Microscopic origins of the swim pressure and the anomalous surface tension of active matter

Abstract

The unique pressure exerted by active particles -the "swim" pressure -has proven to be a useful quantity in explaining many of the seemingly confounding behaviors of active particles. However, its use has also resulted in some puzzling findings including an extremely negative surface tension between phase separated active particles. Here, we demonstrate that this contradiction stems from the fact that the swim pressure is not a true pressure. At a boundary or interface, the reduction in particle swimming generates a net active force density -an entirely self-generated body force. The pressure at the boundary, which was previously identified as the swim pressure, is in fact an elevated (relative to the bulk) value of the traditional particle pressure that is generated by this interfacial force density. Recognizing this unique mechanism for stress generation allows us to define a much more physically plausible surface tension. We clarify the utility of the swim pressure as an "equivalent pressure" (analogous to those defined from electrostatic and gravitational body forces) and the conditions in which this concept can be appropriately applied. arXiv:1912.11727v1 [cond-mat.soft]

Capillary Interfacial Tension in Active Phase Separation

“…For active systems showing bulk liquid-vapor phase separation it has been debated, on the basis of numerical and analytical studies, how to define the liquid-vapor interfacial tension [33][34][35][36][37][38][39]. One of the key results of this Letter is to confirm that no unique definition is possible.…”

Reverse osmotic effect in active matter

“…2(a)]. Since particles exert a local osmotic pressure osmo = nζ D T = nk B T at each point in space [29], the pressure exerted by active particles is higher on the wall than far from it. This extra pressure can be understood in the context of an enhanced diffusivity.…”

Phase Diagram of Active Brownian Spheres: Crystallization and the Metastability of Motility-Induced Phase Separation

“…Further motivating the study of active crystallization is the knowledge that for finite run lengths, ABPs exhibit a distinct geometric transition that has garnered considerable interest: the so-called motility-induced phase separation (MIPS) [12,[18][19][20]. This uniquely nonequilibrium phenomenon requires no interparticle attraction, yet appears to be a genuine liquid-gas transition, with no evidence of rotational symmetry breaking between the coexisting phases in 3D [21][22][23][24][25]. The apparent and conspicuous absence of an ordered phase for activities in the vicinity of the MIPS phase boundary raises the intriguing question: Does the crystallization transition disappear as the system departs from equilibrium?…”