3-D computer simulations reveal diffusional behavior

Fran Granville, Senior Associate Editor - March 1, 2012

A team of researchers at the Georgia Institute of Technology is exploring how nanoparticles move and diffuse on a surface or in a fluid under nonideal to extreme conditions. Rigoberto Hernandez, a professor at the university’s school of chemistry and biochemistry, investigates these relationships by studying 3-D particle-dynamics simulations on high-performance computers. His findings focus on the movements of a spherical probe among static needles (Reference 1).

Hernandez and his former doctoral student, Ashley Tucker, assembled the rod-like scatterers in one of two states during his simulations: disordered, or isotropic, and ordered, or nematic. When disordered, the nanorods point in various directions and typically diffuse normally in all directions. When every rod points in the same direction, the particle, on average, diffuses more in the same direction as the rods than against the grain of the rods. In this nematic state, the probe’s movement mimics the elongated shape of the scatterers.

3-D computer simulations reveal diffusional behavior image

Surprisingly, however, the particles sometimes diffuse faster in the nematic environment than in the disordered environment. That is, the channels left open between the ordered nanorods don’t just steer nanoparticles along a direction; they also enable them to speed right through. As the density of the scatterers increases, the channels become more crowded. The particle diffusing through these assemblies slows dramatically in the simulation. Nevertheless, the nematic scatterers continue to accommodate faster diffusion than do disordered scatterers, according to the researchers.

“These simulations bring us a step closer to creating a nanorod device that allows scientists to control the flow of nanoparticles,” says Hernandez. “Blue-sky applications of such devices include the creation of new light patterns, information flow, and other microscopic triggers.” For example, if scientists need a probe to diffuse in a specific direction at a particular speed, they could trigger the nanorods to move into a specified direction. When they need to change the particle’s direction, they could trigger scatterers to rearrange into a different direction. The trigger could even be the absence of sufficient nanoparticles in a given part of the device. The ensuing reordering of the nanorods would then drive a repopulation of nanoparticles that would then be available to perform a desired action, such as to stimulate light flow.

The National Science Foundation-funded work targets a better understanding of the motion of particles within complex arrays at the nanoscale. The work has significant longterm implications on device fabrication and performance at such scales.

Georgia Institute of Technology

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