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Past years' colloquia

Eisenbud Lecture Series in Mathematics and Physics

Berko Symposium

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Department Colloquia

Martin Weiner Lecture Series
Department of Physics Colloquium
4:00 pm, Abelson 131
Refreshments at 3:30pm outside Abelson 131

Spring 2015 Colloquia

Thursday, February 5 
Special Colloquium at 1pm in Abelson 131

Karen Kasza, Sloan-Kettering Institute
Spatiotemporal control of the active forces that shape living tissues

Abstract: The ability of multicellular tissues to physically change shape, move, and grow is a key feature of life.  These behaviors are often accomplished by local movements or rearrangements of cells within the tissues. Many cell movements are actively driven by contractile forces generated in cells by the motor protein myosin II. During embryonic development, these forces are patterned to orient cell movements, resulting in changes in tissue shape and structure that build functional tissues and organs. To uncover how force-generation by myosin drives cell movement and determines the physical behavior of tissues, I use the fruit fly embryo as a model system, where polarized patterns of myosin activity orient cell movements and rapidly elongate the embryo. I will discuss how studying
embryos generated with engineered myosin variants allows us to dissect mechanisms underlying tissue behavior. In particular, I will describe how myosin variants with enhanced activity accelerate cell movement but, surprisingly, also alter the spatial pattern of forces and result in reduced tissue elongation. These experiments reveal that the levels and patterns of forces are controlled by the same biological cue and suggest a trade-off between the speed and orientation of cell movements within tissues. These studies of how forces shape the fruit fly embryo shed light not only on physical principles at work in active, living materials but also on how defects in cell movements contribute to human birth defects and tumor metastasis.

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Tuesday, February 10

Scott Forth, Rockefeller University
Examining the Mechanics of Dynamic Microtubule Networks

Abstract: Cells utilize dynamic biopolymer networks to carry out mechanical tasks during diverse processes such as cell division, migration, and development. For example, the microtubule-based mitotic spindle performs the physical work of segregating duplicated chromosomes into two daughter cells. The components that make up these networks are not typically at equilibrium, but instead operate under load. The mitotic spindle achieves a ‘steady state’ size and shape despite the components that make up the dense network undergoing constant motion, experiencing large forces, and turning over on rapid timescales. It has been challenging to understand how the forces that move microtubules and chromatin are regulated within these micron-sized dynamic networks by groups of diverse nanometer-sized proteins. Dr. Forth will present his postdoctoral work that directly addresses these problems by employing single molecule biophysical methods, including optical trapping and TIRF microscopy. His work reveals that proteins involved in cell division can generate frictional resistance as they move along microtubules, and that this friction can be harnessed within spindles to help ensure successful DNA segregation. Additionally, he has shown that motor proteins that push microtubules can also generate braking forces that scale with microtubule sliding velocity. Together these results can help us link the biophysical properties of essential proteins with their function in dividing cells.

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Thursday, February 12    
Special colloquium, 1pm in Gzang 122

Wylie Ahmed, Institute Curie
Active mechanics in living oocytes reveals molecular - scale kinetics

Abstract: Living cells actively generate internal forces at the molecular scale to coordinate intracellular organization and transport. This requires force generation by molecular motors (myosin, kinesin,dynein) to drive motion and organization in the crowded intracellular environment. During meiosis,unfertilized eggs (i.e. oocytes) must actively position their nuclear structures to facilitate division for healthy development. This requires precise spatial and temporal coordination of molecular motors and the cytoskeleton. However, precise quantification of molecular motor force kinetics in their native environment is lacking. Here we show that force kinetics of myosin-V in living oocytes is revealed by measurement and modeling of active fluctuations and intracellular mechanics. We find that molecular force generation by myosin-V drives the oocyte cytoplasmic-skeleton out-ofequilibrium and actively softens the environment. Our results reveal that in-vivo myosin-V in oocytes generates 0.4 pN of force, with a power-stroke duration of 300 ms, and drives vesicle motion at 320 nm/s. Our results demonstrate that active forces generated in a living cell can tune intracellular mechanical properties and that in-vivo force kinetics at the molecular level can be extracted using our experimental and theoretical framework.

