Shear-induced orientational ordering in an active glass former

Dense assemblies of self-propelled particles that can form solid-like states also known as active or living glasses are abundant around us, covering a broad range of length scales and timescales: from the cytoplasm to tissues, from bacterial biofilms to vehicular traffic jams, and from Janus colloids to animal herds. Being structurally disordered as well as strongly out of equilibrium, these systems show fascinating dynamical and mechanical properties. Using extensive molecular dynamics simulation and a number of distinct dynamical and mechanical order parameters, we differentiate three dynamical steady states in a sheared model active glassy system: 1) a disordered state, 2) a propulsion-induced ordered state, and 3) a shear-induced ordered state. We supplement these observations with an analytical theory based on an effective single-particle Fokker–Planck description to rationalize the existence of the shear-induced orientational ordering behavior in an active glassy system without explicit aligning interactions of, for example, Vicsek type. This ordering phenomenon occurs in the large persistence time limit and is made possible only by the applied steady shear. Using a Fokker–Planck description with parameters that can be measured independently, we make testable predictions for the joint distribution of single-particle position and orientation. These predictions match well with the joint distribution measured from direct numerical simulation. Our results are of relevance for experiments exploring the rheological response of dense active colloids and jammed active granular matter systems.

Biofilm self-patterning: mechanical force landscaping drives a collective reorientation cascade

During development, cells often self-organize into distinctive patterns with long-range orientational order. However, the mechanism by which long-range order emerges through complex interactions, particularly in the prokaryotic domain, remains elusive. Here we report, in growing Vibrio cholerae biofilms, a reorientation cascade consisting of cell verticalization in the core and radial alignment in the rim, generating a pattern reminiscent of a blooming aster. Single-cell imaging combined with agent-based simulations reveal that cell verticalization and radial alignment are spatiotemporally coupled, each generating the driving force for the other, to cause a dynamic cascade of differential orientational ordering. Such self-patterning is absent in nonadherent mutants but can be restored through opto-manipulation of growth. A two-phase active nematic model is developed to elucidate the mechanism underlying biofilm self-patterning, which offers insights into the control of organization in complex bacterial communities.

Effect of Bjerrum pairs on electrostatic properties in an electrolyte solution near charged surfaces: A mean-field approach

In this paper, we investigate the consequences of ion association, coupled with the considerations of finite size effects and orientational ordering of Bjerrum pairs as well as ions and water...