Prerna Sharma

A high-precision study of hindered diffusion near a wall

A high-precision study of hindered diffusion of a sphere due to its proximity to a solid interface was performed using an optical tweezer combined with digital holography microscopy and a phase-sensitive detection technique. The study provides a confirmation, with high accuracy and no adjustable parameters, of Faxen’s law which describes the variation of the diffusivity of a sphere as a function of its distance from the wall in a Newtonian liquid due to hydrodynamic effects. This general technique is useful for application in microfluidics and lubrication of small devices with moving parts.

Three-dimensional real-time imaging of bi-phasic flow through porous media.

We present a scanning laser-sheet video imaging technique to image bi-phasic flow in three-dimensional porous media in real time with pore-scale spatial resolution, i.e., 35 μm and 500 μm for directions parallel and perpendicular to the flow, respectively. The technique is illustrated for the case of viscous fingering. Using suitable image processing protocols, both the morphology and the movement of the two-fluid interface, were quantitatively estimated. Furthermore, a macroscopic parameter such as the displacement efficiency obtained from a microscopic (pore-scale) analysis demonstrates the versatility and usefulness of the method.

A Nyquist analysis of glassy dynamics, aging, and discrete basins of attraction in a small system

The process of attachment of a deformable polystyrene colloidal particle to a planar substrate shows features of slow relaxation and aging, typically observed in many-body disordered and glassy systems. This slow dynamics is attributed to the presence of polymeric tethers constituting a few internal degrees of freedom. The effects of these few degrees of freedom are revealed by a Nyquist analysis of the data that demonstrate the existence of only a few discernible basins of attraction as the system reaches the ground state and thus provides a minimal model of complexity and glassy dynamics.

Fragmentation of viscous fingers in porous media: A three dimensional video microscopic study

A compact viscous finger in a porous medium destabilizes to form a droplet phase at large flow rates. We present a three dimensional video imaging to study this destabilization phenomenon. The images show that a system-spanning finger is a connected network of a large number of pore-scale fingers which fragment at high flow rates to destabilize the network. The volume occupancy of the droplet distribution in the steady state, arising out of events of break-up and coalescence in the pore scale, shows a non-monotonic behavior, naturally separating sub-pore scale droplets from those which span many pores. An analysis of the motion of the interface between the displacing and the displaced fluids provides evidence that the frequency of the pore scale break-up events increases with the flow rate. For low flow rates, these events are mainly localized near the leading edge of the front. However, for higher flow rates, the pore-scale break-up events extend well-behind the leading edge of the front which prevents the growth of a compact finger and instead promotes the formation of a droplet phase.