When a dense suspension is squeezed from a nozzle, droplet detachment can occur similar to that of pure liquids. While in pure liquids the process of droplet detachment is well characterized through self-similar profiles and known scaling laws, we show here the simple presence of particles causes suspensions to break up in a new fashion.

Using high-speed imaging, we find that detachment of a suspension drop is described by a power law; specifically we investigate the scaling of the neck radius near breakup. We demonstrate data collapse in a variety of particle/liquid combinations, packing fractions, solvent viscosities, and initial conditions. We argue that this scaling is a consequence of particles deforming the neck surface, thereby creating a pressure that is balanced by inertia, and show how it emerges from topological constraints that relate particle configurations with macroscopic Gaussian curvature. This new type of scaling, uniquely enforced by geometry and regulated by the particles, displays memory of its initial conditions, fails to be self-similar, and has implications for the pressure given at generic suspension interfaces.

The split personality emerges from the fact that dense suspensions appear viscous to the naked eye and indeed exhibit a neck shape similar to that of pure liquids of similar viscosity, while at the same time their necks thin with time to breakup as if they were totally non-viscous like water.

1. •Marc Z. Miskin and Heinrich M. Jaeger, “Droplet Formation and Scaling in Dense Suspensions”, Proc. Nat’l Acad. Sci. 109, 4389-4394 (2012). pdf file
   

2. •article by Steve Koppes in the UofC Chronicle, with background about the work and expanding on the split personality aspects
   

The figure above contrasts droplet formation in dense suspensions and pure liquids. (A) Images of a suspension droplet made from 850 μm zirconium dioxide suspended in water, detaching from a 14.5-mm diameter nozzle. The symmetric profile maintains itself until the neck is only one particle thick, and the small liquid bridge adjoining particles ruptures (see left movie below). (B) Schematic of our experimental configuration, which uses two cameras to achieve a spatial resolution of 4.4 μm and a temporal resolution of 10^(−4)s. (C and D) Comparison of 145 μm zirconium dioxide suspended in water (C) and pure 50 cst silicone oil (D), both exiting a 4.7-mm diameter nozzle (see middle and right movies, below). Each panel is an order of magnitude closer to breakup at time τ = 0. Note the asymmetry and increased elongation exhibited by the pure fluid.

Left: This video, lighted from the front, shows a droplet of water containing zirconium dioxide particles measuring 850 microns in diameter detaching from a nozzle. The suspension neck maintains a symmetric profile until the neck gradually narrows to a width of only one particle, when the liquid surrounding the particles ruptures.

Middle: Viscous silicon oil detaching from a nozzle. This video is lighted from the back and combines data from two cameras (see sketch of set-up in panel (b) of the figure above). The droplet neck maintains a symmetric profile at first, but then becomes asymmetric as the neck becomes ever longer.

Right: This video contrasts the behavior of the silicon oil with that of a droplet of water containing zirconium dioxide particles measuring 145 microns in diameter. This video and its companion in the middle were taken under the same experimental conditions (back light, same nozzle diameter)  Furthermore, the oil and the suspension shown in this movie both have the same nominal viscosity. Yet the two behave completely differently. The profile of the detaching droplet neck remains symmetric right up to the final moment of rupture and it becomes thinner over time in a manner unlike the pinch off of the viscous fluid.