Computations in Science Seminars

Previous Talks: 2019

Jan 2019
16
Wed 12:15
Margaret Gardel, University of Chicago
e-mail:
Host: Arvind Murugan ()
Organizer: Steven Strong ()
Controlling the Shape of Cells within Tissue

Mature epithelial tissues have distinct cellular architecture, which is maintained despite externally applied forces, wounding, and cell division or death. Here we investigate how a model tissue develops and maintains cellular structure by quantifying single cell dynamics and cell shape in newly formed monolayers of MDCK cells. Cells initially aggregate through a process resembling wound healing into a confluent monolayer with elongated cells that remain motile. After formation, individual monolayers evolve over time to reach a similar final state with more hexagonal cell shapes and arrested dynamics, resembling mature epithelial tissues. By quantifying cell trajectories, we observe glassy dynamics controlled by cell shape, which have been previously predicted by vertex models. On substrates of different stiffness, monolayers form and evolve with different cell number density but the same relationship between cell shape and speed suggesting that the dynamics are density independent. We find when inhibiting several regulators of the actin cytoskeleton that cell speed and shape remain correlated but the correlation is shifted toward more elongated cell shapes. The magnitude of this shift differs for each inhibitor but velocity correlation length decreases proportionately to the change in final cell shape. We show that these results can be recapitulated in vertex models which incorporate polarization coupling between neighboring cells. Our results demonstrate that multicellular coordination of cell motility plays an important role in regulation of cell shape and determination of final tissue structure.

Jan 2019
23
Wed 12:15
Shmuel Rubinstein, Harvard
e-mail:
Host: William Irvine ()
Organizer: Grayson Jackson ()
The physics of crushing and smashing: Cascades and cataclysmic change

Many of the big problems we are facing involve far from equilibrium systems that entail a cataclysmic change. Climate, turbulence and earthquakes, developmental biology, evolution and even aging and death. These phenomena are rare (sometimes occurring only once) and are entirely irreversible. While understanding the physics of such irreversible processes is of both fundamental and practical importance, these problems also pose unique challenges. These challenges, as they manifest in turbulence, were beautifully portrayed by Richardson:

“Big whirls have little whirls that feed on their velocity, and little whirls have lesser whirls and so on to viscosity” Lewis Fry Richardson (1922)

In his short verse, Richardson captures the essence of the turbulent cascade—the conveyance of kinetic energy across scales that underlies the universal dynamics of turbulent flows. Indeed, such conveyance of important physical quantities (energy, stress, frustration and even information) down and up a vast range of scales underlines the dynamics of many systems. The same applies to how a multi-contact frictional interface will form and break or how correlated defect structures determine the strength of a space-rocket, how an intricate network of creases will form when we crumple a thin sheet or when soda can is smashed. The challenge in understanding these systems is in capturing the events as they occur, keeping up with the dynamics on all scales and at all times. Here, I will review our work on several key irreversible system and introduce the new tools we developed to address their unique evolution and discuss the interesting physics we learned.

Jan 2019
30
Wed 12:15
CANCELLED: Xiang Cheng, University of Minnesota
Rescheduled for Feb. 27, 2019
Feb 2019
6
Wed 12:15 PM
Andrej Košmrlj, Princeton University
e-mail:
Host: Arvind Murugan ()
Organizer: Elizabeth Lee ()
Phase separation in multicomponent liquid mixtures

Multicomponent systems are ubiquitous in nature and industry. While the physics of binary and ternary liquid mixtures is well-understood, the thermodynamic and kinetic properties of N-component mixtures with N>3 have remained relatively unexplored. Inspired by recent examples of intracellular phase separation, we investigate equilibrium phase behavior and morphology of N-component liquid mixtures within the Flory-Huggins theory of regular solutions. In order to determine the number of coexisting phases and their compositions, we developed a new algorithm for constructing complete phase diagrams, based on numerical convexification of the discretized free energy landscape. Together with a Cahn-Hilliard approach for kinetics, we employ this method to study mixtures with N=4 and 5 components. In this talk I will discuss both the coarsening behavior of such systems, as well as the resulting morphologies in 3D. I will also mention how the number of coexisting phases and their compositions can be extracted with Principal Component Analysis (PCA) and K-Means clustering algorithms. Finally, I will discuss how one can reverse engineer the interaction parameters and volume fractions of components in order to achieve a range of desired packing structures, such as nested "Russian dolls" and encapsulated Janus droplets.

Feb 2019
13
Wed 12:15
Jörn Dunkel, MIT
e-mail:
Host: William Irvine ()
Organizer: Peter Chung ()
Wrinkles and spaghetti

Buckling and fracture are ubiquitous phenomena that, despite having been studied for centuries, still pose many interesting conceptual and practical challenges. In this talk, I will summarize recent experimental and theoretical work that aims to understand the role of curvature and torsion in wrinkling and fragmentation processes. First, we will show how changes in curvature can induce phase transitions [1] and topological defects [2] in the wrinkling patterns on curved elastic surfaces. In the second part, we will revisit an observation by Feynman who noted that spaghetti appears to fragment into at least three (but hardly ever two) pieces when placed under large bending stresses. Using a combination of experiments, simulations and analytical scaling arguments, we will demonstrate how twist can be used to control binary fracture of brittle elastic rods [3].

[1] Nature Materials 14, 337 (2015) [2] PRL 116: 104301 (2016) [3] PNAS 115: 8665 (2018)