Beyond Dipolar Activity: Quadrupolar Stress Drives Collapse of Nematic Order on Frictional Substrates

TL;DR

This study reveals that quadrupolar stress, influenced by substrate interactions and hydrodynamic screening, causes the collapse of nematic order in active matter, offering new control methods via environmental modifications.

Contribution

It introduces the concept that quadrupolar activity, rather than dipolar, governs active nematic behavior on substrates, supported by experiments and simulations.

Findings

Quadrupolar activity leads to hierarchical folding in active nematics.

Friction and quadrupolar stress cause collapse of nematic order.

Environmental changes can control active matter dynamics.

Abstract

The field of active nematics has traditionally employed descriptions based on dipolar activity, with interactions that align along a single axis. However, it has been theoretically predicted that interactions with a substrate, prevalent in most biological systems, would require novel forms of activity, such as quadrupolar activity, that are governed by hydrodynamic screening. Here, by combining experiments and numerical simulations, we show that upon light-induced solidification of the underlying medium, microtubule-kinesin mixtures undergo a transformation that leads to a biphasic active suspension. Using an active lyotropic model, we prove that the transition is governed by screening effects that alter the dominant form of active stress. Specifically, the combined effect of friction and quadrupolar activity leads to a hierarchical folding that follows the intrinsic bend instability of the active nematic layer. Our results demonstrate the dynamics of the collapse of orientational order in active nematics and present a new route for controlling active matter by modifying the activity through changing the surrounding environment.