A study of dense suspensions climbing against gravity

Abstract

Dense suspensions have previously been shown to produce a range of anomalous and gravity-defying behaviors when subjected to strong vibrations in the direction of gravity. These behaviors have previously been interpreted via analogies to inverted pendulums and ratchets, language that implies an emergent solid-like structure within the fluid. It is therefore tempting to link these flow instabilities to shear jamming (SJ), but this is too restrictive since the instabilities can also be observed in systems that shear thicken but do not shear jam. As an alternative perspective, we re-frame earlier ideas about “racheting” as a “negative viscosity” effect, in which the cycle-averaged motion of a vibrated fluid is oriented opposite to the direction implied by the cycle-averaged stresses. Using ideas from the Wyart and Cates modeling framework, we predict that such a “negative viscosity” can be achieved in shear flows driven by oscillating stress with both square and sinusoidal wave forms. We extend this same modeling approach to study falling films in a vibrating gravitational field, where we similarly find it is possible to attain an overall flow opposite the direction of gravity. Preliminary experimental findings are also provided in support of the modeling work. • Vertically vibrated dense suspensions can climb against gravity. • We explain this phenomena in terms of an “effective” negative viscosity. • Mechanism is explored in detail using simple modeling tools. • Preliminary experiments reveal climbing on vertically oriented surfaces. • Experimental observations are consistent with model predictions.