# Current Research

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**Autonomous propulsion at the nanoscale: Osmotic Motors**

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**Bulk Rheology of Suspensions**

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**Granular Flows**

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**Hydrodynamic interactions among colloidal particles in confinement**

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**Single particle motion in colloids: force-induced diffusion**

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**Autonomous propulsion at the nanoscale: Osmotic Motors**

Osmotic propulsion and osmotic nanomotors power the motion of nanoscale and microscale objects by using surface catalytic reactions -- so-called catalytic nanomotors. A surface chemical reaction creates a local concentration gradient of the reactive species which generates a net osmotic force on the motor. The motor is able to harness the ever present random thermal motion via a chemical reaction to achieve directed *autonomous* motion. This research demonstrates that such an 'osmotic' motor is possible and addresses such questions as: How fast can the motor move? How large of a force can it generate? How much 'cargo' can it carry? How much fluid can it pump? How can its motion be controlled and directed? What is the efficiency of such an osmotic motor?

**Bulk Rheology of Suspensions**

Many suspension flows can lead to variations in particle volume fraction, thus making the particle phase compressible on a macroscopic scale. The stress in such a flow is characterized by an effective bulk viscosity in addition to the effective shear viscosity of the suspension. We have derived explicit expressions to compute the bulk viscosity for all volume fractions of suspended rigid particles and for all expansion rates. Using Stokesian Dynamics simulations, we can compute the bulk viscosity in various flow situations for all concentrations and incorporate it into macroscopic models of two-phase flows.

**Granular Flows**

Colloidal dispersions and granular matter represent two extremes in particulate flows---viscous effects dominate in the former, particle inertia in the latter. The two extremes can be connected via the Stokes number---a dimensionless number describing the competition between viscous and particle inertia forces. In this research, we investigate rheological implications of the Stokes number, with particular interest in understanding how Stokes number effects might explain quantitative differences between segregation in dry and wet granular flows.

**Hydrodynamic interactions among colloidal particles in confinement**

This research focuses on the modeling of hydrodynamic interactions among colloidal particles in confinement, the development of fast numerical algorithms for simulating the dynamics of colloidal dispersions in confined geometries and the application and analysis of light scattering techniques to measure the anisotropic diffusivities of confined suspensions.

**Single particle motion in colloids: force-induced diffusion**

This work in the area of active, non-linear microrheology began with development of the theory that predicts particle fluctuations in such systems; we used these results to develop analytical expressions for the diffusive motion of an externally forced probe driven through a colloidal dispersion, the force-induced microdiffusivity. This anisotropic tensor may be related to normal stress difference, and this is the subject of future investigation, along with the effects of hydrodynamic interactions.