A high-temperature and -pressure microscope cell to observe colloidal behaviors in subcritical and supercritical water: Brownian motion of colloids near a wall

A high-temperature and -pressure cell for in situ optical microscopy was constructed, and used to study behavior of polystyrene latex (d = 2.092 μm) in water at temperatures between 298 and 573 K at constant pressure of 24.6 MPa. Measured diffusion coefficients of the latex near the quartz surface were smaller than those predicted by Stokes–Einstein equation, and the deviation became larger as temperature was raised. Analysis considering the wall drag effect revealed that the colloids get closer to the wall as temperature is increased. This was ascribed to reduction of buoyant force due to decrease of water density at high temperatures (from 1008 kg/m3 at 298 K to 742.7 kg/m3 at 573 K at 24.6 MPa).