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Colloquium Schedule

Spring 2019

The Evolution of Equilibrium: Soft Matter Physics in Biology

Alison Sweeney, Yale University
Wed., Oct. 2, 2019, 4:30 PM in Science Center 199

Condensed matter physics concerns itself with the interplay between phases of matter predicted by physical theory and the realization of those organizations of matter in nature and in experiment. When it comes to soft condensed matter, arguably the best possible laboratory for finding novel phases of soft matter is in the evolution of life on Earth. This talk will focus on the identification of two arguably novel phases of matter predicted by soft matter theory in evolved systems, that of "patchy colloids" in squid lenses, and of "spatially modulated phases" in pollen grains.

A Crisis in Cosmology?

Tristan Smith, Swarthmore College
Wed., Oct. 30, 2019, 4:30 PM in Science Center 199

I will summarize the Hubble tension - a statistically significant disagreement between several different ways of inferring the current rate of the expansion of the universe - and my sabbatical work trying to search for a theoretical model to resolve this tension. Planned space and ground observations promise to shed light on this tension and either establish it as a clear disagreement with our standard cosmological model or as an indication that we have significantly underestimated the systematic uncertainties of a range of observations.

The Serpent’s Maw: A Physics Perspective on Mouth Function and the Dynamics of Hydra Regeneration

Eva-Maria Collins, Swarthmore College
Fri., Nov. 15, 2019, 12:30 PM in Science Center 199

Named after the monster from the Greek mythology, the freshwater polyp Hydra is famous for its regenerative abilities. While the real Hydra is a few mm small, inconspicuous looking animal, it outperforms the fictional creature by its ability to regenerate from small tissue pieces or from cell aggregates after disintegration into individual cells. This process of self-organization of an initially near-uniform cell ball into an animal with a well-defined head-foot body axis poses fundamental questions regarding the cross-talk of biochemical and physical signaling driving organismal patterning.

Hydra’s self-organizing properties following disintegration into individual cells was first recognized more than 40 years ago. However, what drives cell sorting during regeneration remained debated as existing studies failed to distinguish between different driving mechanisms. Using a combined experimental-theoretical approach, we have recently settled this debate and shown that tissue interfacial tensions drive cell sorting. Furthermore, once sorting is complete or excised tissue pieces have rounded up, the Hydra sphere hollows out and undergoes osmotically driven shape oscillations. The sphere eventually breaks shape symmetry to form an ellipsoid, defining the future head-foot polarity of the adult polyp. It has been proposed that the shape oscillations are necessary for symmetry breaking and successful regeneration. Existing mathematical models assume that a shift in the frequency of shape oscillations of regenerating Hydra spheres coincides with symmetry breaking and axis specification. Recent work in my group breaks this link and suggests that the oscillation pattern is not indicative of symmetry breaking. Instead, the oscillation pattern shift is a direct consequence of mouth function and its use in osmoregulation. As the link between oscillation dynamics and axis specification was a key assumption in current mathematical models of Hydra regeneration, our results require that we reexamine the mechanisms driving pattern formation.