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Issue 26, 2021

Phase behavior and surface tension of soft active Brownian particles

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Abstract

We study quasi two-dimensional, monodisperse systems of active Brownian particles (ABPs) for a range of activities, stiffnesses, and densities. We develop a microscopic, analytical method for predicting the dense phase structure formed after motility-induced phase separation (MIPS) has occurred, including the dense cluster's area fraction, interparticle pressure, and radius. Our predictions are in good agreement with our Brownian dynamics simulations. We, then, derive a continuum model to investigate the relationship between the predicted interparticle pressure, the swim pressure, and the macroscopic pressure in the momentum equation. We find that formulating the point-wise macroscopic pressure as the interparticle pressure and modeling the particle activity through a spatially variant body force – as opposed to a volume-averaged swim pressure – results in consistent predictions of pressure in both the continuum model and the microscopic theory. This formulation of pressure also results in nearly zero surface tension for the phase separated domains, irrespective of activity, stiffness, and area fraction. Furthermore, using Brownian dynamics simulations and our continuum model, we showed that both the interface width and surface tension, are intrinsic characteristics of the system. On the other hand, if we were to exclude the body force induced by activity, we find that the resulting surface tension values are linearly dependent on the size of the simulation, in contrast to the statistical mechanical definition of surface tension.

Graphical abstract: Phase behavior and surface tension of soft active Brownian particles

Supplementary files

Article information


Submitted
05 Mar 2021
Accepted
04 Jun 2021
First published
07 Jun 2021

Soft Matter, 2021,17, 6337-6351
Article type
Paper

Phase behavior and surface tension of soft active Brownian particles

N. Lauersdorf, T. Kolb, M. Moradi, E. Nazockdast and D. Klotsa, Soft Matter, 2021, 17, 6337 DOI: 10.1039/D1SM00350J

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