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Topological Sound and Flocking on Curved Surfaces

Suraj Shankar, Mark Bowick, and Cristina Marchetti’s paper has appeared in Physical Review X with a highlight in Physics

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Mark Bowick to give a talk at KITP

"Fragile Objects: The Hard Science of Soft Matter "

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Congratulations to Kazage Utuje

Kazage successfully defended his thesis, titled “Continuum Models of Collective Migration in Living Tissues.”

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Soft Matter is a multi-disciplinary field that explores the science of easily deformable matter. The low cost of deforming soft matter means that entropic contributions play a very important role in the statistical mechanics of soft materials.  When entropic fluctuations dominate soft matter systems often exhibit markedly different behavior than hard systems. While most hard materials expand when heated many flexible polymers instead shrink when heated since there are many more coiled configurations available than stretched configurations. Thus they have a negative coefficient of thermal expansivity. Polymerized or elastic membranes exhibit a remarkable extended phase described by a rough surface embedded in three-dimensional flat space. The stability of this extended membrane phase arises from thermal fluctuations, which act like isotropic corrugations to stiffen the membrane to bending on long length scales. If you pull on a sheet of rubber longitudinally it shrinks transversely. The fractional shrinking compared to the fractional extension is measured by the so-called Poisson ratio. In contrast, when you pull on a flexible elastic membrane it expands in the transverse direction! The entropic fluctuations are ironed out in the direction under tension and this spread in the transverse direction.