Morphogenesis is a developmental process through which plants and animals acquire their shape and form. This talk will focus on tissue mechanics underlying massive transformations in the shape of epithelial tissues during gastrulation of animal embryos. This transformation of tissue is often described in terms of a (visco-elastic) flow of cells driven by active, myosin motor generated, internal forces. Yet, despite their apparent “fluidity” epithelial tissue can support shear stress, so a deeper examination is in order. This talk will argue that morphogenetic transformations of tissue can be understood in terms of an adiabatic remodeling of the force balance governed by myosin motor activity and controlled by feedback. The ability of cells to directly control active cytoskeletal tension -much like muscles! - deprives tissue mechanics of a constitutive relation, pulling the rug under conventional elasticity as a starting point for a theory. This talk will formulate an alternative continuum mechanics of Active Solids, starting with prescribed internal active tension, that will exhibit “emergent elasticity” on large scales and provide a principled description of active plastic flow as controlled “morphing” transformation of shape that takes place on the force-balanced manifold. This new theory of Active Solids will also provide an example of how the study of biological phenomena pushes the envelope of Physics beyond what we have learned from textbooks.

Many energy, environmental, industrial, and microfluidic processes rely on the viscous flow of polymer solutions through porous media. These fluids are typically shear-thinning; however, these solutions can unexpectedly flow thicken when forced through confined, tortuous spaces such as in porous media. The reason why has been a puzzle for over half a century. In this talk, I will describe how by directly visualizing the flow in a transparent 3D porous medium, we have found that this anomalous flow thickening reflects the onset of an elastic instability in which the fluid exhibits chaotic velocity fluctuations reminiscent of inertial turbulence, despite the vanishingly small Reynolds number. In addition to characterizing this fascinating flow state, we have found that this phenomenon can be harnessed for improving mixing and the efficiency of flow-mediated chemical reactions — with implications for a broad range of processes that are typically limited by poor mixing.

Replaced by Center for Living Systems student-organized special lecture.