Jason Boynewicz Qualifying Exam

Abstract: Brownian motion is a foundational problem in statistical mechanics and historically proved the atomistic theory of matter. Recent experimental advances have allowed tracking a Brownian particle at timescales well below its momentum relaxation time in Newtonian fluids. While past studies have extensively analyzed correlation functions and the mean squared displacement for Brownian particles with equilibrium initial conditions, we condition our experimental trajectories to average only over specific initial velocities. In the case the initial velocity of the Brownian particle is close to zero, we observe a super-ballistic power law scaling of that is unique to incompressible Newtonian fluids. This provides experimental confirmation for a theory of Brownian motion in liquids with arbitrary initial velocities. To reach even shorter timescales and observe compressible effects, we are developing a new pump-probe scheme for sub-nanosecond particle tracking. Measurement of Brownian motion in the compressible regime will provide a test of fluid mechanics at previously unexplored spatial and temporal scales and could provide insight into the physical origins of viscosity in dense fluids.