Seminars and Colloquia - Spring 2008
Tuesday, January 22, 2008
Martin Weiner Lecture Series
Physics Colloquium
Dr. Yohai Roichman, New York University
On Topology and Non-equilibrium Transport - Colloidal Particles in Optical Force Fields
host: Seth Fraden
Is there an intrinsic connection between the topology of a force field, and the dynamics of particles that are driven by this field? An ideal experiment to examine this question is a colloidal particle driven by complex optical force fields. I will describe the experimental methods to produce such complex optical traps, and demonstrate the geometrical nature of the optical forces using the topological view of Pancharatnam-Berry and polarization holonomy of Poynting-Nisbet-Wolf. Using these optical traps and accurate 3D tracking, we demonstrated how changes in the topology of the applied force results in formation of different stochastic behaviors. In particular, we found that even the simplest experiment with optical tweezers behaves as a stochastic engine, and never reach equilibrium. These experiments with optical forces direct us toward a new approach to classify non-equilibrium processes by geometrical tools.
Refreshments will be served outside Abelson 131 at 3:30pm
Monday, January 28, 2008
Condensed Matter Seminar
Dan Reeves, Brandeis University
"Cookie Monster's Secret: How floppy arms can recruit ligands to a reactive site"
Abelson 229
12:30-1:30pm
Tuesday, January 29, 2008
Martin Weiner Lecture Series
Physics Colloquium
No colloquium this week.
Refreshments will be served outside Abelson 131 at 3:30pm
Tuesday, January 29, 2008
High Energy Theory Seminar
1:30pm
"Ads/QCD in 4D and in 2D"
Professor Emanuel Katz, Boston University
Abelson 333
Wednesday, January 30, 2008
High Energy Theory Seminar
1:30pm
"Topology from Cosmology
Dr. Per Berglund, UNH Durham
Abelson 333
Monday, February 4, 2008
Condensed Matter Seminar
Hector Gonzalez-Ochoa, Brandeis University
"Spontaneous Processes in Emulsions"Abelson 229
12:30-1:30pm
Tuesday, February 5, 2008
Martin Weiner Lecture Series
Physics Colloquium
Professor Ted Postol, MIT
"The US Missile Defense and Its European Components: The Scientific and Technical Facts and their Implications for US and European Security"
During 2007 the United States has been presenting briefings and other documents to the European allies that extensively describe the Bush Administration's plans to deploy missile defenses aimed at defending Europe from postulated long-range ballistic missile attacks from Iran. The US plan proposes to deploy a large X-band radar called the EMR (European Midcourse Radar) in the Czech Republic, a launch site of 10 Ground-Based Interceptors, modified from 3-stages to 2, and a Forward Based X-Band Radar (FBX) at some as yet undisclosed location near Iran.
Refreshments will be served outside Abelson 131 at 3:30pm
Monday, February 11, 2008
Condensed Matter Seminar
Ya Liu, Brandeis University
"Shape change and segregation of polymers in 2D confinement"
12:30-1:30pm
Abelson 229
Tuesday, February 12, 2008
Theory Seminar
1:30pm
Abelson 333
Professor Ruben Minasian, CNRS/CEA Saclay
TBA
Tuesday, February 12, 2008
Martin Weiner Lecture Series
Physics Colloquium
4pm
Abelson 131
Professor Ted Solomon, Bucknell University
"Pattern formation and front propagation in reacting systems with chaotic mixing".
Abstract: We present experiments on the dynamics of the Belousov-Zhabotinsky (BZ) chemical reaction in vortex flows that exhibit chaotic mixing. The BZ reaction is well-known as an oscillatory reaction that can display chaotic time-dependence. In the absence of any fluid flow, the BZ system produces target and spiral patterns, similar to those found in a wide variety of reaction-diffusion systems in physics, biology and chemistry. We explore how these patterns are altered in the presence of a chain or array of oscillating and drifting vortices. Experiments show that the patterns that form mimick those associated with chaotic mixing in these flows. Furthermore, we find that if long-range chaotic mixing is superdiffusive with Levy flight trajectories, large-scale synchronization of the oscillating pattern is observed. Other experiments show that fronts propagating in an oscillating vortex flow mode-lock to the frequency of the oscillation. Cellular flows are also found to "freeze" the motion of an interface in the presence of an imposed "wind."
