Impact of concentrated colloidal suspension drops on solid surfaces

When droplets of high concentration wet powder impact on a solid surface, the large stresses that build up upon impact may convert them to a stable system of dry granules. Dilation/jamming has been proposed to explain such powder granulation processes. Stress causes dilation of particles through the droplet surface, against capillary pressure, roughening the surface on the scale of the constituent particles. Under the right conditions of stress magnitude and particle concentration, the droplet jams internally in response to capillary pressure, forming a mechanically stable granule. This remains a tentative model of granulation, which despite its importance in process industries ranging from minerals to foods to detergents, is still imperfectly understood. This work presents the preliminary results of drop impact experiments of a suspension of near hard-core colloidal particles, with the purpose to investigate the impact morphology in the presence of shear thickening or jamming, which may be induced by the large velocity gradients arising upon drop impact. In particular, drops of a suspension of nearly hard-core particles in octadecene (volume fraction: approximately 60%) impacting on substrates of different wettability are studied experimentally by high-speed imaging, for impact Weber numbers ranging between 26 and 262. Upon impact, these drops do not exhibit inertial spreading, which is observed for other Newtonian and non-Newtonian fluid drops. On wettable surfaces (glass), impact is followed by capillary-driven spreading at the same rate observed in power-law fluids (Starov's law), while on less wettable surfaces (PTFE) the colloidal suspension drops relax to achieve the shape of a spherical cap, but do not spread. This peculiar impact morphology, and in particular the absence of inertial spreading, is interpreted as a consequence of dilatancy and jamming occurring upon impact.