Simple Newtonian liquids, such as water, splash and form droplets in a way that seems familiar (for the kind of surprises found even in these cases, see the research by our colleagues Sid Nagel and Wendy Zhang). But what happens when a non-Newtonian liquid breaks apart? Here non-Newtonian refers to a type of liquid behavior where the viscosity is not constant but depends on the stresses or shear rates applied.  Examples are  shear-thinning fluids, which only flow at large applied stress and, if they exhibit a yield stress, can be almost solid-like at rest. Or they can be shear-thickening fluids, which behave the other way around and turn solid-like at large applied stress.  In all of these cases, the way they break apart into droplets or form splats on a flat, hard surface can be strikingly different from common expectations.

A particularly simple way to introduce non-Newtonian behavior to a liquid is adding solid particles.  Better understanding of the behavior of droplet formation in such suspensions is important for applications including inkjet printing (particle concentrations up to 30%) or DNA micro-arraying.  From a fundamental science point of view, the rupture of a single volume filled with matter to produce two unconnected volumes, a transition between two distinct topologies, plays a key role in phenomena ranging from the breakup of nano-jets to black-string instabilities in general relativity. Droplet formation in simple liquids is particularly notable because it exhibits many of the exotic features of a topological transition, such as singularities and scaling, while being accessible enough for direct experimental examination.  When particles are present, new physics emerges because the deformation of the suspending liquid has to accommodate the particle rearrangements. The resulting feedback between geometry and dynamics gives rise to a scaling behavior very much unlike that found in pure liquids of comparable viscosity.

Our group has been investigating the formation of droplets and the evolution of the associated “necks” as the singular breakup moment is approached, as well as the “splats” that result when these drops hit a surface. We study a range of non-Newtonian fluids, from suspensions to liquid metals. This effort makes extensive use of high-speed video imaging up to several 100,000 frames per second.

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