Direct observation of particle microstructure in concentrated suspensions during the falling‐ball experiment

Abstract

It is demonstrated in this study that suspension microstructure and mean fall velocity can be obtained simultaneously in real time during the falling‐ball experiment using nuclear magnetic resonance imaging (NMRI). When the size of the suspending particles is significantly different from that of the falling ball (at least by a factor of 2), the motions of both the falling ball and the surrounding neutrally buoyant particles can be tracked by monitoring the proton NMR signals from the suspending liquid. Using a 50 vol % suspension of approximately 700 μm diameter spheres and a 9 mm diameter cylinder, the suspension microstructure around a 4.76 mm (3/16 in.) diameter falling ball was found to be significantly perturbed by the motions of the ball, whereas the fall velocity decreases with time. The suspension concentration increases at the leading edge and decreases behind the falling ball. These effects diminish as the diameter of the falling ball decreases to 1.59 mm (1/16 in.). Possible mechanisms to account for the observed changes in microstructure are discussed and include many‐body hydrodynamic and boundary effects.