Refreshments will be served outside Abelson 131 at 3:30pm
Host: Irv Epstein
Tuesday, February 19, 2008
No Physics Colloquium. Midterm Recess.
Monday, February 25, 2008
Condensed Matter Seminar
No Seminar today
Abelson 229
12:30-1:30pm
Tuesday, February 26, 2008
Martin Weiner Lecture Series
Physics Colloquium
Dr. Joanna Aizenberg, Department of Chemistry, Harvard University
Title: Sea Sponges: A Textbook in Materials Physics
In the course of evolution, living organisms have developed unique physical mechanisms that control the formation of sophisticated hierarchical biological materials with superior properties. I will focus on biologically-formed inorganic structures and discuss the underlying physical principles and control mechanisms that lead to the generation of length scales, symmetry, hierarchy, polymorphism, non-equilibrium structures, design optimization and multifunctionality. In particular, I will show that a primitive organism - sea sponge - can teach us a valuable lesson in nearly every aspect of materials physics, including nanotechnology, mechanics, optics, crystallography, crystal growth, fluid dynamics and magnetism.
Refreshments will be served outside Abelson 131 at 3:30pm
Monday, March 3, 2008
Condensed Matter Seminar
12:30-1:30pm
Abelson 229
Frederic Gobeaux, Brandeis University
"Type I Collagen in the Dense State: Isotropic/Cholesteric Transition, Fibrillogenesis and Mineralization"
Tuesday, March 4, 2008
Martin Weiner Lecture Series
Physics Colloquium
Jan Liphardt, UC Berkeley Physics Dept.
"Plasmonics, radiating nanowires, and light-powered E. coli"
In the last decade, a variety of new materials such as quantum dots and semiconductor nanowires have been developed. Now, methods are needed to efficiently and accurately integrate these pieces into larger heterostructures that collect, emit, and control light. I will discuss several complementary ways of putting small things together, including self-assembly, optical trapping, DNA hybridization, and the use of re-programmed bacteria. Then, I'll discuss the physics of what happens when certain combinations of materials interact with light. Examples will include tuning of the electrical field using plasmon coupling and nonlinear optical processes occurring inside optically trapped potassium niobate nanowires.
http://physics.berkeley.edu/research/liphardt
Refreshments served at 3:30pm outside Abelson 131.
Host: Mike Hagan
Thursday, March 6, 2008
Special Condensed Matter Seminar
11:30am-12:30pm
Abelson 229
Dr. Norbert Muller, Institute of Organic Chemistry, Johannes Kepler University"
"Nuclear Magnetic Spin Noise--Fundamental Properties and Application Potential"
host: Al Redfield
MARCH MEETING
Monday, March 10-14, 2008
Tuesday, March 11, 2008
Martin Weiner Lecture Series
Physics Colloquium
No colloquium today. March meeting.
Tuesday, March 18, 2008
Theory Seminar
1:30pm
"TBA"
Dr. Pavel Kovtun, MIT and University of Victoria
Abelson 333
Tuesday, March 18, 2008
Eisenbud Lectures in Mathematics and Physics
4pm
Abelson 131
Professor Andrei Okounkov, Princeton University
The Algebra of Random Surfaces
Refreshments after colloquium at Volen Bridge
Wednesday, March 19, 2008
Eisenbud Lectures in Mathematics and Physics
4pm
Abelson 131
Professor Andrei Okounkov, Princeton University
The Algebra of Random Surfaces
Thursday, March 20, 2008 (note different time than previous days)
Eisenbud Lectures in Mathematics and Physics
2:30 pm
Abelson 131
Professor Andrei Okounkov, Princeton University
The Algebra of Random Surfaces
Monday, March 24, 2008
Condensed Matter Seminar
12:30-1:30pm
Tim Sanchez
Abelson 229
"Actin Under Confinement: Rings, Twist, and Supercoiling"
Tuesday, March 25, 2008
Martin Weiner Lecture Series
Physics Colloquium
Matthieu Wyart, Harvard University
Soft Modes, Rigidity and Relaxation in Amorphous Solid.
