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Instability of an active fluid jet

Takuji Ishikawa1,*, Thanh-Nghi Dang2, and Eric Lauga3

  • 1Department of Biomedical Engineering, Tohoku University, Sendai 980-8579, Japan
  • 2Faculty of Mechanical Engineering, RWTH Aachen University, 52056 Aachen, Germany
  • 3Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, United Kingdom

  • *t.ishikawa@tohoku.ac.jp
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Phys. Rev. Fluids 7, 093102 – Published 13 September, 2022

DOI: https://doi.org/10.1103/PhysRevFluids.7.093102

Abstract

The breakup of a fluid jet into droplets has long fascinated natural scientists, with early research dating back to the 19th century. Infinitesimal perturbations to a jet grow because of surface tension, which eventually leads to breakup of the jet into droplets (Rayleigh-Plateau instability). Although this classical phenomenon has long been studied, it is not clear how it is modified when the fluid is replaced by an active fluid. In this study, we investigate instabilities of a jet of an active fluid. The active fluid is modeled by a suspension of microswimmers that propel themselves by generating surface tangential velocities, i.e., squirmers. The squirmers are assumed to be bottom-heavy and heavier than the surrounding fluid, so that a downward jet of the active fluid self-assembles under gravity. Hydrodynamic interactions between squirmers are computed using the Stokesian dynamics method, in which near-field hydrodynamics are accurately calculated. We find that jets of active fluids are unstable, with different unstable modes between pullers and pushers. For an active fluid of pullers, the jet breaks up into droplets in a varicose manner reminiscent of a Newtonian fluid. For pushers, however, the jet buckles and undergoes a waving (sinuous) instability. The physical mechanisms for these two instabilities can be understood by an inspection of the stress fields in the jets, and parametric study reveals the importance of hydrodynamic interactions in the instabilities. Although both gravity and bottom-heaviness play an essential role in realizing the downward jet, their influence on the instability is found to be limited. Our findings help reveal new features of the collective properties of active fluids.

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Phys. Rev. Fluids 7, 093102– Published 13 September, 2022
  • Received 27 April 2022
  • Accepted 26 August 2022
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