Colloidal cluster formation using evaporating droplets on superhydrophobic substrates

The formation of particle conglomerates is a crucial step in the food industry (e.g. milk powder) or in the pharmaceutical industry to extract active agents as penicillin, enzymes, etc, which is typically done using spray drying processes. Using superhydrophobic substrates, contact lines are free to move and droplets maintain an almost spherical shape during most of their lifetime, mimicking the drying process in an aerosol in a more controlled way. This feature has been exploited to fabricate controllable three-dimensional colloidal structures (1,2), which are formed due to the evaporation-driven assembly of the colloidal particles. Different colloidal structures can be formed: from flattened structures generated due to the eventual failure of the superhydrophobic state, to almost perfect spherical macro-colloidal structures, passing through pseudo-spherical structures suffering from buckling instabilities (2,3). Surprisingly, the final structure's shape depends strongly on the colloidal particle concentration and on the contact line motion, and only marginally on the droplet's evaporation rate.

In this work, we will explore the role of both the type of colloidal particles and the superhydrophobic substrate's microstructure features on the final colloidal structure and on the contact line motion, as well as their relationship.

(1) Marin, A et al. (2012) PNAS 109(41): 16455-16458.

(2) Seyfert, C. et al. (2021). Soft matter, 17(3), 506-515.

(3) Tsapis, N.et al. (2005). PRL 94(1), 018302.