Xiao-guang Ma

Colloidal diffusion over a periodic energy landscape

A two-layer colloidal system is developed for the study of colloidal diffusion over a two-dimensional periodic energy landscape. The energy landscape is made from the bottom layer of colloidal spheres forming a honey-comb crystalline pattern above a glass substrate. The corrugated surface of the bottom colloidal crystal provides a gravitational potential field for the diffusing particles in the top layer. The obtained population probability histogram P(x, y) of the diffusing particles is used to fully characterize the energy landscape U(x, y) via the Boltzmann distribution. The dynamical properties of the diffusing particle, such as its escape time tR and diffusion coefficient D are simultaneously measured from the particles trajectories. The long-time diffusion coefficients D is found to be in good agreement with the theory for all colloidal samples studied. The experiment demonstrates the applications of this newly constructed colloidal energy landscape.

Colloidal dynamics over a tilted periodic potential: Forward and reverse transition probabilities and entropy production in a nonequilibrium steady state

We report a systematic study of the forward and reverse transition probability density functions (TPDFs) and entropy production in a nonequilibrium steady state (NESS). The NESS is realized in a two-layer colloidal system, in which the bottom-layer colloidal crystal provides a two-dimensional periodic potential U_{0}(x,y) for the top-layer diffusing particles. By tilting the sample at an angle with respect to gravity, a tangential component of the gravitational force F is applied to the diffusing particles, which breaks the detailed balance (DB) condition and generates a steady particle flux along the [1,0] crystalline orientation. While both the measured forward and reverse TPDFs reveal interesting space-time dependence, their ratio is found to be independent of time and obeys a DB-like relation. The experimental results are in good agreement with the theoretical predictions. This study thus provides a better understanding on how entropy is generated and heat is dissipated to the reservoir during a NESS transition process. It also demonstrates the applications of the two-layer colloidal system in the study of NESS transition dynamics.