No video available.

Tuesday, February 17
Note time: 12noon in Abelson 131

Al Sanchez, Harvard University
Non linear dynamics and phase space behavior of interacting
biological components: From genes to ecosystems

Abstract: Biology operates across a very wide range of scales and levels of organization: molecules organize into cells, cells organize into organisms, and organisms organize into ecosystems. One of the main goals of biological physics is to find general laws and organizing principles that govern the behavior of biological systems across these scales. In this talk I will overview my recent work characterizing the dynamics and phase space behavior of two seemingly very different types of biological systems: gene networks and microbial ecosystems. In spite of their very different levels of organization, we found that many ecological and genetic networks are described by the same class of bifurcations and thus share similar dynamic behavior. First I will discuss our recent work on the behavior of gene networks near a critical bifurcation. By experimentally mapping their phase space dynamics, we found many surprising features of these systems. For instance, when they operate close to their critical point, cellular metabolic switches could be turned on and off not only by their specific biochemical signals, as the common textbook models suggest, but also by a large number of universal stressors such as high temperature. In the second part of this talk, I will discuss how we experimentally mapped the phase space dynamics of an ecological "cooperator-freeloader" ecosystem. Visual inspection of this phase space reveals the fate of microbial cooperator populations when they are invaded by freeloaders, and it also tells us how this invasion alters ecosystem level properties such as its productivity and resilience. Finally, we found that this ecosystem is also described by a catastrophic bifurcation, and that critical slowing down can forecast its proximity to the tipping point.

No video available.

Tuesday, February 24

Xuefeng Wang, University of Illinois, Urbana-Champaign
Cellular Forces Measured and Controlled by DNA-based Molecular Force Sensor & Modulator

Abstract: Mammalian cells are remarkable force processors. Cells adhere through membrane protein integrins and generate forces on integrins to probe the local environment. These forces regulate many fundamental cellular functions such as cell adhesion, proliferation, migration, and ultimately stem cell differentiation and cancer progression. Because of its critical importance, integrin force has long been a central topic in the field of cell mechanics. To study integrin forces at the molecular level, I developed a DNA-based force sensor and modulator termed tension gauge tether (TGT) which quantitatively reports and controls cellular forces on integrin molecules. Using TGT, I systematically studied the ranges and physiological roles of integrin forces and discovered two distinct force regimes: cell membrane generates ~40 pN molecular force on integrins to regulate cell adhesion and spreading, while actomyosin generates >54 pN molecular force on integrins to regulate cell polarization and migration. This work demonstrated the versatility of integrins and shed light on the mechanism how cells vary integrin forces to regulate different cellular functions. TGT was also applied to study other mechano-sensitive proteins such as cadherins and Notch receptors. Overall, TGT provides a novel avenue for the study of cell mechanics at the molecular level. In the future, I will apply TGT to study a series of mechanical–involved cellular processes such as kinase activation, durotaxis, stem cell differentation and endocytosis.

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Thursday, February 26
Special Colloquium, 1pm in Abelson 131
Benjamin Rogers, Harvard University
Sculpting phase diagrams: freezing by heating, switchable crystals, and more

Abstract: Physicists and chemists have long searched for ways to turn disordered states of matter (fluids) into ordered states (crystals). We know from everyday experience that such transitions can be triggered through changes in temperature: water freezes to become ice, and crystals like rock candy form when a hot solution cools. Colloidal suspensions--solutions of particles with diameters ranging from tens of nanometers to micrometers--also exhibit strikingly similar phase transitions under the right circumstances. In this talk, I will present experiments showing that these colloidal phase transitions can be controlled in unfamiliar ways. Using grafted DNA strands to induce specific attractions between the particles, and free DNA strands that compete to bind with the grafted ones, I will show that it is possible to create suspensions with exotic phase behavior, such as arbitrarily wide gas-solid coexistence, re-entrant melting, and even reversible transitions between different crystal phases. This work could prove especially useful in nanotechnology, where a central goal is to manufacture materials and devices by growing them directly from solution.

