(461c) Convection and Capillarity Induced Pattern Formation in the Spreading of a Concentrated Suspension of Rigid Spheres over a Liquid-Air Interface

Spreading phenomena play an important role in several natural and industrial processes, and hence, several theoretical and experimental studies have focused on developing a deeper understanding of the physics of spreading. This paper focuses on the spreading of fluids with rigid spherical inclusions. Past literature in this area can be broadly categorized on the basis of three variables: substrate fluidity, the volatility of the continuous phase of the suspension, and the volume fraction of particles in the mixture. For example, studies of non-volatile single-phase liquids on air-solid interfaces lie in the regime of zero substrate fluidity, zero particle volume fraction and zero volatility. Investigations of the coffee ring effect on rigid surfaces span the regime of substrates with zero fluidity, dilute suspensions and significant volatility. Our work focuses on the extremely simple limit of high substrate fluidity, moderate particle volume fractions (10% to 40%) and zero volatility. We have examined the spreading of a drop of a concentrated suspension of PMMA particles in silicone oil, a non-volatile suspending medium, on an air – glycerol interface.

During the initial period of spreading, the behavior of a suspension is similar to that of a single-phase fluid. The spreading front remains circular, and the area occupied by the spreading suspension is found to be a linear function of time, the latter being consistent with a scaling analysis for the spreading of a Newtonian fluid. As the front approaches a critical radius, the particles are found to be in a monolayer configuration near the advancing suspension front, and the particle area fraction in the monolayer is spatially non-uniform. Beyond the critical radius, the spreading front bursts, ejecting small particle chains that are swept away by the spreading front. After this initial burst, two different outcomes are observed. The spreading suspension either disintegrates completely into particle fragments, or reconfigures into a two-dimensional network of particles. These observations will be explained in the presentation by invoking a combination of convection and capillarity induced particle motion. The effects of particle size, particle volume fraction, drop volume and substrate viscosity on the details and outcome of the spreading process will also be elucidated.