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Tunable Long Range Forces Mediated by Self-Propelled Colloidal Hard Spheres

Ran Ni, Martien A. Cohen Stuart, and Peter G. Bolhuis
Phys. Rev. Lett. 114, 018302 – Published 7 January 2015
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    Abstract

    Using Brownian dynamics simulations, we systematically study the effective interaction between two parallel hard walls in a 2D suspension of self-propelled (active) colloidal hard spheres, and we find that the effective force between two hard walls can be tuned from a long range repulsion into a long range attraction by changing the density of active particles. At relatively high densities, the active hard spheres can form a dynamic crystalline bridge, which induces a strong oscillating long range dynamic wetting repulsion between the walls. With decreasing density, the dynamic bridge gradually breaks, and an intriguing long range dynamic depletion attraction arises. A similar effect occurs in a quasi-2D suspension of self-propelled colloidal hard spheres by changing the height of the confinement. Our results open up new possibilities to manipulate the motion and assembly of microscopic objects by using active matter.

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    • Received 8 September 2014

    DOI:https://doi.org/10.1103/PhysRevLett.114.018302

    © 2015 American Physical Society

    Authors & Affiliations

    Ran Ni1,2,*, Martien A. Cohen Stuart2, and Peter G. Bolhuis1

    • 1Van’t Hoff Institute for Molecular Sciences, Universiteit van Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
    • 2Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands

    • *rannimail@gmail.com

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    Vol. 114, Iss. 1 — 9 January 2015

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    • Figure 1
      Figure 1

      (a) Effective forces Fσ/kBT as a function of wall-to-wall distance r at density ρbulkσ2=0.4 with various activity f. (b) Fσ/kBT and the average density ρ¯(r) inside the confinement −0.5r≤x≤0.5r, −0.5W≤y≤0.5W as a function of r with fσ/kBT=40 and ρbulkσ2=0.4. (c) Fσ/kBT as a function of r at ρbulkσ2=0.4 with various Gaussian size polydispersities s. (d) Reduced density distribution log10[ρ(x,y)/ρbulk] of active particles with fσ/kBT=40 at ρbulkσ2=0.4, with a wall-to-wall distance r/σ=2.1,5,10, and 20. (e) Typical snapshots of systems for density distributions in (d), respectively. (f) F/(ρbulkσkBT) for fσ/kBT=40 as a function of wall-to-wall distance r for various particle densities. (g) Fσ2/(WkBT) for a self-propelled ideal particle system with fσ/kBT=40 and ρbulkσ2=0.2 for various wall sizes W. The lines are fits with ∼−exp(−r/ξ). (h) Fσ2/(WkBT) for ideal particles with density ρbulkσ2=0.2 for various self-propulsions and a wall size W/σ=80. (i) Reduced density distribution log10[ρ(x,y)/ρbulk] for ideal particle systems with fσ/kBT=40 and r/σ=2.2,5,10, and 20, where the wall size is W/σ=10, and ρbulkσ2=0.4.

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    • Figure 2
      Figure 2

      (a) Simple model with one ideal particle (grey square) moving in a 2D square lattice, with two parallel impenetrable walls (dark squares). (b)–(d) Reduced density distributions ρ(x,y)/ρbulk for kswap=0.2,0.5, and 1, respectively. (e) Effective force between two walls F/v0 for various kswap as a function of r. The solid lines are the fits with ∼−exp(−r/ξ). (f) Fitted interaction range ξ in (e) as a function of 1/kswap.

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    • Figure 3
      Figure 3

      (a) Main: Fσ2/(WkBT) as a function of wall-wall separation r/σ for fσ/kBT=40 in quasi-2D confinement with a varying height H/σ, at fixed 2D number density Nσ2/A=0.4. Inset: a typical system snapshot with two walls as yellow bricks. (b) Fσ3/(WHkBT) as a function of r/σ for fσ/kBT=40 and varying H/σ, where the 3D number density is fixed at Nσ3/(AH)=0.2.

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