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Active turbulence in a gas of self-assembled spinners

  1. Alexey Snezhkoa,1
  1. aMaterials Science Division, Argonne National Laboratory, Argonne, IL 60439;
  2. bInstitute of Complex Systems, Forschungszentrum Jülich, 52425 Jülich, Germany;
  3. cInstitute for Advanced Simulation, Forschungszentrum Jülich, 52425 Jülich, Germany;
  4. dDepartment of Biomedical Engineering, Pennsylvania State University, University Park, PA 16802
  1. Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved October 23, 2017 (received for review June 5, 2017)

Significance

Turbulent fluid motion is widespread in nature and is observed across diverse length and time scales, ranging from high-Reynolds number hydrodynamics to active fluids, such as bacterial suspensions and cytoskeletal extracts. It is recognized as one of the unsolved challenges in theoretical physics. Here, we explore out-of-equilibrium magnetic colloidal particles at liquid interfaces that exhibit complex collective behavior, resulting in emergence of an active spinner phase. Self-assembled spinners (active spinning without self-propulsion) induce vigorous vortical flows, demonstrating the properties of a 2D hydrodynamic turbulence. Our findings provide insight into the behavior of active spinner liquids and ways to control the collective dynamics and transport in active colloidal materials.

Abstract

Colloidal particles subject to an external periodic forcing exhibit complex collective behavior and self-assembled patterns. A dispersion of magnetic microparticles confined at the air–liquid interface and energized by a uniform uniaxial alternating magnetic field exhibits dynamic arrays of self-assembled spinners rotating in either direction. Here, we report on experimental and simulation studies of active turbulence and transport in a gas of self-assembled spinners. We show that the spinners, emerging as a result of spontaneous symmetry breaking of clock/counterclockwise rotation of self-assembled particle chains, generate vigorous vortical flows at the interface. An ensemble of spinners exhibits chaotic dynamics due to self-generated advection flows. The same-chirality spinners (clockwise or counterclockwise) show a tendency to aggregate and form dynamic clusters. Emergent self-induced interface currents promote active diffusion that could be tuned by the parameters of the external excitation field. Furthermore, the erratic motion of spinners at the interface generates chaotic fluid flow reminiscent of 2D turbulence. Our work provides insight into fundamental aspects of collective transport in active spinner materials and yields rules for particle manipulation at the microscale.

Footnotes

  • ?1To whom correspondence may addressed. Email: snezhko{at}anl.gov or g.gompper{at}fz-juelich.de.
  • Author contributions: A.S. designed research; G.K., S.D., R.G.W., G.G., and A.S. performed research; G.K., S.D., R.G.W., G.G., I.S.A., and A.S. analyzed data; and G.K., S.D., R.G.W., G.G., I.S.A., and A.S. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1710188114/-/DCSupplemental.

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