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Fluctuations and Structure at Active Interfaces

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ADIW04 - Anti-Diffusion in Multiphase and Active Flows

The simplest class of active soft matter may be modelled at the particle level by a system of self-propelled Brownian particles (ABPs) and at the hydrodynamic level by the (active) model B+ for the dynamics of a conserved scalar order parameter field. Studies revealed surprising collective behavior, notably phase separation in systems of purely repulsive particles, known as motility induced phase separation (MIPS) [1]. Even for these systems fundamental questions, such as the surface tension [2] and the scaling of interfacial fluctuations [3] were addressed only recently. Early work based on computer simulations of phase separated ABPs in two dimensions (2D) reported an interfacial roughness compatible with equilibrium Edwards-Wilkinson behaviour [4,5] in contrast with results from non-linear field theory that predict the existence of a MIPS universality class (qKPZ) away from the critical region [3]. One suggestion is that the system of ABPs undergoes bubbly phase separation [6,3] for which no theoretical predictions are available. Here we consider phase separation of ABPs on a 2D lattice and perform simulations using rejection-free Kinetic Monte Carlo (KMC) methods [7] to access long length and time scales, over a wide range of parameters: at fixed activity system sizes L = 10 to 6500 (in lattice units) and at fixed L activities from near-critical to deep in the phase separated system (persistence length lp = 50 to 230).   We found different regimes: lp/L>1, where w(t) saturates with L, and lp/L MIPS . Finally, near the critical point the auto-correlation function exhibits a fat tail. Clearly, further work is needed (and is underway) to understand the rich behaviour of active interfaces. References

E. Cates and J. Tailleur, Motility-Induced Phase Separation, Annu. Rev. Condens. Matter Phys., 6, 219 (2015). Fausti, E. Tjhung, M. E. Cates, and C. Nardini, Capillary Interfacial Tension in Active Phase Separation, Phys. Rev. Lett., 127, 068001; Erratum Phys. Rev. Lett. 128, 219901 (2021) Besse, G. Fausti, M. E. Cates, B. Delamotte, and C. Nardini, Interface Roughening in Nonequilibrium Phase-Separated Systems, Phys. Rev. Lett., 130, 187102 (2023). Bialké, J. T. Siebert, H. Löwen, and T. Speck, Negative interfacial tension in phase-separated active Brownian particles, Phys. Rev. Lett., 115, 098301 (2015). Patch, D. M. Sussman, D. Yllanes, and M. C. Marchetti, Curvature-dependent tension and tangential flows at the interface of motility-induced phases, Soft Matter, 14, 7435–7445 (2018). Tjhung, C. Nardini, and M. E. Cates, Cluster phases and bubbly phase separation in active fluids: Reversal of the ostwald process, Phys. Rev. X 8 , 031080 (2018). Neta, M. Tasinkevych, M. M. Telo da Gama, C. S. Dias, Wetting of a solid surface by active matter, Soft Matter, 17, 2468-2478 (2021). Surfaces and Interfaces of Soft Active Matter, J. M. Cordeiro, MSc Thesis, University of Lisbon, 2023

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