating of nanoparticle suspensions. We demonstrate that there is a
sweet spot in the parameter space of solids concentration and withdrawal speed where monolayer coatings can be achieved.
Analysis of the experimental data suggests that in order to obtain such coatings, the substrate needs to be withdrawn rapidly
enough to overcome pinning of the liquid-air meniscus on the particles, but not so rapidly that a continuous liquid film is entrained.