Condensed Matter Theory

Abstract:

We consider the tracer diffusion Drr that arises from the run-and-tumble motion of low Reynolds number swimmers, such as bacteria. Assuming a dilute suspension, where the bacteria move in uncorrelated runs of length λ, we obtain an exact expression for Drr for dipolar swimmers in three dimensions, hence explaining the surprising result that this is independent of λ. We compare Drr to the contribution to tracer diffusion from entrainment. © 2013 American Physical Society.

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.

Abstract:

Continually moving cilia on the surface of marine organisms provide a natural defense against biofouling. To probe the physical mechanisms underlying this antifouling behavior, we integrate the lattice Boltzmann and immersed boundary methods and undertake the first computational studies of the interactions between actuated, biomimetic cilia and a model swimmer. We find that swimmers are effectively "knocked away" from the ciliated surface through a combination of steric repulsion and locally fluctuating flows. In addition, the net flow generated by the collective motion of the entire ciliary array was important for significantly reducing the times spent by relatively slow swimmers near the surface. The results reveal that active ciliated layers can offer a means to resist a wide range of species with a single surface. © 2013 American Chemical Society.