Prof Ramin Golestanian

Abstract:

We present a simple method to control the position of ellipsoidal magnetic particles in microchannel Poiseuille flow at low Reynolds number using a static uniform magnetic field. The magnetic field is utilized to pin the particle orientation, and the hydrodynamic interactions between ellipsoids and channel walls allow control of the transverse position of the particles. We employ a far-field hydrodynamic theory and simulations using the boundary element method and Brownian dynamics to show how magnetic particles can be focussed and segregated by size and shape. This is of importance for particle manipulation in lab-on-a-chip devices.

Abstract:

The effect of conformational fluctuations of modular macromolecules, such as enzymes, on their diffusion properties is addressed using a simple generic model of an asymmetric dumbbell made of two hydrodynamically coupled subunits. It is shown that equilibrium fluctuations can lead to an interplay between the internal and the external degrees of freedom and give rise to negative contributions to the overall diffusion coefficient. Considering that this model enzyme explores a mechanochemical cycle, we show how substrate binding and unbinding affects its internal fluctuations, and how this can result in an enhancement of the overall diffusion coefficient of the molecule. These theoretical predictions are successfully confronted with recent measurements of enzyme diffusion in dilute conditions using fluorescence correlation spectroscopy.

Abstract:

Designing microscopic and nanoscopic self-propelled particles and characterising their motion have become a major scientific challenge over the past few decades. To this purpose, phoretic effects, namely propulsion mechanisms relying on local field gradients, have been the focus of many theoretical and experimental studies. In this review, we adopt a tutorial approach to present the basic physical mechanisms at stake in phoretic motion, and describe the different experimental works that led to the fabrication of active particles based on this principle. We also present the collective effects observed in assemblies of interacting active colloids, and the theoretical tools that have been used to describe phoretic and hydrodynamic interactions.