442 research outputs found

### Reduced Persistence Length and Fluctuation-Induced Interactions of Directed Semiflexible Polymers on Fluctuating surfaces

We consider directed semiflexible polymers embedded in a fluctuating surface
which is governed by either surface tension or bending rigidity. The attractive
interactions induced by the fluctuations of the surface reduce the rigidity of
the polymers. In particular, it is shown that for arbitrarily stiff parallel
polymers, there is a characteristic separation below which they prefer to bend
rather than stay linear. The out-of plane fluctuations of the polymer, screen
out the long-range fluctuation-induced forces, resulting in only a short-ranged
effective attraction.Comment: REVTEX, one postscript figur

### Distribution of Interacting Ionic Particles in Disordered Media

Equilibrium distribution of interacting ionic particles in a charged
disordered background is studied using the nonlinear Poisson-Boltzmann
equation. For an arbitrarily given realization of the disorder, an exact
solution of the equation is obtained in one dimension using a mapping of the
nonlinear Poisson-Boltzmann equation to a self-consistent Schrodinger equation.
The resulting density profile shows that the ions are delocalized, despite what
the equivalent Schrodinger formulation in one dimension would suggest. It is
shown that the ions are not distributed so as to locally neutralize the
background, presumably due to their mutual interactions

### Emergent Run-and-Tumble Behavior in a Simple Model of Chlamydomonas with Intrinsic Noise

Recent experiments on the green alga Chlamydomonas that swims using
synchronized beating of a pair of flagella have revealed that it exhibits a
run-and-tumble behavior similar to that of bacteria such as E. Coli. Using a
simple purely hydrodynamic model that incorporates a stroke cycle and an
intrinsic Gaussian white noise, we show that a stochastic run-and-tumble
behavior could emerge, due to the nonlinearity of the combined
synchronization-rotation-translation dynamics. This suggests the intriguing
possibility that the alga might exploit nonlinear mechanics---as opposed to
sophisticated biochemical circuitry as used by bacteria---to control its
behavior.Comment: 5 pages, 2 composite figures (made of 12 separate EPS files

### Anomalous Diffusion of Symmetric and Asymmetric Active Colloids

The stochastic dynamics of colloidal particles with surface activityâ€”in the form of catalytic reaction or particle releaseâ€”and self-phoretic effects are studied analytically. Three different time scales corresponding to inertial effects, solute redistribution, and rotational diffusion are identified and shown to lead to a plethora of different regimes involving inertial, propulsive, anomalous, and diffusive behaviors. For symmetric active colloids, a regime is found where the mean-squared displacement has a superdiffusive
t
3
/
2
behavior. At the longest time scales, an effective diffusion coefficient is found which has a nonmonotonic dependence on the size of the colloid

### Fluctuation-Induced Interactions between Rods on Membranes and Interfaces

We consider the interaction between two rods embedded in a fluctuating
surface which is governed by either surface tension or rigidity. The
modification of fluctuations by the rods leads to an attractive long-range
interaction that falls off as $1/R^4$ with their separation. The orientational
dependence of the resulting interaction is non-trivial and may lead to
interesting patterns of rod-like objects on such surfaces.Comment: Revtex, 10 pages, one figur

### Casimir Dispersion Forces and Orientational Pairwise Additivity

A path integral formulation is used to study the fluctuation-induced
interactions between manifolds of arbitrary shape at large separations. It is
shown that the form of the interactions crucially depends on the choice of the
boundary condition. In particular, whether or not the Casimir interaction is
pairwise additive is shown to depend on whether the ``metallic'' boundary
condition corresponds to a ``grounded'' or an ``isolated'' manifold.Comment: 6 pages, RevTe

### Coherent Hydrodynamic Coupling for Stochastic Swimmers

A recently developed theory of stochastic swimming is used to study the
notion of coherence in active systems that couple via hydrodynamic
interactions. It is shown that correlations between various modes of
deformation in stochastic systems play the same role as the relative internal
phase in deterministic systems. An example is presented where a simple swimmer
can use these correlations to hunt a non-swimmer by forming a hydrodynamic
bound state of tunable velocity and equilibrium separation. These results
highlight the significance of coherence in the collective behavior of
nano-scale stochastic swimmers.Comment: 6 pages, 3 figure

### Path Integral Approach to the Dynamic Casimir Effect with Fluctuating Boundaries

A path integral formulation is developed for the dynamic Casimir effect. It allows us to study arbitrary deformations in space and time of the perfectly reflecting (conducting) boundaries of a cavity. The mechanical response of the intervening vacuum is calculated to linear order in the frequency-wavevector plane, using which a plethora of interesting phenomena can be studied. For a single corrugated plate we find a correction to mass at low frequencies, and an effective shear viscosity at high frequencies that are both anisotropic. The anisotropy is set by the wavevector of the corrugation. For two plates, the mass renormalization is modified by a function of the ratio between the separation of the plates and the wave-length of corrugations. The dissipation rate is not modified for frequencies below the lowest optical mode of the cavity, and there is a resonant dissipation for all frequencies greater than that. In this regime, a divergence in the response function implies that such high frequency deformation modes of the cavity can not be excited by any macroscopic external forces. This phenomenon is intimately related to resonant particle creation. For particular examples of two corrugated plates that are stationary, or moving uniformly in the lateral directions, Josephson-like effects are observed. For capillary waves on the surface of mercury a renormalization to surface tension, and sound velocity is obtained

### Small object limit of Casimir effect and the sign of the Casimir force

We show a simple way of deriving the Casimir Polder interaction, present some
general arguments on the finiteness and sign of mutual Casimir interactions and
finally we derive a simple expression for Casimir radiation from small
accelerated objects.Comment: 13 pages, late

### Phase Dependent Forcing and Synchronization in the three-sphere model of Chlamydomonas

The green alga {\it Chlamydomonas} swims with synchronized beating of its two
flagella, and is experimentally observed to exhibit run-and-tumble behaviour
similar to bacteria. Recently we studied a simple hydrodynamic three-sphere
model of {\it Chlamydomonas} with a phase dependent driving force which can
produce run-and-tumble behaviour when intrinsic noise is added, due to the
non-linear mechanics of the system. Here, we consider the noiseless case and
explore numerically the parameter space in the driving force profiles, which
determine whether or not the synchronized state evolves from a given initial
condition, as well as the stability of the synchronized state. We find that
phase dependent forcing, or a beat pattern, is necessary for stable
synchronization in the geometry we work with.Comment: 17 pages, 10 composite figures (made of 32 separate files

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