Physics Colloquia – Fall 2007

(Fridays 2:00 PM. GN 101)

Titles link to the abstracts.
Date Speaker Title
Sept 7
 J.S. Faulkner, F.D. Medina, etc
Sept 14
 Robin Jordan (FAU)
Sept 21
 Korey Sorge (FAU)
Sept 28
 Gregg Fields (FAU)
Oct 5
 Prem Chapagain (FIU)
Oct 12
 Karl D. von Ellenrieder (FAU)
Oct 19
 Angela Guzman (FAU)
Oct 26
 Vincent Naudot (FAU)
Nov 2
 James Roros, Host (FAU)
Nov 9
 Dan Needleman (Harvard)
Nov 16
 Zvonimir Dogic (Brandeis)
Nov 30
 Vinothan Manoharan (Harvard)
     

 

Colloquium Abstracts

A memorial for Dr. Robert Stetson [Grand Palm Room of the Student Union]
J.S. Faulkner, F.D. Medina, etc, Sept 7
Dr. Robert F. Stetson, a founding faculty member of the physics department and retired Professor Emeritus (1997), passed away at his Boca Raton home on June 2. A memorial service will be held on Friday, Sept. 7, from 2 to 3 p.m. in the Grand Palm Room of the Student Union on the Boca Raton campus. Friends and colleagues will gather to remember Dr. Stetson's work and his dedication to FAU. For those who wish to make a gift in his memory, tax-deductible donations can be made in Dr. Stetson's name to the FAU Foundation, designating the physics department account. For more information about the memorial service, please contact Melissa Troshinsky 7-2762.
 
Some (amusing) things you may not know about pi
Robin Jordan (FAU), Sept 14
In this talk I take a slightly irreverant and whimsical look at pi, which as all scholars know is the ... "quantitas, in quam cum multiplicetur diameter, proveniet circumferentia".
 
Flips and Flops: The Study of Artificial Antiferromagnets (CANCELLED)
Korey Sorge (FAU), Sept 21
Traditional antiferromagnetic (AFM) materials are notoriously hard to study. Not only are there many different kinds of AFM order (spin orientation at specific lattice sites), but it can change as a function of temperature or field. In the 1980s, a collection of magnetic multilayers became an elegant probe of antiferromagnetism in an ``adjustable'' system---the artificial antiferromagnet. This discovery not only lead to a simpler way of studying this difficult problem, but revolutionized the computer hard drive industry. In this talk, I would like to give insight into what an artificial antiferromagnet is, how they are studied, and their connection to traditional antiferromagnetic materials.
 
Mechanism and Inhibition of Collagenolytic Matrix Metalloproteinases
Gregg Fields (FAU), Sept 28
Collagen serves as a structural scaffold and a barrier between tissues, and thus collagen catabolism (collagenolysis) is required to be a tightly regulated process in normal physiology. In turn, the destruction or damage of collagen during pathological states plays a role in tumor growth and invasion, cartilage degradation, or atherosclerotic plaque formation and rupture. Only a small number of proteases have been identified capable of efficient processing of triple-helical regions of collagens. Several members of the zinc metalloenzyme family, specifically matrix metalloproteinases (MMPs), possess collagenolytic activity. A mechanistic understanding of the cleavage of intact collagens has been pursued for many years; the results of such studies could lead to the development of truly selective MMP inhibitors. Our laboratory has developed triple-helical peptide (THP) substrates and inhibitors for MMPs, with the goal of using these model systems to dissect collagenolytic behavior. Studies of MMP/THP interactions by biophysical methods [NMR spectroscopy and hydrogen/deuterium exchange mass spectrometry (HDX MS)] in combination with site-specific mutagenesis and kinetic analyses have allowed us to more precisely determinate the roles of MMP regions and residues in the binding, unwinding, and hydrolysis of triple-helical structures. These results have also led to a "conformational entropy shift" hypothesis explaining how MMPs process collagen without input from an external energy source. Ultimately, we are utilizing information about collagenolytic mechanisms to design inhibitors that target proteases implicated in cancer progression (MMP-2, MMP-9, and MT1-MMP) while sparing proteases with host-beneficial functions (MMP-3 and MMP-8).
 
