Self-propelling nanoparticles push larger objects together

Computer simulation reveals remarkable collective effect of active colloidal nanoparticles

8 January 2015

Researchers at the universities of Amsterdam and Wageningen predict the existence of a new effective force in active nanomaterials. In computer simulations they discovered the ability of randomly moving self propelling nanoparticles to collectively move larger microscopic objects. They speculate that the active nano particles can be used to realise high-tech materials with complex structures, in a 'natural' fashion. Their research has been published by Physical Review Letters on 7 January.

It's always a fascinating sight: a flock of starlings or a school of fish, moving collectively as if guided by an invisible hand. Both are examples of 'active systems' of which the behaviour is determined by the collective efforts of smaller self-propelling units. Such systems can be found everywhere: from starlings and fish to bacteria.

Examples of active systems in nature: birds (photo Walter Baxter), fish (photo Gordon Firestein) and bacteria (photo Jürgen Berger and Gregory Velicer).

Examples of active systems in nature: birds (photo Walter Baxter), fish (photo Gordon Firestein) and bacteria (photo Jürgen Berger and Gregory Velicer).

Inspired by nature, scientists have developed active materials consisting of small dispersed nanoparticles, propelling themselves for instance by means of a chemical reaction at their surface. Such active systems can show unexpected behaviour not observed in comparable dispersions of passive particles.

Simulation 

Researchers of the Amsterdam Center for Multiscale Modeling at the Van 't Hoff Institute for Molecular Sciences (University of Amsterdam) now have explored the effect these active nanoparticles have on larger (though still microscopic) passive objects. In this they cooperated with the Laboratory for Physical Chemistry and Colloid Science at Wageningen University. By means of computer simulation they discovered that the collective behaviour of the particles (simulated as dense, hard spheres) can result in very large forces between the passive objects.

Surprisingly, the scope of these forces significantly exceeds expectations based on the size of the active particles. Furthermore, it is possible to invert forces from attractive to repulsive, simply by modifying the concentration of the active particles.

Memory effect

The researchers explain their finding by postulating a 'memory effect' regarding the active nanoparticles. Because of rotational inertia these need some time to 'forget' their direction of movement. In interaction with the larger passive particles this leads to collective effects resulting in the rather strong forces.

Simulation Active Nanoparticles

Active nanoparticles (in blue), propelling themselves in a random direction (red arrows). Because the particles have a tendency to maintain their direction they accumulate around and in between two larger rectangular objects. This leads to a repulsive force between these objects, driving them apart. In case of lower particle densities the resulting forces changes from repulsive to attractive. This is an effective attraction resulting from a reduced particle density between the objects in comparison to the overall particle density.

The researchers speculate that active particles can be used to generate new highly structured high-tech materials through self-assembly mimicking natural processes. They are now contemplating applications in the field of micro-fluidics, where systems are quasi two-dimensional.

Publication

Ran Ni, Martien A. Cohen Stuart, and Peter G. Bolhuis: Tunable Long Range Forces Mediated by Self-Propelled Colloidal Hard Spheres Phys. Rev. Lett. 114, 018302 – Published 7 January 2015 DOI: 10.1103/PhysRevLett.114.018302

Published by  HIMS