Recent experimental studies have demonstrated that cellular motion can be directed by topographical gradients, such as those resulting from spatial variations in the features of a micropatterned substrate. This phenomenon, known as topotaxis, is especially prominent among cells persistently crawling within a spatially varying distribution of cell-sized obstacles. In this article we introduce a toy model of topotaxis based on active Brownian particles constrained to move in a lattice of obstacles, with space-dependent lattice spacing. Using numerical simulations and analytical arguments, we demonstrate that topographical gradients introduce a spatial modulation of the particles' persistence, leading to directed motion toward regions of higher persistence. Our results demonstrate that persistent motion alone is sufficient to drive topotaxis and could serve as a starting point for more detailed studies on self-propelled particles and cells. arXiv:1908.06078v1 [cond-mat.soft]

Active particle assemblies can exhibit a wide range of interesting dynamical phases depending on internal parameters such as density, adhesion strength or self-propulsion.

The structure of nuclear pores has been the object of considerable investigation, but how nuclear pores are arranged on the nuclear surface is still less studied. Here, we analyze super-resolution images of the surface of Xenopus laevis oocytes nuclei during development and characterize the arrangement of nuclear pore using tools commonly used to study atomic structural and topological features of ordinary matter. To interpret the experimental results, we perform numerical simulations of octagonal clusters mimicking typical pore shapes and find structures that are in excellence agreement with experiments. The statistical features of the geometrical arrangement does not depend on the type of interaction between the pores, attractive or repulsive, but only on their octagonal geometry. We conclude that the observed arrangement of the pores is mainly is dominated by their octagonal symmetry.

Active particle assemblies can exhibit a wide range of interesting dynamical phases depending on internal parameters such as density, adhesion strength or self-propulsion. Active self-rotations are rarely studied in this context, although they can be relevant for active matter systems, as we illustrate by analyzing the motion of Chlamydomonas reinhardtii algae under different experimental conditions. Inspired by this example, we simulate the dynamics of a system of interacting active disks endowed with active torques. At low packing fractions, adhesion causes the formation of small rotating clusters, resembling those observed when algae are stressed. At higher densities, the model shows a jamming to unjamming transition promoted by active torques and hindered by adhesion. Our results yield a comprehensive picture of the dynamics of active rotators, providing useful guidance to interpret experimental results in cellular systems where rotations might play a role.