We poorly understand the microscopic properties of amorphous solids, such as transport, force propagation or even the nature of their mechanical stability. These questions are related to the presence of soft modes in their vibrational spectrum. We explain the nature of these modes in repulsive, short-range systems. This enables to derive a microscopic criterion of rigidity which extends a previous result of Maxwell. This implies that rigidity is not a local property,but is characterized by a length which depends on the packing geometry, and which can be large and even diverge, e.g. near the random close packing. We argue that this description applies to granular media, silica and colloidal glasses. We propose a description of the glass transition in hard sphere systems in terms of these soft modes. This leads to several predictions, in particular a non-trivial power law scaling characterizing the packing geometry in the glass phase, that we check numerically.
Refreshments will be served outside Abelson 131 at 3:30pm
CANCELLED DUE TO ILLNESS
Tuesday, April 1, 2008
Martin Weiner Lecture Series
Physics Colloquium
Professor Sam Hess, Department of Physics, University of Maine, Orino
"Imaging Living Cells with Nanometer Resolution: Breaking the Diffraction Barrier"
Diffraction limits resolution in normal light microscopy to 200-250 nm, but much of biology occurs at shorter length scales. Recently, methods based on single-molecule localization have demonstrated resolution below the diffraction limit.
Fluorescence photoactivation localization microcsopy (FPALM) uses optical control of the density of visible fluorescent molecules to achieve resolution of up to 20 nm in biological samples. Photoactivatable fluorescent molecules are activated bybrief pulses of a high-energy (405 nm) laser, applied at low intensity such that a sparse subset of molecules reaches the active state. Inactive molecules are essentially non-fluorescent before activation. Molecules in the active state are essentially normal fluorescent molecules, which can be imaged by illumination with a longer wavelength (called the readout beam) and by detection with a high-sensitivity camera. Active molecules are imaged for a few frames, and then bleach under thereadout beam. Iterative cycles of activation, readout, and bleaching are repeated until as many molecules as possible have been imaged. Acquired images are analyzed to measure the position (localize) molecules in each frame. The plotted positions ofall localized molecules is then the FPALM image. Examples from living and fixed cells, crystalline surfaces, and other samples will be presented.
Refreshments will be served outside Abelson 131 at 3:30pm
Host: Zvonimir Dogic
Friday, April 4, 2008
Soft Condensed Matter Seminar
Abelson 229
12:00pm
"Protein as model colloid or the physics of dynamical arrest
Dr. Thomas Gibaud
Ecole Normale Superierure de Lyon, France
Monday, April 7, 2008
Condensed Matter Seminar
Mitch Mailmann
"2D Packings of Frictionless Anisotropic Grains"
Abelson 229
12:30-1:30pm
Tuesday, April 8, 2008
Martin Weiner Lecture Series
Physics Colloquium, 4:00pm
Professor Phillip Geissler, UC Berkeley Chemistry Department
Title: “Origins of kinetic frustration in biomolecular rearrangements: Slowly folding proteins and sluggish relaxation of overstretched DNA”
Slow dynamics in soft matter generically arise from large free energy barriers or long wavelength motions. The specific nature of such barriers and motions, however, can be quite elusive in complex macromolecular systems, where rearrangements of interest may involve very many atoms or molecules acting in concert. A famous example in biophysics is the slow folding of poorly designed proteins. It has long been imagined that a polypeptide with random amino acid sequence would encounter diverse kinetic traps, stabilized by strong non-native
interactions, en route to its folded state; while the folding of well-designed sequences would minimally involve non-native contacts. We have tested several influential implications of this "principle of minimal frustration" for a lattice model of heteropolymer folding. By thoroughly examining folding pathways of a diverse ensemble of sequences, we have found that certain predictions are remarkably robust but that their conceptual underpinnings appear incomplete. A second example of subtle kinetic frustration involves the response of double-stranded DNA to strong tensile force. Debates on the microscopic nature of the overstretched state have pitted denaturation against conversion to a putative "S" form of hybridized DNA. We have devised a lattice model of DNA overstretching and simulated its dynamics numerically. Our results, whose qualitative features depend very weakly on model parameters, indicate that the participation of both denatured strands and S-DNA are required to explain patterns of hysteresis observed in experiments.