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Tuesday, March 3

No colloquium. APS Meeting.

Monday, March 9 

Dustin Kleckner, University of Chicago 
Vortex Knots in Fluids and Superfluids
Note time and place: 1pm in Gzang 122

Abstract:  Linked and knotted field lines appear as important features in many physical fields, e.g., tangled vortex lines in turbulent flows or braided loops of plasma on the surface of the sun.  In idealized examples, the topology (knottedness) of these field lines may never change, and is associated with a conserved quantity.  Real dissipative fields, however, go through ‘reconnections’ which may untie knots and raise difficult questions about the role of topologically conserved quantities.  I will describe novel techniques we have developed to generate vortex knots in experimental fluids and simulated superfluids.  Our first glimpses into the behavior of these elemental knots show that their dynamics can be understood in terms of system-independent geometric principles.

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Tuesday, March 10

Markus Deserno, Carnegie Mellon Physics
Measuring Elastic Membrane Properties in Computer Simulations
Host: Prof. Michael Hagan

Abstract: One of the fascinating aspects of fluid lipid membranes is that on length scales not much bigger than their thickness they can be described exceedingly well by a completely geometric Hamiltonian. Such a continuum-level theory features two main input parameters, namely the moduli describing curvature elasticity. These are called "(mean) bending modulus" and "Gaussian curvature modulus", and their values are not predicted by the effective macroscopic theory. Instead, one either measures them in experiment, or one conducts computer simulations of realistic or coarse-grained membrane models which aim at extracting these values. In this talk I will present novel ways for doing the latter. I show that actively buckling a membrane leads to a clean and accurate signal for the mean bending modulus. The very beautiful and ancient theory of Euler Elastica helps to analyze the results, but we have to go beyond it when the membrane enters a gel phase. For the Gaussian modulus one needs to find a way to beat the insidious Gauss-Bonnet theorem, and a protocol that monitors the closure of open vesicle patches permits exactly that. I will also show how our results sometimes fit into existing knowledge and guesses, and how they sometimes trigger perplexing questions for which we do not know the answer yet.

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Tuesday, March 17

Robert J. Wood, Harvard University
Manufacturing, actuation, sensing, and control for robotic insects
Host: Prof. Aparna Baskaran

Abstract: As the characteristic size of a flying robot decreases, the challenges for successful flight revert to basic questions of fabrication, actuation, fluid mechanics, stabilization, and power - whereas such questions have in general been answered for larger aircraft. When developing a robot on the scale of a housefly, all hardware must be developed from scratch as there is nothing "off-the-shelf" which can be used for mechanisms, sensors, or computation that would satisfy the extreme mass and power limitations. With these challenges in mind, this talk will present progress in the essential technologies for insect-scale robots and the first flight experiments with robotic insects.

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Tuesday, March 24

Sanjoy Mahajan, MIT
Flipping the classroom to improve STEM learning
Host:  Prof. Jane' Kondev

Abstract: A distressing amount of evidence indicates that our students learn little of what we teach them, having great difficulty transferring their knowledge to new problems and situations.  I will give examples. A remedy is to flip the classroom---to ask students to start learning out of class and to use the classroom to foster higher-level learning. This valuable approach, enabled first by Gutenberg in the 15th century with the development of cheap books, can be enhanced with 21st-century digital technology (including for MOOCs). I will discuss a framework based on cognitive psychology, the ICAP framework, which is valuable for planning flipped classrooms that foster deep learning.  I will illustrate the framework with examples from teaching mathematics, electrical engineering, and physics.  In keeping with the first "O" (open) in MOOC, I will illustrate the framework and its digital implementation with freely licensed, nonproprietary tools, available for all to use, improve, and share.

