Driscoll Lab

Instabilities in freely flowing dense suspensions


Impact of a 3 mm colloidal suspension drop at 2 m/s at various volume fractions (top: 44%, middle: 48%, bottom: 50%). As the volume fraction of silica particles is increased, the impact changes from liquid-like to completely solid-like.

Research Highlight: "Watch this strange fluid act like a solid and liquid at the same time"

Video summary: 2019 DFD Gallery of fluid motion entry

Recently, we have begun a new direction exploring free-surface flows of colloidal suspensions. We are studying colloidal suspensions not in bulk and sealed in a sample chamber, but those that are unbounded and free to flow. As illustrated by the videos at left and right, the unusual mechanical response of this material strongly alters the dynamics of the destabilizing fluid. These types of systems couple together the complex problem of multiphase flow and the rich rheology of colloidal fluids.

We have begun our free-surface studies by looking at the splashing colloidal drops and the bursting of thin colloidal films. These two systems represent classic, well-studied hydrodynamic problems, which we use as a starting point for understanding how the non-Newtonian rheology of a dense suspension can change bulk behavior. Both drops and bubbles experience extremely high flow rates, allowing us to probe suspension rheology at much higher shear rates than are typically accessible via standard rheometry. This lets us probe regimes of behavior that are not accessible by other means, and explicitly study how boundaries contribute to rheological phenomena such as shear jamming.


Bursting of a thin liquid film composed of colloidal suspension stabilized with surfactant. A variety of instabilites can appear as the film breaks apart, from wrinkling, to folding, to cracking.

Driven colloidal suspensions


A cascade of instabilities: A shock front quickly develops in the uniform suspension, and then this shock becomes unstable, emitting fingers whose wavelength is set by the particle-floor gap. As the instability continues to evolve, the fingertips can break off into stable, self-sustained structures.

Hydrodynamic trappping of a microroller by an individual obstacle. Flucuations are required to enable trapping, as the attractor point lies inside a seperatix in the flow field generated by the microroller. Thus the microrollers have no path to reach the attractor unless they are aided by a random kick that pushes them inside the speratrix.

Research Highlight: "A surprising way to trap a microparticle"

Research Highlight: "Colloids: A microscopic army"

2016 DFD Gallery of Fluid Motion entry

We have been exploring emergent structure in a driven system of colloidal particles, magnetic microrollers, which are activated by a rotating magnetic field. These particles sediment near to the bottom floor of the sample chamber due to gravity, but hover slightly above it because of thermal flucuations. When the particles are rotated with an external field, this leads to both translation and strong advective flows, even very far from the particle. This flows create strong collective effects which can strongly modify suspension flow, and lead to a rich array of emergent structure in this system, from shocks, to fingering patterns, to stable hydrodynamically-bound clusters. We study these emergent structures using a variety of computational and experimental techniques, including Stokesian dynamics simulations (in collboration with Blaise Delmotte, Aleks Donev, and Brennan Sprinkle), and a coupled magnetic-optical imaging system. We find that in all cases, the size scale of the emergent structures is coupled to a single geometric parameter: the height the rollers hover above the wall.

In order to leverage colloidal swimmers in microfluidic applications, it is crucial to understand their interaction with surface features such as obstacles. We observe hydrodynamic trapping in our system, that is the microrollers can be caught and trapped as they move past an obstacle. This trapping has two remarkable features: it only happens in the wake of the obstacle and, more importantly, it can only occur with Brownian motion. While noise is usually needed to escape traps in dynamical systems, in this case it is the only means to reach a hydrodynamic attractor. We are currently exploring the interaction between microrollers and more complex structured environements, and how these interactions can be used for shaping materials.

Material Failure


Ladder Fracture picture.
A PEGDA/PEGMEA hydrogel rupturing due to imbibement-induced stresses. Our work identified a new control parameter for this complex rupture process, illustrating the importance of considering dynamically changing material propoerties.
Honeycomb Fracture picture.
Experimental images of meta-material failure. Left: A rigid lattice is rapidly broken in two by a running crack. Right: As the material becomes more and more floppy (easier to shear than stretch), failure becomes slow and diffuse.

A material's response to forcing is ultimately controlled by its structure, although these connections are not always well understood, especially in soft materials. Fracture offers a unique way to probe a material; how something falls apart can reveal what was holding it together.

My group is currently exploring these ideas in a hydrogel system (PEGDA/PEGMEA), which allows facile control of elastic moduli by adjusting the crosslinker mol fraction. We have focused understanding rupture processes induced by swelling, a highly underexplored regeime. Our recent work has highlighted that to understand material failure in these conditions, one must consider temporally varying material propoerties.


previous work with with Sid Nagel (UChicago), Bryan Chen, Thomas Beuman, Vincenzo Vitelli (Leiden University)

Research Highlight: "Physicists gain insight into why materials break"

Solids can break in many ways, from slow, plastic deformation to rapid and catastrophic shattering. In collaboration with the Vitelli group, I studied the failure of meta-materials; structures that can designed to have tunable elastic properties. We showed that as these materials approach a rigidity transition, their failure behavior changes dramatically, from slow, diffuse failure to a rapid, running crack. Understanding and controlling this failure behavior offers both a new tool for measuring soft material properties as well as a way to design materials with tunable, engineered failure properities.

These ideas also translate to a model 1D system: a perforated geometry, created by introducing holes into a solid sheet. Even in this simple model system, a dramatic transition in failure mode occurs as a function of geomtery: rapid failure via a running crack is completely replaced by random breaking. Thus, by choosing appropriate geometric parameters, the entire dynamics and structure of the failure process can be controlled.