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Register a new userNon-conservative forces and effective temperatures in active polymers
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We follow the dynamics of an ensemble of interacting self-propelled semi-flexible polymers in contact with a thermal bath. We characterize structure and dynamics of the passive system and as a function of the motor activity. We find that the fluctuation-dissipation relation allows for the definition of an effective temperature that is compatible with the results obtained by using a tracer particle as a thermometer. The effective temperature takes a higher value than the temperature of the bath when the effect of the motors is not correlated with the structural rearrangements they induce. Our data are compatible with a dependence upon the square of the motor strength (normalized by the average internal force) and they suggest an intriguing linear dependence on the tracer diffusion constant times the density of the embedding matrix. We show how to use this concept to rationalize experimental results and suggest possible innovative research directions.
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We use molecular dynamics simulations to study the dynamics of an ensemble of interacting self-propelled semi-flexible polymers in contact with a thermal bath. Our intention is to model complex systems of biological interest. We find that an effective temperature allows one to rationalize the out of equilibrium dynamics of the system. This parameter is measured in several independent ways -- from fluctuation-dissipation relations and by using tracer particles -- and they all yield equivalent results. The effective temperature takes a higher value than the temperature of the bath when the effect of the motors is not correlated with the structural rearrangements they induce. We show how to use this concept to interpret experimental results and suggest possible innovative research directions.
To develop a minimal model for a cell moving in a crowded environment such as in tissue, we investigate the response of a liquid drop of active matter moving on a flat rigid substrate to forces applied at its boundaries. Our model incorporates active stresses due to a prescribed orientation profile of the cytoskeleton, coupling with the substrate, surface tension and imposed boundary forces. We find a highly non-linear response to forces that we characterise using the drop velocity, its shape, and the traction between the drop and the substrate. There are two main modes of motion: a long and thin drop with zero traction in the bulk, mostly occurring under strong stretching forces, and a parabolic drop with finite traction in the bulk, mostly occurring under strong squeezing forces. There is a sharp transition between these two modes as a function of the applied forces and indications of drop break-up where large forces stretch the drop in opposite directions.
We study numerically the phases and dynamics of a dense collection of self-propelled particles with soft repulsive interactions in two dimensions. The model is motivated by recent in vitro experiments on confluent monolayers of migratory epithelial and endothelial cells. The phase diagram exhibits a liquid phase with giant number fluctuations at low packing fraction and high self-propulsion speed and a jammed phase at high packing fraction and low self-propulsion speed. The dynamics of the jammed phase is controlled by the low frequency modes of the jammed packing.
Recently, Huang, Wu and Florin posted a Comment (0806.4632v1) on our preprint (0804.0730v1) describing nonequilibrium circulation of a colloidal sphere trapped in a optical tweezer. The Comment suggests that evidence for toroidal probability currents obtained from experiments and simulations in the original posting should be considered inconclusive. The authors concerns are based on two claims: (1) that Brownian dynamics simulations of the trapped particles motions reveal no statistically significant circulation, and (2) that a realistic description of the radiation pressure acting on the trapped sphere is inconsistent with the motion we have described. In this Reply, we demonstrate both of these claims to be incorrect, and thus the original results and conclusions in 0804.0730v1 to be still valid.
Using monomer-resolved Molecular Dynamics simulations and theoretical arguments based on the radial dependence of the osmotic pressure in the interior of a star, we systematically investigate the effective interactions between hard, colloidal particles and star polymers in a good solvent. The relevant parameters are the size ratio q between the stars and the colloids, as well as the number of polymeric arms f (functionality) attached to the common center of the star. By covering a wide range of qs ranging from zero (star against a flat wall) up to about 0.75, we establish analytical forms for the star-colloid interaction which are in excellent agreement with simulation results. A modified expression for the star-star interaction for low functionalities, f < 10 is also introduced.
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