Simultaneous measurements of thermal-gradient driven and Brownian motion in glycerol-water solutions using multiple particle tracking microrheology in ground and microgravity conditions

Thermophoresis, the directed motion of particles along a temperature gradient, is a technique used for separating molecules and colloidal particles. However, few methods exist that can simultaneously measure local fluid properties and thermophoretic motion. This hinders the optimization of thermophoretic separation in complex fluids, where local rheology is temperature-dependent. We use two-dimensional (2D) multiple particle tracking microrheology with a 1D temperature gradient to measure thermal-gradient driven motion in the x-direction and temperature-dependent viscosity from Brownian motion in the y-direction. We measure viscosity and particle motion in Newtonian glycerol-water solutions in ground (0–50 w/w%) and microgravity conditions (20 and 30 w/w%) on the International Space Station (ISS) and compare our results to tabulated data. Microgravity eliminates buoyancy-induced recirculation, enabling measurement of only thermophoretic motion. Viscosities measured in the y-direction in ground and microgravity conditions agree with tabulated values. In ground conditions, measured particle velocities in the x-direction scale with viscosity and thermal gradient, consistent with recirculation. In microgravity, particle motion is Brownian in both directions. Although the apparent Soret coefficient values in microgravity indicate thermophoretic motion, the absence of directed motion and no measurable difference between motion in the x- and y-directions show that thermophoresis is undetectable. This is likely due to an insufficient temperature gradient achievable in our experiments on the ISS. Our method enables measurements of local viscosity orthogonally to an applied temperature gradient and can be used to measure spatial variations in local rheology, which is essential for quantifying thermal-gradient driven motion in complex fluids.