Mar
13
Teaser:

This talk will describe new results on the properties of colloidal crystals, both on their solidification and on their melting.  It will describe how hard-sphere like colloids crystallize, and will explore the huge discrepancy between the nucleation rates predicted by theory and measured in simulation and those measured experimentally.  The discrepancy can be as large as 150 orders of magnitude!  A simple modification to the theory, suggested by experiment, is able to account for this behavior and to rectify the discrepancy....

Mar
12
Teaser:

The viruses that infect bacteria have a hallowed position in the development of modern biology, and once inspired Max Delbruck refer to them as "the atom of biology".  Recently, these viruses have become the subject of intensive physical investigation.  Using single-molecule techniques, it is actually possible to watch these viruses in the act of packing  and ejecting their DNA.    This talk will begin with a general introduction to viruses and their life cycles and will then focus on simple physical arguments about the forces that attend viral DNA packaging and ejection, predictions about the ejection process and single-molecule measurements...

Mar
10
Teaser:

In this exciting event, three lectures will be presented from world renown Chef Jose Andres and Harvard Physics Professors Michael P. Brenner and David A. Weitz.  Awards will also be presented to the top Dekalb County high school student...

Mar
09
Teaser:

Membrane proteins are critical components of all cells, controlling, e.g., signaling, nutrient exchange, and energy production, and are the target of over half of all drugs currently in production.  At an early stage of their synthesis, nearly all membrane proteins are directed to a protein-conducting channel, the SecY/Sec61 complex, which permits access to the membrane via its lateral gate.  By combining molecular dynamics simulations with cryo-electron microscopy data, we recently resolved the first structure of a membrane-protein-insertion intermediate state of SecY bound to a translating ribosome, with a transmembrane (TM) segment...

Mar
08
Teaser:

We study the effect of electron-electron interaction on the resistivity of a metal where umklapp scattering is either not effective or suppressed. This can happen in cases such as in a metal near a Pomeranchuk quantum phase transition or in a system with low density of carriers, e.g., the surface states of three-dimensional (3D) topological insulators. In such cases, one must consider both interactions and disorder to obtain a finite and T dependent resistivity. The existence of the Fermi-liquid (T^2) term in resistivity of a two-dimensional (2D) metal, as we show, then depends on 1) dimension (2D vs 3D), 2) geometry (concave vs convex), and 3) topology (simply vs multiply...

Mar
08
Teaser:

Topological states of matter have quantum entangled ground states characterized by topological quantum numbers rather than symmetry
breaking. Inspired by the discovery of topological insulators, I describe recent progress in finding a variety of new classes of topological materials
in semiconductors and superconductors. Potential applications in electronics and quantum computation will be briefly discussed.

Mar
07
Teaser:

The ribosome translates the genetic information encoded in messenger RNA into protein. Folded structures in the coding region of an mRNA represent
a kinetic barrier that lowers the peptide elongation rate, as the ribosome must disrupt structures it encounters in the mRNA to allow translocation to the next codon. Such structures are exploited by the cell to create diverse strategies for translation regulation. Although strand separation activity is inherent to the ribosome, requiring no exogenous helicases, its mechanism is still unknown. By using a single-molecule optical tweezers assay to follow in real time the codon-by-codon translation of mRNA...

Mar
06
Teaser:
The ability to effectively control a fluid would enable many exciting technological advances, such as the design of quieter, more efficient aircraft.  Model-based feedback control is a particularly attractive approach, but the equations governing the fluid, although known, are typically too complex to apply standard tools for dynamical systems analysis or control synthesis.  This talk addresses model reduction techniques, which are used to simplify existing models, to obtain low-order models tractable enough to be used for analysis and control, while retaining the essential physics.
Mar
05
Teaser:

In 1998 two rival teams of astronomers studying exploding white dwarf stars, called type Ia supernovae, came to the surprising conclusion that the expansion of the Universe is speeding up. This discovery of "the accelerating Universe" ushered in a revolution in our cosmological understanding. I will describe the steps leading to this discovery, and how observations of supernovae from telescopes on the ground and in space can be used to trace the history of cosmic expansion. The continued study of these stellar explosions will shed light on the mysterious "dark energy" that dominates and drives our accelerating Universe.

Feb
24
Teaser:

Diffusion of single molecules and organelles in living cells has attracted considerable interest. The motion so essential for intra- and intercellular transport, regulation, and signaling, and hence for the life within cells exhibits surprising deviations from normal Brownian motion. Using optical tweezers combined with single particle tracking inside living cellular organisms we study intracellular diffusion of nano-sized organelles inside living cells. The temperature increase caused by absorption by the laser light as well as the potential physiological damage are important also to consider and will be addressed [1,2]. Lipid granules inside living...

Feb
24
Teaser:

In ordinary solids, acoustic shocks are extreme mechanical phenomena: they occur when rigid materials are subjected to violent impacts. But in soft materials things are different. Granular media, foams and polymer networks can all be prepared in a state of vanishing rigidity in which even the tiniest perturbation elicits an extreme mechanical response. When that happens these materials are not just soft, they have become fragile.

In this talk, we present simulations in which two-dimensional jammed granular packings are dynamically compressed, and demonstrate that the elementary excitations are strongly nonlinear shocks, rather than ordinary...

Feb
23
Teaser:

Trapped attractive atomic Bose-Einstein condensates (BECs) in three spatial dimensions are known to exist for some finite time only. This is because the gas is prone to self-collapse, due to the attractive nature of the interaction. The 'mainstream' way to describe the state of the condensate is a mean-field (MF) theory, that assumes total condensation of the system.  In this talk I will introduce the notion of fragmentation, in contrast to coherence, and show that the states of definite angular momentum of the 3D many-body system cannot be condensed MF states. With this at hand, I examine the impact of the angular momentum to the stability of the attractive gas and show that there...

Feb
22
Teaser:

One of the fundamental problems in biology is understanding how phenotypic variations arise in individuals. Phenotypic variation is generally attributed to genetic or environmental factors. However, in several important cases, phenotypic variations are observed even among genetically identical cells in homogeneous environments. Recent research indicates that such `non-genetic individuality' can arise due to intrinsic stochasticity in the process of gene expression. Correspondingly there is a need to develop a framework for quantitative modeling of stochastic gene expression and its regulation. Of particular interest is modeling of regulation by non-coding...

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