Department Colloquia
The Physics Department Colloquia are held on Tuesdays at 4 p.m. in Abelson 131.Fall 2019
September 3, 2019
September 10, 2019
Alexander Hensley, "Measuring crystal nucleation and growth of DNA-grafted colloidal particles"
Abstract: Crystallization is a phase transition in which a fluid of particles forms a solid lattice. In nature it produces diamonds and snowflakes, while in industry it leads to the crystalline silicon used in electronic circuits. Micrometer scale particles can also form crystals. For example particles coated in DNA can form a variety of crystals whose structures and properties can be prescribed by the choice of DNA sequences. Is the crystallization of these artificial particles governed by the same physics as the crystallization of diamonds and silicon? I will present progress on an experimental study of the nucleation and growth of colloidal crystals due to DNA hybridization. Specifically, I will describe a microfluidics-based approach in which we produce hundreds of monodisperse, isolated droplets filled with colloidal particles and then track the formation of crystals within each drop as a function of time. We find that the initial nucleation of crystals from a supersaturated solution involves overcoming a free-energy barrier. Furthermore, we find that free-energy barrier to crystallization decreases drastically with increasing temperature and exhibits evidence of heterogeneity in the barrier height, which we hypothesize results from variations in the interaction strengths between particles. We also find that once nucleated, the crystals grow at a rate that is limited by the diffusive flux of colloidal particles to the growing crystal surface. These findings may help us to devise strategies to tune the nucleation rates and crystal growth kinetics independently, which will be helpful as we try to engineer higher quality or more complex self-assembled structures.
Luke Korley, "Lux-Zeplin: The Search for Dark Matter"
Abstract: The Lux-Zeplin experiment will search for dark matter particle interactions with 7 active tonnes of liquid xenon surrounded by 17.5 tonnes of Gd doped organic scintillator, 4850 feet deep in the Black Hills. I will introduce the LZ detector; and give a brief overview of ongoing efforts in the mine and at home in preparation for the first science run.
September 17, 2019
Matthew Headrick, Brandeis University
Abstract: In trying to understand quantum gravity at a fundamental level, one of the most confusing questions is where the degrees of freedom are. So-called holographic dualities help with this question, by showing that certain quantum gravity theories are equivalent to conventional quantum field theories, in which we understand in principle where the degrees of freedom are and how they interact. Using such dualities, a new way of understanding entanglement in quantum gravity, involving so-called “bit threads”, has recently been developed. From this point of view, space becomes a kind of channel for carrying entanglement of fundamental degrees of freedom. We will explain what holographic dualities are, what bit threads are, and what they might tell us about the nature of space in quantum gravity.
September 24, 2019
Daniel J. Cohen, Princeton University
Host: Guillaume Duclos
Abstract: Tissues are wonderfully complex communities of soft, living agents. Emergent behaviors arising from cell-cell interactions contribute to critical behaviors such as collective motion, healing, and coordinated growth. Tools that can harness or bias these processes allow us a new way to manipulate living tissue dynamics with broad fundamental and biomedical applications. Our work draws from soft matter biophysics, swarm dynamics, and engineering to develop new experimental approaches to programmatically bias key collective cell behaviors—effectively allowing us to ‘herd’ cells. I will discuss two of our core approaches—Outside-In steering of collective cell migration, and Inside-Out guidance of cellular activity. In the case of Outside-In control, we apply an external stimulus in the form of a DC electric field to a population of cells to induce migration through an obscure phenomenon known as ‘electrotaxis’. Our latest electric bioreactor allows us to literally program collective cell migration. We hope to someday use this to accelerate healing processes. For Inside-Out control, we flip the paradigm and focus on mimicking an agent in the group (e.g. a cell). Here, we use a cell-mimetic biomaterial that can infiltrate a tissue to manipulate growth dynamics or spy on the tissue from within. So far, we have demonstrated successful integration with epithelial and cardiac tissues, and we are now exploring the broader space of cell-cell interactions for new targets that we hope will help improve implant integration and efficacy.
October 1, 2019
October 8, 2019
Pierre-Thomas Brun, Princeton University
Host: Guillaume Duclos
Abstract: The talk is concerned with the directed control of fluidic instabilities to program shapes. While instabilities are traditionally regarded as a route towards failure in engineering, I aim to follow a different path; taming fluidic instabilities and harnessing the patterns and structures they naturally form. This methodology capitalizes on the inherent periodicity, scalability, versatility and robustness of mechanical instabilities. This new design paradigm – building with instabilities – calls for an improved understanding of instabilities and pattern formation in complex media. While stability analysis is a classic topic in mechanics, little is known on the so called inverse problem: finding the optimal set of initial conditions and interactions that will be transmuted into a target shape without direct external intervention. While the epicenter of the research is fundamental, utilizing instabilities to structure soft materials opens new research directions in the study of the behavior and deformations of architected soft materials, inspired by natural soft-materials that self-assemble into well defined structures to display remarkable properties. More broadly, the talk is rooted on the basis of recognizing model experiments as a valuable and powerful tool for discovery and exploration, in turn seeding the development of formal and predictive models.
October 15, 2019
October 29, 2019
Aleksandra Walczak, École Normale Supérieure, Paris
Host: Guillaume Duclos
November 26, 2019
Spring 2020
January 14, 2020
January 21, 2020
January 28, 2020
February 4, 2020
February 11, 2020
February 18, 2020
February 25, 2020
March 3, 2020
April 7, 2020
April 14, 2020
April 28, 2020
Nima Arkani-Hamed, Institute for Advanced Study
Title TBD