Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published January 2020 | Submitted + Published + Supplemental Material
Journal Article Open

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


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.

Additional Information

© 2020 American Physical Society. Received 6 June 2019; published 16 January 2020. A.K.O. acknowledges support by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144469 and an HHMI Gilliam Fellowship. J.F.B. acknowledges support by the National Science Foundation under Grant No. CBET-1803662.

Attached Files

Published - PhysRevE.101.012604.pdf

Submitted - 1912.11727.pdf

Supplemental Material - SM_omar_surface_tension_2019_PRE.pdf

Supplemental Material - dropletdynamics.mp4

Supplemental Material - slabpreparation.mp4


Files (65.4 MB)
Name Size Download all
265.3 kB Preview Download
25.3 MB Download
37.2 MB Download
1.2 MB Preview Download
1.4 MB Preview Download

Additional details

August 19, 2023
October 18, 2023