Efficiency limits of the three-sphere swimmer

Babak Nasouri, Andrej Vilfan, and Ramin Golestanian
Phys. Rev. Fluids 4, 073101 – Published 8 July 2019
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Abstract

We consider a swimmer consisting of a collinear assembly of three spheres connected by two slender rods. This swimmer can propel itself forward by varying the lengths of the rods in a way that is not invariant under time reversal. Although any non-reciprocal strokes of the arms can lead to a net displacement, the energetic efficiency of the swimmer is strongly dependent on the details and sequences of these strokes, and also the sizes of the spheres. We define the efficiency of the swimmer using Lighthill's criterion, i.e., the power that is needed to pull the swimmer by an external force at a certain speed, divided by the power needed for active swimming with the same average speed. Here, we determine numerically the optimal stroke sequences and the optimal size ratio of the spheres, while limiting the maximum extension of the rods. Our calculation takes into account both far-field and near-field hydrodynamic interactions. We show that, surprisingly, the three-sphere swimmer with unequal spheres can be more efficient than the equally sized case. We also show that the variations of efficiency with size ratio is not monotonic and there exists a specific size ratio at which the swimmer has the highest efficiency. We find that the swimming efficiency initially rises by increasing the maximum allowable extension of the rods, and then converges to a maximum value. We calculate this upper limit analytically and report the highest value of efficiency that the three-sphere swimmer can reach.

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  • Received 15 May 2019

DOI:https://doi.org/10.1103/PhysRevFluids.4.073101

©2019 American Physical Society

Physics Subject Headings (PhySH)

Fluid Dynamics

Authors & Affiliations

Babak Nasouri1, Andrej Vilfan1,2,*, and Ramin Golestanian1,3,†

  • 1Max Planck Institute for Dynamics and Self-Organization (MPIDS), 37077 Goettingen, Germany
  • 2Jožef Stefan Institute, 1000 Ljubljana, Slovenia
  • 3Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3PU, United Kingdom

  • *andrej.vilfan@ds.mpg.de
  • ramin.golestanian@ds.mpg.de

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Issue

Vol. 4, Iss. 7 — July 2019

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