Scott Waitukaitis

High-speed tracking of rupture and clustering in freely falling granular streams

Thin streams of liquid commonly break up into characteristic droplet patterns owing to the surface-tension-driven Plateau–Rayleigh instability. Very similar patterns are observed when initially uniform streams of dry granular material break up into clusters of grains, even though flows of macroscopic particles are considered to lack surface tension. Recent studies on freely falling granular streams tracked fluctuations in the stream profile, but the clustering mechanism remained unresolved because the full evolution of the instability could not be observed. Here we demonstrate that the cluster formation is driven by minute, nanoNewton cohesive forces that arise from a combination of van der Waals interactions and capillary bridges between nanometre-scale surface asperities. Our experiments involve high-speed video imaging of the granular stream in the co-moving frame, control over the properties of the grain surfaces and the use of atomic force microscopy to measure grain–grain interactions. The cohesive forces that we measure correspond to an equivalent surface tension five orders of magnitude below that of ordinary liquids. We find that the shapes of these weakly cohesive, non-thermal clusters of macroscopic particles closely resemble droplets resulting from thermally induced rupture of liquid nanojets.

Impact-activated solidification of dense suspensions via dynamic jamming fronts

Although liquids typically flow around intruding objects, a counterintuitive phenomenon occurs in dense suspensions of micrometre-sized particles: they become liquid-like when perturbed lightly, but harden when driven strongly. Rheological experiments have investigated how such thickening arises under shear, and linked it to hydrodynamic interactions or granular dilation. However, neither of these mechanisms alone can explain the ability of suspensions to generate very large, positive normal stresses under impact. To illustrate the phenomenon, such stresses can be large enough to allow a person to run across a suspension without sinking, and far exceed the upper limit observed under shear or extension. Here we show that these stresses originate from an impact-generated solidification front that transforms an initially compressible particle matrix into a rapidly growing jammed region, ultimately leading to extraordinary amounts of momentum absorption. Using high-speed videography, embedded force sensing and X-ray imaging, we capture the detailed dynamics of this process as it decelerates a metal rod hitting a suspension of cornflour (cornstarch) in water. We develop a model for the dynamic solidification and its effect on the surrounding suspension that reproduces the observed behaviour quantitatively. Our findings suggest that prior interpretations of the impact resistance as dominated by shear thickening need to be revisited.

Origami multistability: from single vertices to metasheets.

We show that the simplest building blocks of origami-based materials-rigid, degree-four vertices-are generically multistable. The existence of two distinct branches of folding motion emerging from the flat state suggests at least bistability, but we show how nonlinearities in the folding motions allow generic vertex geometries to have as many as five stable states. In special geometries with collinear folds and symmetry, more branches emerge leading to as many as six stable states. Tuning the fold energy parameters, we show how monostability is also possible. Finally, we show how to program the stability features of a single vertex into a periodic fold tessellation. The resulting metasheets provide a previously unanticipated functionality-tunable and switchable shape and size via multistability.

Dynamic jamming fronts

We describe a model experiment for dynamic jamming: a two-dimensional collection of initially unjammed disks that are forced into the jammed state by uniaxial compression via a rake. This leads to a stable densification front that travels ahead of the rake, leaving regions behind it jammed. Using disk conservation in conjunction with an upper limit to the packing fraction at jamming onset, we predict the front speed as a function of packing fraction and rake speed. However, we find that the jamming front has a finite width, a feature that cannot be explained by disk conservation alone. This width appears to diverge on approach to jamming, which suggests that it may be related to growing lengthscales encountered in other jamming studies.

Results and Conclusions

The resistance of dense suspensions to intruding objects has typically been associated with shear thickening, but in this thesis we have shown that dynamic jamming of the suspension is the cause. In this final chapter, we review and gather all of the evidence we have found to support this claim, as well as give an outlook for future questions and experiments.

In situ granular charge measurement by free-fall videography

We present the design and performance characterization of a new experimental technique for measuring individual particle charges in large ensembles of macroscopic grains. The measurement principle is qualitatively similar to that used in determining the elementary charge by Millikan in that it follows individual particle trajectories. However, by taking advantage of new technology we are able to work with macroscopic grains and achieve several orders of magnitude better resolution in charge to mass ratios. By observing freely falling grains accelerated in a horizontal electric field with a co-falling, high-speed video camera, we dramatically increase particle tracking time and measurement precision. Keeping the granular medium under vacuum, we eliminate air drag, leaving the electrostatic force as the primary source of particle accelerations in the co-moving frame. Because the technique is based on direct imaging, we can distinguish between different particle types during the experiment, opening up the possibility of studying charge transfer processes between different particle species. For the ∼300 μm diameter grains reported here, we achieve an average acceleration resolution of ∼0.008 m/s(2), a force resolution of ∼500 pN, and a median charge resolution ∼6× 10(4) elementary charges per grain (corresponding to surface charge densities ∼1 elementary charges per μm(2)). The primary source of error is indeterminacy in the grain mass, but with higher resolution cameras and better optics this can be further improved. The high degree of resolution and the ability to visually identify particles of different species or sizes with direct imaging make this a powerful new tool to characterize charging processes in granular media.

Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing

Water droplets skid across hot surfaces, hovering imperceptibly as they undergo rapid vaporization. Elastic solids are now shown to exhibit a variant of this behaviour, engaging in sustained bouncing by coupling vapour release to elastic deformation. The Leidenfrost effect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover1,2. Although well understood for liquids1,2,3,4,5,6,7,8,9,10,11,12,13,14 and stiff sublimable solids15,16,17,18, nothing is known about the effect with materials whose stiffness lies between these extremes. Here we introduce a new phenomenon that occurs with vaporizable soft solids—the elastic Leidenfrost effect. By dropping hydrogel spheres onto hot surfaces we find that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, we uncover high-frequency microscopic gap dynamics at the sphere/substrate interface. We show how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapour and sustain the bouncing. Our findings suggest a new strategy for injecting mechanical energy into a widely used class of soft materials, with potential relevance to fields such as active matter, soft robotics and microfluidics.

From nanoscale cohesion to macroscale entanglement: Opportunities for designing granular aggregate behavior by tailoring grain shape and interactions

The packing arrangement of individual particles inside a granular material and the resulting response to applied stresses depend critically on particle-particle interactions. One aspect that recently received attention are nanoscale surface features of particles, which play an important role in determining the strength of cohesive van der Waals and capillary interactions and also affect tribo-charging of grains. We describe experiments on freely falling granular streams that can detect the contributions from all three of these forces. We show that it is possible to measure the charge of individual grains and build up distributions that are detailed enough to provide stringent tests of tribo-charging models currently available. A second aspect concerns particle shape. In this case steric interactions become important and new types of aggregate behavior can be expected when non-convex particle shapes are considered that can interlock or entangle. However, a general connection between the mechanical response...

Freely Accelerating Impact into Cornstarch and Water Suspensions

This chapter is dedicated to an experimental study of the response of dense suspensions of cornstarch and water to surface impact.

Dynamic Jamming Fronts in a Model 2D System

In the previous chapter, it was shown that the behavior of the suspension directly below the rod suggests that the suspension solidifies as a consequence of impact. This process was reminiscent of a snowplow. In this chapter, we construct a model experiment to study this snowplow solidification.