Elucidating Interfacial Instability of Apolar Active Suspensions

Biological processes in living systems, such as cell division, endocytosis, and mechanotransduction, often exhibit intriguing non-equilibrium physics and phenomena of self-driven microparticles. While there are extensive studies on understanding large-scale, turbulent-like collective dynamics in bulk, interfacial behaviors due to active particle self-assembly still need to be explored. This research studies the interfacial instability and pattern formation in a wall-bounded, three-dimensional apolar active-fluid film that interfaces with a Newtonian liquid. The active fluid phase comprises immotile "extensor" particles that exert extensile dipolar stresses on the fluid. In the dilute and semi-dilute regimes, we combine discrete particle simulations and linear stability analysis to elucidate the physical mechanisms of the emerging blob patterns at the particle-fluid interface during the initial transient. We estimate the system's time and length scales by measuring the auto-correlation functions of discrete particles and extracting topological structures from the reconstructed smooth field of the orientation-order parameter. We proceed to perform linear stability analysis for a coarse-grained hydrodynamic liquid crystal model to reveal the finite-wavelength nature of the numerically observed unstable interfacial dynamics. We show that the interplay between particle activity, hydrodynamic-nematic coupling, and film thickness determines the growing structures of the interfacial particle blobs.