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Tuesday, March 31

Daniel Prober, Yale University
Graphene Nanobolometers for Ultrasensitive Far-Infrared Detection
Host: Prof. Bulbul Chakraborty

Abstract: Graphene has recently been proposed as an ultrasensitive THz photon detector for space-based astronomy observations.  We have studied the thermal properties of monolayer graphene for this application, and done extensive modeling of the detection processes.  One would employ superconducting contacts to achieve energy confinement in the graphene.  Recently we have studied experimentally the energy loss processes in graphene down to T = 0.1 K.  The space-based observatories that could employ such detectors will be discussed, as well as the science that can be done with these observatories.

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Tuesday, April 7

No colloquium. Passover/Spring Recess.

Tuesday, April 14

Paul Townsend, Cambridge University
Host: Albion Lawrence
What happened to Einstein's Brane?

Abstract: In 1905 Einstein abolished  the "luminiferous aether’’ of 19th century electromagnetic theory;  in his new theory (which came to be known as Special Relativity) it was an unnecessary hypothesis. However, that doesn’t mean that it doesn’t exist! To be consistent with Special Relativity, an "aethereal’’ medium must have a tension (negative pressure) that is equal to its energy density. In 1917, two years after presenting  his theory of General Relativity in its final form, Einstein proposed (although it took Lemaitre to see this) that the vacuum is precisely such a  medium; it’s tension is Einstein’s "cosmological constant’’ and its energy,  now called ``dark energy’’, appears to constitute (according to current consensus) about 70% of the energy of the observable universe. But what does this have to do with electrodynamics? General Relativity is contained in string theory, and the various possible string theories are unified by M-theory. The constituents of M-theory, as currently understood, are extended objects of various dimensions known as "branes’'. Their characteristic feature is a tension equal to their energy density, and photons are disturbances of some (effective) 3-brane, which is therefore the ``luminiferous aether’’. Dark matter `lives’ on other branes, and the cosmological constant has an interpretation as 3-brane tension.  We may be living on Einstein's brane! His brain remains in a jar of formaldehyde.  This talk will survey some of the basic physics of strings and branes, with an attempt to put them into an Einsteinian context.

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Tuesday, April 21

Klaus Schmidt-Rohr, Brandeis University
Dynamics and Nanostructure of Polymer Materials from NMR and Scattering Analysis

Abstract: The properties of polymer materials, including ultradrawn fibers and fuel-cell membranes, are strongly affected by their nanometer scale structure and molecular motions. Since these systems exhibit a large degree of disorder, diffraction methods are of limited use, and advanced solid-state nuclear magnetic resonance (NMR) techniques, complemented by small-angle scattering analysis, can provide unique insights. Surprising large-amplitude motions of chains in crystallites of many polymers have been identified as thermally activated helical jumps, using two-dimensional exchange NMR. The concomitant displacements of the chains along their axes result in chain diffusion between crystalline and amorphous regions. This affects macroscopic properties such as creep and ultradrawability. -- In hydrated Nafion, a fluoropolymer that is the benchmark material for proton exchange membranes in H2/O2 fuel cells, we have investigated the long contentious nanometer-scale structure. 13C and 19F NMR results have highlighted the significant stiffness of the perfluorinated polymer backbone. Using an algorithm based on 3D numerical Fourier transformation that yields the scattered intensity I(q) for any model implemented on a cubic lattice, small-angle scattering data of hydrated Nafion from the literature have been quantitatively simulated. The characteristic ”ionomer peak” is attributed to long, locally parallel but otherwise randomly packed water channels of ~2.4-nm diameter surrounded by the partially hydrophilic sidebranches, forming inverted-micelle cylinders. Simulations for a dozen other models of Nafion do not match the scattering data.  The water-channel model is the first without constrictions of ~1.2 nm diameter and can explain the fast diffusion of water and protons through Nafion.  

Tuesday, April 28

No colloquium. Brandeis Friday.