Refreshments will be served outside Abelson 131 at 3:30pm
Host: Mike Hagan
Monday, April 14, 2008
Condensed Matter Seminar
Silke Henkes
Abelson 229
12:30-1:30pm
"Evolution with interacting genes"
Tuesday, April 15, 2008
Astrophysics Seminar
12 noon
Abelson 229
"Spitzer Observation of Brightest Cluster Galaxies"
Professor Chris O'Dea
Rochester Institute of Technology
Abstract: I present Spitzer IRAC and MIPS observations of 62 brightest cluster galaxies (BCGs) with optical line emission located in the cores of X-ray luminous clusters. We find that at least half of these sources have signs of excess infrared emission. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding several systems dominated by an AGN, the excess mid-infrared emission in the remaining brightest cluster galaxies is likely related to star formation. The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star forming galaxies. The gas depletion time scale is about 1 Gyr. The physical extent of the infrared excess is consistent with that of the optical emission line nebulae. This supports the hypothesis that the star formation occurs in molecular gas associated with the emission line nebulae and with evidence that the emission line nebulae are mainly powered by ongoing star formation. We find a correlation between mass deposition rates estimated from the X-ray emission and the star formation rate estimated from the infrared luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates suggesting that the re-heating of the ICM is generally very effective in reducing the amount of mass cooling from the hot phase but does not eliminate it completely.
Tuesday, April 15, 2008
Martin Weiner Lecture Series
Physics Colloquium
Professor Irwin Shapiro, Harvard-Smithsonian Center for Astrophysics
Title: "Frame Dragging: A Half Century Hunt for an Elusive Effect"
Refreshments will be served outside Abelson 131 at 3:30pm
Host: David Roberts
Tuesday, April 22, 2008
NO COLLOQUIUM--Passover and Spring Recess
Monday, April 28, 2008
Condensed Matter Seminar
Abelson 229
12:30-1:30pm
Henry Fu, Brown University
"Swimming in viscoelastic fluids and gels"
Abstract: The swimming of microorganisms in Newtonian fluids has been,and still remains, an active area of research. However, in many cases the natural environment which swimmers move through are non-Newtonianfluids or even gels. For example, mammalian sperm swim through mucus in the female reproductive tract. In this talk I focus on swimming through viscoelastic fluids and gels. First, the forces exerted by a viscoelasticmedium are different from those exerted by a Newtonian fluid. I will address how this affects swimming shapes and speeds of flexibleswimmers such as sperm. Second, the time-reversibility of the Stokesflow equations for low Reynolds number constrains what types of swimming motions are effective in Newtonian fluids (Purcell's "Scallop theorem"). I will discuss how this is altered in nonlinearly viscoelasticfluids, and whether any new swimming strategies become available toswimmers in viscoelastic media. Finally, I will describe issues that arise for swimmers moving through viscoelastic gels, which are solids rather than fluids.
Tuesday, April 29, 2008
NO COLLOQUIUM--Brandeis Friday
Wednesday, May 14, 2008
Biophysics Seminar
Abelson 229
12 Noon
"Stability and structural transformations in discoidal High-density lipoproteins"
Sangeeta Benjwal
Boston University
Host: Azadeh Samadani
Tuesday, June 3
Condensed Matter Seminar
Abelson 229
12:00 noon sharp (not 12:10pm)
"Bacteria as Collloids"
Professor Wilson Poon
The University of Edinburgh
Host: Seth Fraden
Bacteria constitute the most basic form of life on earth today. In terms of physical size, bacteria inhabit the colloidal realm: a few hundreds of nanometers to a few microns. Brownian motion and the associated physics of colloids therefore determine many aspects of bacterial behaviour, but these aspects are often ignored by microbiologists. The 'active' nature of bacteria (their self motility) also means that we can expect certain rather intriguing differences. In this talk, I will review some basic biology of bacteria, and discuss how we may expect colloidal physics to feature in bacterial living, pointing out similarities and differences using simple scaling arguments. In particular, I will highlight what I believe to be major research opportunities, including a case study of bacterial motility in viscoelastic media.