Prof Ramin Golestanian

Abstract:

The effect of crowding on the run-and-tumble dynamics of swimmers such as bacteria is studied using a discrete lattice model of mutually excluding particles that move with constant velocity along a direction that is randomized at a rate α. In stationary state, the system is found to break into dense clusters in which particles are trapped or stopped from moving. The characteristic size of these clusters predominantly scales as α-0.5 in both one and two dimensions. For a range of densities, due to cooperative effects, the stopping time scales as T1d0.85 and as T2d0.8, where Td is the diffusive time associated with the motion of cluster boundaries. Our findings might be helpful in understanding the early stages of biofilm formation. © 2014 American Physical Society.

Abstract:

We study a continuum model of an extensile active nematic to show that mesoscale turbulence develops in two stages: i) ordered regions undergo an intrinsic hydrodynamic instability generating walls, lines of strong bend deformations; ii) the walls relax by forming oppositely charged pairs of defects. Both creation and annihilation of defect pairs reinstate nematic regions which undergo further instabilities, leading to a dynamic steady state. We compare this with the development of active turbulence in a contractile active nematic. © Copyright EPLA, 2013.

Abstract:

We present a number of arguments to demonstrate that a quantum analog of the Cherenkov effect occurs when two nondispersive half spaces are in relative motion. We show that they experience friction beyond a threshold velocity which, in their center-of-mass frame, is the phase speed of light within their medium, and the loss in mechanical energy is radiated through the medium before getting fully absorbed in the form of heat. By deriving various correlation functions inside and outside the two half spaces, we explicitly compute this radiation and discuss its dependence on the reference frame. © 2013 American Physical Society.