Viscosity of high temperature liquids via passive microrheology

An instrument employing passive microrheology has been developed for measuring the viscosity of liquids at high temperatures. The viscometer features a dark-field optical microscope, a custom high temperature laser transmission furnace, and flame-sealed capillaries containing microsphere suspensions of test liquids. The Brownian trajectories of individual microspheres were captured in long image sequences and analyzed for their mean square displacement, which provides viscosity via the Stokes-Einstein-Sutherland relation. The viscometer was validated at room temperature with glycerol-water mixtures and at high temperature with water and molten nitrate salt. The measured viscosity was in good agreement with the literature values of each liquid across all temperatures studied (20-450 °C). The measured diffusion coefficient and liquid viscosity achieved <1% and 2%-3.3% uncertainty, respectively, where the latter was limited by the coefficient of variation of the microsphere size distribution. The viscometer could reach temperatures of up to 760 °C, <1 mL sample sizes, high throughput capability with ≈1 min acquisition time, and low cost sample vessels. Importantly, the viscometer recovers the dynamic viscosity without requiring knowledge of either material properties nor a calibration liquid.