Packing defects and the width of biopolymer bundles

The formation of bundles composed of actin filaments and cross-linking proteins is an essential process in the maintenance of the cells' cytoskeleton. It has also been recreated by in-vitro experiments, where actin networks are routinely produced to mimic and study the cellular structures. It has long been observed that these bundles seem to have a well defined width distribution, which has not been adequately described theoretically. We propose here that packing defects of the filaments, quenched and random, contribute an effective repulsion that counters the cross-linking adhesion energy and leads to a well defined bundle width. This is a two-dimensional strain-field version of the classic Rayleigh instability of charged droplets

Signatures of motor susceptibility in the dynamics of a tracer particle in an active gel

We study a model for the motion of a tracer particle inside an active gel, exposing the properties of the van Hove distribution of the particle displacements. Active events of a typical force magnitude give rise to non-Gaussian distributions, having exponential tails or side-peaks. The side-peaks appear when the local bulk elasticity of the gel is large enough and few active sources are dominant. We explain the regimes of the different distributions, and study the structure of the peaks for active sources that are susceptible to the elastic stress that they cause inside the gel. We show how the van Hove distribution is altered by both the duty cycle of the active sources and their susceptibility, and suggest it as a sensitive probe to analyze microrheology data in active systems with restoring elastic forces.Comment: 4 pages, 4 figures and supplemental information (5 pages, 4 figures

Active Particle Models for Animal Behavior based on Effective Nonreciprocal Forces and Potentials

Modelling animal behavior using active-particle models is a major current challenge. The unique properties of animals mean that such models require the use of new types of effective interactions between the particles, and specifically effective forces that do not obey the usual conservation laws of Newtonian mechanics. These include nonreciprocal forces that break conservation of energy and momentum. We demonstrate here two very different animal behavior systems where such nonreciprocal effective forces naturally arise: the first is when animals form contests, as many animals do, such as fighting over some localized resource. The second system is of cohesive swarms, that are maintained by long-range adaptive attraction. These examples show that theoretical modelling in terms of active particles, interacting with effective nonreciprocal forces and potentials, expands the scope of active-particle research as well as helps to explain complex phenomena in animal behaviour.Comment: Contributed chapter to the book "Active Particles" (Volume 4