A manuscript entitled “Ordering of colloidal hard spheres under gravity: From monolayer to multilayer” by Ph.D. student Ziwei Guo, 3rd-year Emory undergraduate Peiyao Wu, and J. T. Kindt has been accepted for publication in the RSC journal Soft Matter.  The paper is based on Monte Carlo simulations by Peiyao and Ziwei that map out how perfectly spherical colloidal particles are packed when resting on a hard surface, as a function of Péclet number (Pe) which describes the relative strength of gravitational and thermal energies.  At high Pe, there is a greater energy cost to placing a particle in a second layer, and a denser packing of the base monolayer will need to be achieved before upper layers are populated.  We found that at Pe~18 and above, this effect is strong enough that the base layer will freeze into a hexagonally ordered state (hexatic or 2-d solid) before a large fraction of particles are found in second layer.  A slight decrease in Pe to 16 (corresponding to just a 3% decrease in particle diameter) yields a different outcome – at the ordering transition, there are half as many particles in the overlayers as in the base layer.  To understand why such a small change in gravitational energy makes such a drastic difference, we performed calculations to find the free energies of  grain boundaries between ordered domains at varying Pe and overlayer coverage, building on our previous work.   The picture that emerged was that low concentrations of particles in the overlayer promote local disorder in the base layer, tending to suppress the ordering transition, but that beyond a certain coverage the overlayer exerts a more uniform pressure that promotes ordering.

Peiyao and our collaborators in the Weeks group in the Department of Physics are working to understand how the behavior of a real experimental system of silica spheres suspended in water relates to this idealized simulation model.