Lattice Model investigations of Protein Folding and misfolding
Prem Chapagain (FIU), Oct 5
In order for a protein to carry out its biological function, it must fold to the appropriate configuration, the protein's "native state". For many proteins that do not require chaperones, the primary sequence of amino acids contains all the necessary information for the folding process. A protein has a large number of structural degrees of freedom and many competing energy terms. Because of the complexity of the system, computer simulations must chose between simulating either longer chains with reduced models or shorter chains with more detailed models. Experimental, theoretical and computational studies have improved our understanding of the folding process in recent years. I will present some results of Monte-Carlo simulation of a two-helix bundle folding and misfolding using a lattice model for single chain simulations. I will also describe the computer lattice model for simulating the dynamics of two peptide chains and discuss the simulation of GCN4 dimerization.
 
The hydrodynamics of tunniform swimming
Karl D. von Ellenrieder (FAU), Oct 12
Because of their highly efficient propulsion, the swimming motions of dolphin (carangiform locomotion) and tuna (thunniform locomotion) have been of great interest to engineers - especially those hoping to replicate the movement for the design of efficient and maneuverable underwater vehicles. In both carangiform and thunniform swimming modes, the driving forces are predominantly generated by the oscillatory motion of the caudal or tail fin. The front of the animal remains relatively straight and most of the thrust-generating movement is confined to the posterior 33% (carangiform) or 10% (thunniform) of its body. Since thrust production in these two modes is believed to be principally caused by lift forces acting on the tail fin, the flow structure and mechanics of oscillating wings, in the absence of any upstream body, is frequently studied. Here, dye visualizations and Particle Image Velocity (PIV) measurements of the flow around an oscillating wing will be presented.
 
Quantum reflection in evanescent-wave mirrors
Angela Guzman (FAU), Oct 19
Evanescent-wave (EW) mirrors have been considered [1] as a means of measuring atom-surface forces, assuming that the reflecting potential is the sum of the exponentially repulsive dipole potential created by the EW and the attractive atom-wall van der Waals potential, resulting in a potential barrier whose height can be measured [2]. Quantum reflection from a solid surface as observed for low density BECs [3], occurs at atomic velocities very close to zero. The existence of a potential barrier in EW mirrors opens the possibility for effective quantum reflection at non-zero atomic velocities. From the perspective of quantum electrodynamics, Casimir and Polder [4] analyzed a system consisting of a ground state atom in the electromagnetic vacuum inside a cubic cavity with perfectly conducting walls and calculated the second-order perturbation energy of the ground state atom in the vacuum, using as intermediate states the excited states of the atom and the state of the radiation field in which only one light quantum is present. Atoms reflected by the EW mirrors interact however not only with the vacuum but with the reflecting evanescent field, which induces an atomic dipole-moment at the frequency of the applied field. We introduce here a quantum dipole-dipole interaction model to obtain cold atom-dielectric wall interaction potentials in the presence of an EW created by total reflection of a wave. The atom-wall interaction is described by the effective, non-Hermitian, quantum dipole-dipole interaction between the atomic induced dipole and its image dipole in the dielectric. The effective potential has the short distance behavior of the van der Waals potential and the asymptotical behavior of the optical potential but does not correspond to their direct addition. The imaginary part of the effective dipole potential represents spatially modulated losses caused by radiative processes into vacuum modes that do not contribute to the dipole-dipole interaction but rather to random scattering of the reflected atoms. [1] M. Kasevich, K. Moler, E. Riis, E. Sunderman, D. Weiss, and S. Chu, in Atomic Physics 12, edited by J. C. Zorn, R. R. Lewis, and M. K. Weiss (AIP, New York, 1991), vol. AIP Conf. Proc. No. 233, p. 47. [2] A. Landragin et al., Phys. Rev. Lett. 77, 1464 (1996). [3] T. A. Pasquini, Y. Shin, C. Sanner, M. Saba, A. Schirotzek, D. E. Pritchard, and W. Ketterle, Phys. Rev. Lett. 93, 223201 (2004). [4] H. B. G. Casimir and D. Polder, Phys. Rev. 73, 360 (1948).
 
