Highly Resolved Brownian Motion in Space and in Time

Since the discovery of Brownian motion in bulk fluids by Robert Brown in 1827, Brownian motion at long timescales has been extensively studied both theoretically and experimentally for over a century. The theory for short-timescale Brownian motion was also well established in the last century, while experimental studies were not accessible until this decade. This article reviews experimental progress on short-timescale Brownian motion and related applications. The ability to measure instantaneous velocity enables new fundamental tests of statistical mechanics of Brownian particles, such as the Maxwell-Boltzmann velocity distribution and the energy equipartition theorem. In addition, Brownian particles can be used as probes to study boundary effects imposed by a solid wall, wettability at solid-fluid interfaces, and viscoelasticity. We propose future studies of fluid compressibility and nonequilibrium physics using short-duration pulsed lasers. Lastly, we also propose that an optically trapped particle can serve as a new testing ground for nucleation in a supersaturated vapor or a supercooled liquid.