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Issue 23, 2018
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Active fluids at circular boundaries: swim pressure and anomalous droplet ripening

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We investigate the swim pressure exerted by non-chiral and chiral active particles on convex or concave circular boundaries. Active particles are modeled as non-interacting and non-aligning self-propelled Brownian particles. The convex and concave circular boundaries are used to model a fixed inclusion immersed in an active bath and a cavity (or container) enclosing the active particles, respectively. We first present a detailed analysis of the role of convex versus concave boundary curvature and of the chirality of active particles in their spatial distribution, chirality-induced currents, and the swim pressure they exert on the bounding surfaces. The results will then be used to predict the mechanical equilibria of suspended fluid enclosures (generically referred to as ‘droplets’) in a bulk with active particles being present either inside the bulk fluid or within the suspended droplets. We show that, while droplets containing active particles behave in accordance with standard capillary paradigms when suspended in a normal bulk, those containing a normal fluid exhibit anomalous behaviors when suspended in an active bulk. In the latter case, the excess swim pressure results in non-monotonic dependence of the inside droplet pressure on the droplet radius; hence, revealing an anomalous regime of behavior beyond a threshold radius, in which the inside droplet pressure increases upon increasing the droplet size. Furthermore, for two interconnected droplets, mechanical equilibrium can occur also when the droplets have different sizes. We thus identify a regime of anomalous droplet ripening, where two unequal-sized droplets can reach a final state of equal size upon interconnection, in stark contrast with the standard Ostwald ripening phenomenon, implying shrinkage of the smaller droplet in favor of the larger one.

Graphical abstract: Active fluids at circular boundaries: swim pressure and anomalous droplet ripening

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Publication details

The article was received on 18 Feb 2018, accepted on 17 May 2018 and first published on 18 May 2018

Article type: Paper
DOI: 10.1039/C8SM00338F
Soft Matter, 2018,14, 4820-4834

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    Active fluids at circular boundaries: swim pressure and anomalous droplet ripening

    T. Jamali and A. Naji, Soft Matter, 2018, 14, 4820
    DOI: 10.1039/C8SM00338F

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