We aim to develop a resource-efficient particle separation technique to help create sensitive single-step diagnostic tests that quantify the viral load by separating and concentrating viral particles from a biological sample. To drive separation of micron-sized charged colloidal particles from a bulk solution in a small sample, we study thermophoresis, directional motion of particles due to a near surface ion flux in response to a temperature gradient. To capture this motion, we use 2D multiple particle tracking microrheology (MPT), a passive microrheological technique that quantifies Brownian particle motion and relates it to material rheological properties. Using MPT to measure particle motion while a 1D thermal gradient is applied to the sample enables simultaneous measurements of local temperature-dependent fluid viscosity and thermophoretic particle movement. Measuring local rheological properties is crucial for optimizing rheology-dependent thermophoretic separation in complex biological fluids with variable properties. One additional challenge is the recirculation that occurs due to buoyancy-driven flow. Thus, these experiments have been performed both on Earth and on the International Space Station under microgravity conditions. To verify this technique’s accuracy in simultaneously measuring viscosity and particle velocity, we analyze Brownian and thermophoretic motion of particles in Newtonian solutions with varying glycerol concentrations. Viscosities from the measured Brownian component are consistent with tabulated viscosities for glycerol solutions, suggesting the reliability of this method. Experimental thermophoretic diffusion and velocity values are within range of current theoretical estimates, confirming particles are translating due to thermophoresis. While current theories on what drives thermophoresis in complex fluids are not yet experimentally confirmed, this novel technique allows us to characterize thermophoresis in fluids with variable properties. By quantifying how temperature-dependent material properties, such as viscosity, affect the extent of thermophoretic particle diffusion, we can optimize thermophoresis-based separation in complex fluids and advance the design of more robust bioseparations techniques.
This work was supported by the National Science Foundation (NSF) and Center for the Advancement of Science in Space (CASIS).
