Measuring Absolute Velocities from Nonequilibrium Oscillations via Single-Detector 3D Dynamic Light Scattering.

Single-detector 3D dynamic light scattering (3D DLS) emerges as a reliable technique to determine the drift velocity of out-of-equilibrium colloidal particles. In particular, our investigation reveals the appearance of oscillations of a well-defined frequency in the autocorrelation function of the scattered intensity when particles are immersed in a medium exposed to thermally induced convection. These oscillations arise as a consequence of the directed motion of particles due to the convection of the fluid. The experimental results obtained for different colloidal systems are corroborated by a theoretical model and thoroughly validated with fluid dynamics and Brownian dynamics simulations. The excellent agreement between experimental, theoretical and simulation data allows us to provide a solid and comprehensive explanation of the observed physical phenomena. This Letter, using an advanced dynamic light scattering technique, offers insights into the field of nonequilibrium particle dynamics, applicable not only to colloidal suspensions affected by steady-state diffusion-convection but also to other nonequilibrium scenarios, such as systems driven by external fields (e.g., gravitational, electric or magnetic fields).