Larger particles can diffuse faster than smaller ones

Brownian motion, first studied in pollen grains by botanist Robert Brown in 1827, describes the random movement of particles suspended in a liquid medium. A fundamental question is how the structure of their surroundings affects the dynamics of such particles. Previous research has investigated Brownian dynamics in viscous liquids, which have no underlying regular structure.

Lettinga et al. explored the movement of rod-shaped guest particles, both short and long, within liquid crystalline phases of a matrix consisting of layers of short host rods. They aimed to highlight the differences between short particles that fit within the layers and long particles that stick out into two adjacent layers.

“Colloidal rods display a plethora of liquid crystalline phase transitions and are therefore a beautiful playground for these kinds of studies,” said author M. Paul Lettinga.

The researchers used video microscopy to perform single particle tracking on rodlike viruses labeled with fluorescent dyes. Samples were prepared with concentrations of host rods in the range from the nematic to the smectic phase.

Counterintuitively, they found large particles can diffuse faster than small ones. The assumption that large particles always diffuse slower does not hold when the length scale associated with the energy landscape, formed by self-assembled host particles, is smaller than the length scale of the guest particle.

The work may have implications for drug delivery — for example, when considering particle transport through cell membranes.

“The cell membrane has many things in common with a smectic layer, but in those studies, the main issues are always the specific interaction between the particle and hardly the exact size of the particle,” he said. “We show that tuning the size can be crucial.”

Source: “When bigger is faster: A self-Van Hove analysis of the enhanced self-diffusion of non-commensurate guest particles in smectics,” by M. Paul Lettinga, Laura Alvarez, Olivera Korculanin, and Eric Grelet, Journal of Chemical Physics (2021). The article can be accessed at http://doi.org/10.1063/5.0049093 .