Colloidal rods in square confinement

Cortes L. B. G., Gao Y., Dullens R. P. A., Aarts D. G. A. L., "Colloidal liquid crystals in square confinement: isotropic, nematic and smectic phases", J. Phys.: Condens. Matter 29, 2017

Imagine you’ve got a bunch of matches and you need to pack them into a matchbox. That’s an easy challenge, unless each match is constantly moving around (due to Brownian motion) and they need to align along any of the walls of the matchbox. If you only need to fit a handful of rods it’s still ok, but at higher densities the rods will all want to point in one direction, which is impossible given the boundary conditions. At even higher concentrations they want to pack in layers, which forms another complicating factor.

To study this problem experimentally we have developed a system of small silica rods, which are around 5 micrometer long and 0.5 micrometer thick. Under gravity they slowly settle into square boxes of varying dimensions, from several times the rod length up to tens of times, and they form a range of patterns that we can quantify using microscopy down to the almost single particle level. Some of the patterns observed were already predicted in computer simulations, but some other structures are novel and we are currently working on a theory to understand these observations better.

Importantly, such packing problems occur in nature and technology: for example, the packing of DNA or the confinement of actin filaments in biological cells, and liquid crystals in each pixel of your phone’s screen. Our experiments shine light on the possible patterns which form and on the applicability of continuum theories down to these small lengthscales.

A full version of the article can be found here.