In the Losert lab we deal with collective dynamics of many-body systems. Strange behavior often results when the particles that make up a material are not microscopic. For instance, a pile of sand can act like a solid; it supports your weight on the beach. On the other hand, it will flow like a liquid if you pour it out of bucket. This behavior has implications for natural phenomena like earthquakes and blood clots, as well as man-made problems like traffic jams and making toothpaste.

1) Impacts into Granular Materials

with Matt Harrington and Emily Lim

Granular materials, like sand, are inherently fragile. They can appear at rest but a slight disturbance can cre an avalanche or failure within the material.

Considering this fragile state, if you drop (or shoot) something into a granular material, it’s unsurprising that t object will go into the material. But the material is also surprisingly strong: a high energy impact will go into t material relatively less than a low energy impact. In other words, if you hit a granular material harder, it will resist more. Force laws have been measured for these systems, but there is no theory explaining why they work.

To understand this phenomenon, we are taking the perspective of the grains. With refractive index-matched scanning, we are able to see inside a granular material, and track the motions of the grains. Thus, we hope to finally gain insight into the dynamics of granular impact by looking into the microscopics.

2) Rotations in Granular Materials

with Matt Harrington and Michael Lin

In a static granular system, Newton’s laws may be satisfied with a wide variety of contact forces between particles, even for the same particle arrangements. The culprit for this confusion is friction: static friction can vary up to its threshold. Further complicating things, many simulations of granular materials don’t even consider friction, but surely it must play a role in real systems. A key to understanding the role of friction is to look at particle rotations in slow flows. Using refractive index-matching we are addressing this mystery of granular materials, empirically measuring particle rotations.

3) Secondary Flows in Granular Materials

with Naomi Murdoch and Patrick Michel

If you excite a granular material by shaking a jar of it, the material will obey your commands, and flow back and forth. But there are also secondary flows happening. For example, if the particles aren’t all exactly the same size, the large ones will gradually come to the top; the so-called “Brazil nut” effect. Such secondary flows are extremely important for industrial applications, it estimated about 10% of the world’s energy is consumed processing granular materials. By tracking particles, we can measure these secondary flows, a measure their dependence on parameters like particle-particle friction. We can also do extreme things like put an experimental system on a vomit comet to analyze the influence of gravity.

4) Rearrangements in Epithelial Cell Migration

with Rachel Lee, Doug Kelley, and Nick Ouellette

Collections of cells and collections of grains are analogous systems. Of course, they are both many-body systems.

They are both sensitive to parameters such as density. Further, they each have their own particle-particle interactions. Grains interact via classic contact mechanics. Cells have a less well-defined interaction, but it is some recipe involving mechanical and chemical signaling between cells.

But the analogy breaks down: cells are alive, grains are passive.

Using the same data analysis techniques used for granular materials, we can look at the statistics of these many-body active systems and see how they are similar or different from passive systems to gain insight into how collective cell migration works.

5) Rearrangements in Embryogenesis

with Philipp Keller and Fernando Amat

When the cells in an embryo divide and eventually conspire to form a creature, they are crowded, subject to fluid flows, and a host of other chemical and mechanical signals. Yet somehow they are able to sort through this noise and pick up cues about where to go and what kind of cell to become. We investigate mechanisms such as deformations and rearrangements the early development of organisms.