Scientists have found that micron-size particles which are trapped at fluid interfaces exhibit a collective dynamic that is subject to seemingly unrelated governing laws. These laws show a smooth transitioning from long-ranged cosmological-style gravitational attraction down to short-range attractive and repulsive forces. The study is by Johannes Bleibel from the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, and his colleagues.
The authors used so-called colloidal particles that are larger than molecules but too small to be observed with the naked eye. These particles are adsorbed at the interface between two fluids and assembled into a monolayer. This constitutes a 2D model in which particles that are larger than a micron deform the interface through their own weight and generate an effective long-range attraction which looks like gravitation in 2D. Thus, the particles assemble in clusters.
To model long-range forces between particles, the researchers used numerical simulations based on random movement of particles, known as Brownian dynamics. Here, they took advantage of the formal analogy between so-called capillary attraction -- the long-ranged interaction through interface deformation -- and gravitational attraction. They used a particle-mesh method as employed in simulations of what are known as self-gravitating fluids, corresponding to the collapse of a system under its own gravity, traditionally used in cosmological studies.
The authors 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 is used as a template for nanoparticles deposited onto substrates in nanotechnology applications.
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Journal Reference:Bleibel J, Dominguez A, Oettel M, Dietrich S. Collective dynamics of colloids at fluid interfaces. European Physical Journal E, 34:125 DOI: 10.1140/epje/i2011-11125-2