Prof Ramin Golestanian

Abstract:

We present a simple two-state model to understand the size-dependent endocytosis of nanoparticles. Using this model, we elucidate the relevant energy terms required to understand the size-dependent uptake mechanism and verify it by correctly predicting the behavior at large and small particle sizes. In the absence of interactions between the nanoparticles, we observe an asymmetric distribution of sizes with maximum uptake at intermediate sizes and a minimum size cut-off below which there can be no endocytosis. Including the effect of interactions in our model has remarkable effects on the uptake characteristics. Attractive interactions shift the minimum size cut-off and increase the optimal uptake while repulsive interactions make the distribution more symmetric lowering the optimal uptake. © 2011 IOP Publishing Ltd.

Abstract:

We present a simple two-state model to understand the size-dependent endocytosis of nanoparticles. Using this model, we elucidate the relevant energy terms required to understand the size-dependent uptake mechanism and verify it by correctly predicting the behavior at large and small particle sizes. In the absence of interactions between the nanoparticles, we observe an asymmetric distribution of sizes with maximum uptake at intermediate sizes and a minimum size cut-off below which there can be no endocytosis. Including the effect of interactions in our model has remarkable effects on the uptake characteristics. Attractive interactions shift the minimum size cut-off and increase the optimal uptake while repulsive interactions make the distribution more symmetric lowering the optimal uptake.

Abstract:

After a long gap following the classic work of Taylor, there have recently been several studies dealing with hydrodynamic synchronization. It is now apparent that synchronization driven by hydrodynamic interactions is not only possible, but relevant to the efficiency of pumping by arrays of cilia and to bacterial swimming. Recent work has included experiments demonstrating synchronization, both in model systems and between bacterial flagella. The effect has been demonstrated in model swimmers and pumps, and large scale simulations have been used to investigate synchronization of cilia and of sperm cells. In this review article, we summarize the various experimental and theoretical studies of hydrodynamic synchronization, and put them in a framework which draws parallels between the different systems and suggests useful directions for further research. © The Royal Society of Chemistry 2011.

Abstract:

Synchronization of actively oscillating organelles such as cilia and flagella facilitates self-propulsion of cells and pumping fluid in low Reynolds number environments. To understand the key mechanism behind synchronization induced by hydrodynamic interaction, we study a model of rigid-body rotors making fixed trajectories of arbitrary shape under driving forces that are arbitrary functions of the phase. For a wide class of geometries, we obtain the necessary and sufficient conditions for synchronization of a pair of rotors. We also find a novel synchronized pattern with an oscillating phase shift. Our results shed light on the role of hydrodynamic interactions in biological systems, and could help in developing efficient mixing and transport strategies in microfluidic devices.

Abstract:

Using particle-tracking techniques, the translational and rotational diffusion of paralyzed E. coli with and without flagella are studied experimentally. The position and orientation of the bacteria are tracked in the lab frame and their corresponding mean-square displacements are analyzed in the lab frame and in the body frame to extract the intrinsic anisotropic translational diffusion coefficients as well as the rotational diffusion coefficient for both strains. The deflagellated strain is found to show an anisotropic translational diffusion, with diffusion coefficients that are compatible with theoretical estimates based on its measured geometrical features. The corresponding translational diffusion coefficients of the flagellated strain have been found to be reduced as compared to those of the deflagellated counterpart. Similar results have also been found for the rotational diffusion coefficients of the two strains. Our results suggest that the presence of flagella --even as a passive component-- has a significant role in the dynamics of E. coli, and should be taken into account in theoretical studies of its motion.