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Published January 7, 2021 | Published + Submitted
Journal Article Open

The "isothermal" compressibility of active matter

Abstract

We demonstrate that the mechanically defined "isothermal" compressibility behaves as a thermodynamic-like response function for suspensions of active Brownian particles. The compressibility computed from the active pressure—a combination of the collision and unique swim pressures—is capable of predicting the critical point for motility induced phase separation, as expected from the mechanical stability criterion. We relate this mechanical definition to the static structure factor via an active form of the thermodynamic compressibility equation and find the two to be equivalent, as would be the case for equilibrium systems. This equivalence indicates that compressibility behaves like a thermodynamic response function, even when activity is large. Finally, we discuss the importance of the phase interface when defining an active chemical potential. Previous definitions of the active chemical potential are shown to be accurate above the critical point but breakdown in the coexistence region. Inclusion of the swim pressure in the mechanical compressibility definition suggests that the interface is essential for determining phase behavior.

Additional Information

© 2020 Published under license by AIP Publishing. Submitted: 11 September 2020; Accepted: 18 December 2020; Published Online: 6 January 2021. We gratefully acknowledge the support of NVIDIA Corporation with the donation of the Titan V GPU used for this research. S.A.M. acknowledges the support by the Arnold and Mabel Beckman Foundation. J.F.B. acknowledges the support by the National Science Foundation under Grant No. CBET-1803662. Data Availability: The data that support the findings of this study are available from the corresponding author upon request.

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Published - 5.0029364.pdf

Submitted - 2009.11439.pdf

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Created:
August 20, 2023
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October 20, 2023