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Chemically active particles generate spatial gradients in the chemical composition of the solution in which they are immersed. When such particle is in the vicinity of a liquid-fluid interface, the variations in chemical composition along the interafce induce surface tension gradients, this gives rise to Marangoni stresses and drives hydrodynamic flows extending in the solution and thus coupling back to the particle. Here we study a mean-field model for the many-body dynamics of a monolayer of spherically symmetric active particles located at a fluid–fluid interface . Due to the spherical symmetry, the particles do not self-propel; but the long-ranged Marangoni flows, induced by the response of the interface to the activity of the particles, compete with the direct interaction between particles and lead to collective dynamics. We show that, in spite of the intrinsic out-of-equilibrium character of the system, the monolayer evolves to a “pseudoequilibrium” state, in which the Marangoni flows force the coexistence of the thermodynamic phases associated to the direct interaction. In particular, for a soft (1/r^3) repulsive interaction between the particles and a sufficiently large average density of particles, a radially stratified, “onion-like” structure of two-dimensional phase transitions (freezing from liquid to hexatic, and melting from solid to hexatic) should be observable within the
monolayer.

References
1. A. Dominguez and M.N. Popescu, Soft Matter 14, 8017 (2018)
2. A. Dominguez, P. Malgaretti, M. N. Popescu, and S. Dietrich, Phys. Rev. Lett. 116, 078301 (2016)