Physics Colloquia - Spring 2011

(usually Fridays 2:00 PM in PS 109)

Titles link to the abstracts.

Date Speaker Title
Jan 28
Sukanya Chakrabarti (FAU)
Feb 11
Dan Chen (Brandeis Univ.)
Feb 18
Phillip Chang (University of Toronto)
Feb 25
Jonathan Engle (FAU)
Mar 4
Paul Delhaimer (University of Tennessee)
Mar 18
Tapio Ala-Nissila (Aalto University School of Science, Espoo, Finland and Brown University, Providence RI)
Mar 25
Deguo Du (FAU)
Apr 1
Leo Blitz (UC Berkeley)
Apr 8
Ernazar Abdikamalov (LSU and CAltech)
Apr 15
Tanja Godsenschwege (FAU)

Colloquium Abstracts

Finding dark galaxies from their tidal imprints
Sukanya Chakrabarti (FAU), January 28th
Characterizing the ubiquitous dark matter in the universe has proven to be one of the most challenging problems in modern astrophysics. If the dynamical impact of dark-matter dominated dwarf galaxies on the outskirts of galactic disks can be deciphered, we may be able to infer and characterize cold dark matter sub-structure in a fundamentally new way. I show how one can analyze observed disturbances in the outer gas disks of spiral galaxies to quantitatively characterize galactic companions without requiring knowledge of their optical light. This method, which I call "Tidal Analysis", allows one to determine the mass and relative position (in radius and azimuth) of galactic companions purely from analysis of observed disturbances in gas disks. I will first demonstrate the validity of this method by applying it to local spirals with known optical companions to provide a proof of principle. I will then review my earlier work on the Milky Way that prompted the development of this method. Specifically, analysis of observed disturbances on the outskirts of the Milky Way disk favor a 1:100 mass ratio perturber with a close pericentric approach. I will conclude by discussing ongoing work on developing scaling relations between observed HI maps and satellite mass, and our plans for the near-future which include testing the Tidal Analysis method on large samples to determine its statistical viability.
Microrheology of Soft Matter
Daniel Chen (Brandeis University), February 11th
Microrheology is an experimental technique wherein the dynamics of probe particles is used to characterize the mechanical response of the material in which they are embedded. In recent years, microrheology has emerged as a powerful tool to explore the physics of soft matter systems including but not limited to colloidal suspensions, polymeric gels, and suspensions of microorganisms. In this talk I will give an overview of the technique and highlight applications of microrheology in three soft matter systems: an entangled biopolymer solution, a suspension of actively swimming bacteria, and a gel-forming carbon nanotube network.
Shedding Light on the Dark Sector
Phillip Chang (University of Toronto), February 18th
I will discuss my work on two outstanding problems related to the dark sector of the universe: the observed paucity of dark matter substructure in galactic halos and the nature of the dark energy. Regarding the first problem, Lambda-CDM predicts an abundance of substructure that is not matched by the observed number of dwarfs locally. I will discuss ongoing work on utilizing the extended HI disk of galaxies to constrain the population of the dark halos. We use a combination of numerical simulations, linear theory, and new analysis methodology to infer the presence of substructure from the observed disturbance in the HI disk of galaxies. The second problem I will discuss is using stellar evolution to place constraints on the nature of dark energy. One recent suggestion is the dark energy arises from the presence of a scalar field, i.e., a fifth force, that couples to matter like gravity. Locally (in our high density patch of the universe) this scalar field is not observed, but it operates in the low density regions of the universe. I will show that the effect of this scalar field will modified stellar structure and evolution in these parts of the universe and how careful observations of the colors of red giants can put constraints on these theories.
Spin-foams: Path integral and canonical quantum gravity in one
Jonathan Engle (FAU), February 25th
Path integral and canonical quantizations are sometimes presented as different equivalent formulations of quantum mechanics. But this is not the case -- though Path integrals have the strength of allowing space-time covariant expressions for quantum dynamics, if one is careful, one sees they are not in themselves sufficient to define a quantum theory, but are rather dependent on a canonical quantization. Loop quantum gravity is in many ways a highly successful canonical quantization of general relativity, but there is no broad consensus regarding the definition of its dynamics. Spin-foams offer a definition of the dynamics of LQG via path integral. In this approach, the successes of LQG are retained -- predictions of discrete geometric spectra and black hole entropy calculations, for example -- while allowing dynamics to be defined in a space-time covariant way. In this talk, I will present the general arguments motivating the spin-foam approach, the conceptual non-trivialities involved, and give a summary of progress that has been and is being made.
