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Unstable fronts and motile structures formed by microrollers

Nature Physics volume 13, pages 375–379 (2017)Cite this article

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Abstract

Condensation of objects into stable clusters occurs naturally in equilibrium1 and driven systems2,3,4,5. It is commonly held that potential interactions6, depletion forces7, or sensing8 are the only mechanisms which can create long-lived compact structures. Here we show that persistent motile structures can form spontaneously from hydrodynamic interactions alone, with no sensing or potential interactions. We study this structure formation in a system of colloidal rollers suspended and translating above a floor, using both experiments and large-scale three-dimensional simulations. In this system, clusters originate from a previously unreported fingering instability, where fingers pinch off from an unstable front to form autonomous ‘critters’, whose size is selected by the height of the particles above the floor. These critters are a stable state of the system, move much faster than individual particles, and quickly respond to a changing drive. With speed and direction set by a rotating magnetic field, these active structures offer interesting possibilities for guided transport, flow generation, and mixing at the microscale.

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Figure 1: Strong velocity enhancement due to collective effects.
Figure 2: Fingering instability.
Figure 3: Self-sustained critters.
Figure 4: Instability and clustering controlled by hydrodynamics.

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Change history

  • 10 March 2017

    In the version of this Letter orignally published, the Acknowledgements was missing the following sentence: 'We gratefully acknowledge the support of the NVIDIA Corporation with the donation of GPU hardware for performing some of the simulations reported here'. This has been corrected in all versions of this Letter.

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Acknowledgements

This work was supported primarily by the Gordon and Betty Moore Foundation through Grant GBMF3849 and the Materials Research Science and Engineering Center (MRSEC) program of the National Science Foundation under Award Number DMR-1420073. A. Donev and B. Delmotte were supported in part by the National Science Foundation under award DMS-1418706. P. Chaikin was partially supported by NASA under Grant Number NNX13AR67G. We gratefully acknowledge the support of the NVIDIA Corporation with the donation of GPU hardware for performing some of the simulations reported here.

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Author notes

  1. Michelle Driscoll and Blaise Delmotte: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Physics, New York University, New York, New York 10003, USA

    Michelle Driscoll & Paul Chaikin

  2. Courant Institute of Mathematical Sciences, New York University, New York, New York 10012, USA

    Blaise Delmotte & Aleksandar Donev

  3. Department of Chemistry, New York University, New York, New York 10003, USA

    Mena Youssef & Stefano Sacanna

Authors
  1. Michelle Driscoll
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  2. Blaise Delmotte
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  3. Mena Youssef
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  4. Stefano Sacanna
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  5. Aleksandar Donev
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  6. Paul Chaikin
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Contributions

M.D. performed the experiments. B.D. performed the simulations. M.Y. and S.S. synthesized the colloidal particles. M.D., B.D., A.D. and P.C. conceived the project, analysed the results and wrote the paper.

Corresponding authors

Correspondence to Michelle Driscoll or Blaise Delmotte.

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The authors declare no competing financial interests.

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Driscoll, M., Delmotte, B., Youssef, M. et al. Unstable fronts and motile structures formed by microrollers. Nature Phys 13, 375–379 (2017). https://doi.org/10.1038/nphys3970

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