Cosmology in a Petri dish (Vol. 43 No. 2)

image Illustration of a single colloid adsorbed at a fluid interface represented by the dotted line.

Methods to study the formation of our universe are used to understand long-range interactions between particles at the micrometric scale. In the article summarised here, it it is shown that micron-size particles trapped at fluid interfaces exhibit a collective dynamic that is subject to seemingly unrelated governing laws smoothly transitioning from long-ranged cosmological-style gravitational attraction down to short-range attractive and repulsive forces.

The authors used so-called colloidal particles that are larger than molecules but too small to be observed with the naked eye, which are adsorbed at the interface between two fluids and assembled into a monolayer. This constitutes a 2D model in which particles larger than a micron deform the interface through their own weight and generate an effective long-range attraction which looks like gravitation in 2D, and thus assemble in clusters.

To model long-range forces between particles, numerical simulations based on random movement of particles, known as Brownian dynamics, have been used. It takes advantage of the formal analogy between so-called capillary attraction - the long-ranged interaction through interface deformation - and gravitational attraction.

It is also found that this long-range interaction no longer matters beyond a certain length determined by the properties of both the particles and the interface, and short-range forces come into play. This means that for systems exceeding this length, particles first tend to self-assemble into several clusters which eventually merge into a single, large cluster.

The study of monolayer aggregates of micron-size colloids are used in nanotechnology applications.

Collective dynamics of colloids at fluid interfaces
J. Bleibel, A. Dominguez, M. Oettel and S. Dietrich, Eur. Phys. J. E, 34, 125 (2011)