THE INFLUENCE OF VISCOSITY ON THE STRENGTH OF AN AXIALLY STRAINED PENDULAR LIQUID BRIDGE

Pendular liquid bridges between two relatively moving solid particles (dynamic bridges) occur extensively throughout the powder technology industry, with a typical example being binder granulation. By means of a sensitive linear voltage differential force tranducer, the influence of viscosity on the strength of an axially, periodically strained pendular bridge was investigated where bridge strength is defined as the maximum interparticle force developed during one complete sphere oscillation. The relative importance of the theoretically derived dimensionless groups governing bridge strength is experimentally established for systems exhibiting complete solid-liquid wettability. A simplified solution of dynamic bridge strength based on the superposition of lubrication theory and circular approximation is presented. For a small gap distance with sufficient bridge volume and in the limit of small Reynolds number, a good agreement between the experimental and present theoretical axial force response is observed, indicating the importance of the capillary number Ca in determining pendular bridge strength. The present quasi-static solution is zeroeth-order in Ca and gap distance, and is expected to break down with increasing local inertial effects. Such inertial effects are governed by a modified Bond number. In the limit of low Ca, they lead to an increase in bridge strength due to an added mass effect, whereas in the limit of high Ca, they lead to a reduced, shifted force response due to an insufficient rate of vorticity propagation. Under industrially relevant conditions, the strength of the dynamic bridge was observed to exceed the static by over an order of magnitude. Since a significant number of powder processes such as granulation are not static in nature, viscous interactions between particles should be considered whenever a liquid interstitial phase is present. © 1990.