Flow and structure of complex systems

Submerged granular flows

A cylinder partially filled with glass beads and the remainder with a fluid of different viscosities is rotated at different speeds to generate a thin surface flow atop a static rotating bed. The resulting flow of particles is completely immersed in the viscous liquid (like a slurry). The objective of this work is to study the effect of fluid viscosity on the overall flow dynamics of particulate media. Different interstitial fluids having an order of magnitude range of viscosities are employed to attain this objective. Using laser fluorescence and refractive index matching the flow behaviour near the wall and upto 20 particle diameters away from the side walls is observed to elucidate and quantify the wall effects. The observed flow behaviour is compared with quantitative predictions of available continuum models.

A small amount of liquid added to an assembly of particles (e.g. sand) is known to aggolomerate the particles which helps in building sand castles. The agglomeration occurs due to liquid bridges between particles pairs. Such a particle-air-liquid system is encountered in applications like tablet coating, rotary dryers etc. and is known to pose several design/operational issues. We are interested in understanding the flow of such materials under the influence of external forcing, for. e.g. rotating cylinder flow. Using flow visualization techniques and soft particle (DEM) simulations we are trying to elucidate the effect of surface tension, viscosity, volume fraction of added liquid and cylinder rotational speed on the complex flow of such cohesive particles.

Thin film of flowing suspensions (particles immersed in a viscous liquid) are of interest to various industrial applications, viz., paint flow on a substrate, coating of surfaces and natural applications like avalanches, landslides etc. Our interest lies in understanding the flow of a thin film of suspension on a horizontal flat substrate being rotated at very large speeds. Such a system is used in pharmaceutical industries to create drug microparticles of desired sizes. The suspension flowing atop the disc is known to generate intabilities due to competition between centrifugal forces, viscous dissipation and surface tension forces. The overall goal of this work is to understand the correlation between the final droplet size ensuing from the periphery of the rotating plate (which eventually cools to form a microparticle) and the complex thin flow occurring on the rotating plate. We employ flow visualization to study the macroscopic picture of flow as well as microscopic motion of suspended particles flowing within the thin film.

Similar to cohesive flows, addition of a secondary liquid to a suspension of particles in a liquid is known to create agglomerates. It was recently shown that such an agglomeration and the overall strength of the agglomerate is independent of the affinity between particles and secondary added liquid and can happen even at very low volume fractions of particles. We are exploring this behaviour by observing the structure of resulting agglomerate and its dependence on particle and liquid properties using confocal microscopy. Primarily we are interested in observing the percolation threshold at which agglomerates are formed. A detailed understanding of this behaviour can help improve processability of several applications handling suspensions in some form or other.

Suspensions of Laponite particles (size about 20 nm) are known to age over a period of time showing several structural features during the course of aging. In one of the recent works, our collaborator, Prof. Yogesh Joshi and his group from IIT Kanpur, showed that laponite suspension kept in contact with an interface (with air or oil) can show structural arrangements on length scales several orders of magnitude larger than the particle size. More importantly the structural changes are dependent on the fluid at the interface in contact with the suspension. We plan to carry out micro-rheology of such suspensions in a confocal microscope using tracer particles and tracking their motion through laser fluorescence, camera imaging and analysis to elucidate the mechanism of structural ordering