Dynamical complexity and Applications
Vincent Naudot (FAU), Oct 26
We present mathematical backgrounds in dynamical system that are relevant for studying the dynamics of ecological models. The system we consider is derived from Volterra Lotka and describes the evolution of population of predators with their prey in a given ecosystem. We show that the dynamics can be very complicated and exhibits Strange Attractors.
 
An Introduction to CERN and to Particle Phyics
James Roros, Host (FAU), Nov 2
The presentation will be the first part of a three part lecture titled " An Introduction to CERN and to Particle Physics" by Dr.C.H. Llewellyn Smith. (Director-General of CERN at the time of the presentation). Dr. Smith gives an introduction to CERN and its research accelerators (current and planned) and begins an introduction to particle physics and the standard model with its shortcomings which CERN research is addressing.
 
Dynamics in the Metaphase Spindle: Single Molecules and Cell Division
Dan Needleman (Harvard), Nov 9
A wide variety of subcellular structures exist in a non-equilibrium steady state with a constant flux of molecules and energy continuously modifying and maintaining their architecture. A prime example of this is the spindle: a remarkable, self-organizing molecular machine that segregates chromosomes during cell division. The spindle is a highly dynamic structure composed of the protein tubulin, which assembles into long polymers called microtubules, and a variety of other proteins that regulate microtubule nucleation, polymerization, depolymerization, and translocation. We investigated the motions of thousands of individual tubulin molecules in spindles, using a combination of single molecule confocal microscopy and automated particle tracking. This study allowed us to perform the first detailed analysis of microtubule polymerization in spindles. Our single molecule data can be quantitatively explained by a first-passage analysis of a very simple model of microtubule dynamics. These results unambiguously rule out a number of proposed mechanisms of microtubule turnover, leading to surprising implications for models of spindle organization.
 
Self-assembly of biopolymers with a twist
Zvonimir Dogic (Brandeis), Nov 16
I will describe two examples of self-assembly of colloidal structures that are driven by a combination of chiral and entropic forces. In a first set of experiments we study the liquid crystalline phase behavior of a concentrated suspension of helical flagella isolated from Salmonella typhimurium. We demonstrate that the static phase behavior and dynamics of chiral helical rods are very different when compared to simpler achiral hard rods. In a second set of experiments we describe the formation of colloidal membranes in a rod/polymer mixture. The fluctuations of these membranes are very similar to those found in conventional lipid membranes. However, their formation is driven by entropic and not hydrophobic forces. Increasing the strength of the chiral interaction, results in a remarkable transition from a flat two-dimensional membranes into one-dimensional twisted ribbons.
 
Colloidal condensed matter
Vinothan Manoharan (Harvard), Nov 30
The very first Nobel Prize went to J. H. van't Hoff for his discovery that dilute suspensions of small particles in a solvent obey the ideal gas equation of state. Since that time it has been established that suspensions of particles -- colloids -- can also form liquids, crystals, and even glassy phases. Because the particles are much larger than atoms or molecules, they behave classically, and studies of these systems have revealed many universal physical principles governing the formation of condensed phases. In this talk I will discuss experiments on "finite" colloidal systems: small numbers (5-20) of colloidal particles confined to a container and left to assemble. We probe these systems using optical microscopy, light scattering, and a relatively new imaging method called digital holographic microscopy, which allows us to see all the particles in three dimensions, in real space and real time. Although the systems are small and simple, their collective behavior shows some surprising features.