Pendant drop thread dynamics of particle-laden liquids

Temporal evolution of pendant drops toward pinch-off is studied for a particle-laden liquid. Stretching and thinning of the necking region, or thread, are quantified for suspensions of monodisperse noncolloidal particles over a range of particle volume fractions, phi. The stretching is characterized by the length of connected material attached to the orifice as this evolves from the quasistatic drop to bifurcation. Drop formation from suspensions of 0 <=, phi <= 0.4 was studied and compared with the same process for liquids of varying viscosity but the same surface tension as the suspension. Two orifice sizes and two noncolloidal particle sizes, at ratios of orifice to particle diameter of 12.8 and 25.6, were studied at tube-scale Reynolds numbers of Re = 10(-2), capillary numbers of Ca < 0. 1 and Bond numbers of 0. 5 < Bo < 2.5. At large times prior to thread rupture, the added viscosity imparted to the mixture by the particles causes a slower thinning than is observed for the pure liquid. As rupture is approached, thinning at the narrowest part of the thread is more rapid for suspensions than for pure liquid. More viscous pure liquids result in longer thread length at rupture, while the presence of particles, even at small phi, results in shorter connected length at rupture. These observations support a two-stage description of the drop formation process from suspensions. The first stage (far in time from rupture) is described well by a continuum representation of the particle phase resulting in an empirical increased effective shear viscosity, while the second stage (near in time to rupture) requires consideration of the finite-sized and discrete particles. The second stage evolution involves a localized thinning of the thread through a short axial span of dilute or particle-free material, and this localization results in the more rapid thinning and shorter thread lengths at rupture in the suspensions. A clear delineation in time between the first and second stages has not been identified. (C) 2006 Elsevier Ltd. All rights reserved.