Active foams

Recent experimental evidence revealed a new state of active matter in biofilaments and molecular motors mixtures called active foam [1]. These disordered cellular solids are always out of equilibrium – the edges constituting the foam appear and disappear, changing the morphology and topology of the whole network. Here we introduce a classification of such structures based on the dominating orientational order. We focus on two particular classes – nematic and polar active foams. We develop the hydrodynamic model for each foam type and show how the corresponding continuum theories can be derived by coarse-graining the motor-induced microscopic interactions between the filaments. Moreover, we demonstrate that the imbalance between polar and antipolar interactions determines the eventual macroscopic patterns. We investigate the instabilities and the characteristic length scales associated with each model and pin down the mechanisms underlying the foam formation. We also describe the motors' inhomogeneity role in forming "bilayers" – essential building blocks constituting polar active foams. Finally, we discuss potentially related experimental systems where the active foams could be realized.

[1] B. Lemma, N. P. Mitchell, R. Subramanian, D. J. Needleman, and Z. Dogic (2022). Active microphase separation in mixtures of microtubules and tip-accumulating molecular motors. Phys. Rev. X 12(3), 031006