An experimental study of particle effects on drop formation

The formation of drops of particulate suspensions composed of spherical, neutrally buoyant, noncolloidal particles in a viscous liquid is examined experimentally. The suspensions are investigated over a range of particle volume fractions, phi = n pi d(p)(3)/6 where n is the particle number density and d(p) is the particle diameter (d(p) = 212-250 mum in most of the experiments), and for flow through three tubes of outer diameters d = 0.16, 0.32 and 0.64 cm; the corresponding inner diameters are 0.10, 0.22, and 0.53 cm. Drop formation in the dripping mode and the transition from dripping to jetting are investigated. In the dripping mode, the behavior of low-phi suspensions (phi less than or equal to0.10) is markedly different from that of higher-phi suspensions (0.15less than or equal tophiless than or equal to0.40), with a transition in the qualitative behavior over a relatively narrow range of concentrations. Pinch-off structures for low-phi suspensions are similar to that of the pure liquid, consisting of a long slender liquid thread connecting a hemispherical cap of liquid at the orifice to the nearly spherical forming drop; the structures differ from those of pure liquids in that individual particles, or groups of particles, can be captured in the thread at rupture, leading to new modes of satellite drop formation. At higher phi, the presence of large numbers of particles in the thinning thread during necking results in thick conelike structures termed "spindles'' owing to this labeling of structures of similar geometry observed in electrohydrodynamic sprays. Particles are found to substantially suppress the number of satellite drops at higher phi, but the few satellite drops produced are much larger than observed in pure liquid drop formation. The transition from dripping to jetting occurs at a smaller flow rate at finite phi, with much shorter coherent lengths after jetting than in the case of a pure liquid. The transition becomes less abrupt and more difficult to identify at the highest concentrations examined. (C) 2004 American Institute of Physics.