Protecting hydrophobic moieties drives the formation of complex structures in materials and cells
Paul Delhaimer (University of Tennessee), March 4th
Using hydrophilic and hydrophobic interactions between molecules and their environment as a theme, I will discuss research on self-assembling systems called polymersomes and also our recent work on a cellular organelle called lipid droplets. For more than a decade, large polymeric amphiphiles of high molecular weights have been used to assemble bilayer vesicles and micelles called polymersomes. Although the formation energy needed to create these assemblies is relatively high compared to those energies needed to form assemblies from biological amphiphiles such as lipids, once formed, these assemblies are tremendously stable and are useful for physical applications. I will review the physical properties of these assemblies and will then talk about an exciting application for their physical attributes: drug delivery. We have used these cylinders as drug delivery vehicles for epithelial-based tumors. However, their most interesting characteristic is that they circulate in the vasculature of rodents for up to one week. This is the longest circulation time of any synthetic material of comparable size. We think that the shape and stability of the cylinders is what allows for the long circulation time. I will explain how we are taking advantage of this and other properties of the cylinders to advance the field of applied polymer physics. Continuing on this theme I will discuss recent findings on lipid droplets. Lipid droplets have become of great interest recently because of the increase in obesity and diabetes in Western societies. Just as the hydrophobic block of the large polymeric amphiphiles described above is protected when the amphiphiles form polymersomes, the cell has sophisticated ways of sequestering fats from the rest of the cell. Lipid droplets are used as the storage centers for most fats in the cell, but the term storage is misleading. These organelles are highly dynamic in their cellular location and their individual sizes. I will describe the hypotheses of how they are formed and how their contents are converted to energy.
Coarse-Graining Polymers with the MARTINI Force Field
Tapio Ala-Nissila (Aalto University School of Science, Espoo, Finland and Brown University, Providence RI), March 18th
Fully microscopic studies of complex molecules are severely compromised by the length and time scales accessible by classical Molecular Dynamics techniques. To this end, considerable efforts have been taken to develop coarse-grained models, which retain some degree of chemical specifity but are computationally feasible. Over the last few years, a systematic coarse-graining approach called the MARTINI force-field method has been developed to describe biomolecules [1]. The MARTINI force-fields are based on relatively simple chemical units, which suggests that the approach could potentially be used to model other macromolecules, too. In this talk I will describe how the MARTINI approach can be generalized to describe polymer chains, using Polystyrene (PS) as a benchmark case [2]. The new MARTINI model gives a good quantitative description of many of the structural and thermodynamic properties of PS under a variety of conditions, but is computationally almost two orders of magnitude more efficient than fully microscopic models. I will also describe how this new approach can be used to develop a semi-realistic model of a polyester resin, which is commercially used for coil coating applications [3]. 1. S.-J. Marrink et al., J. Phys. Chem. B 111, 7812 (2007). 2. G. Rossi et al., Soft Matter 7, 698 (2011). 3. G. Rossi et al., unpublished (2011).
Towards Understanding of Protein Folding and Misfolding -From Ultrafast Folding Dynamics to Misfolding Disease
Deguo Du (FAU), March 25th
Understanding the underlying folding mechanism of the crucial protein structural units, such as protein secondary structural motifs, will provide profound insights into the folding mechanisms of larger and more complex protein structures. The fast folding kinetics of a number of beta-hairpin, beta-sheet and mini-proteins were studied using laser induced temperature jump technique combined with time-resolved spectroscopies. Furthermore, a kinetic aggregation assay that enables highly sensitive semiquantification of amyloid in cells and tissues was developed. This assay represents a novel approach to the early diagnosis of protein misfolding diseases.
Dark Matter and the Milky Way
Leo Blitz (UC Berkeley), April 1st
Cold Dark Matter simulations of the growth of structure in the Universe make predictions that match the distribution of galaxies rather well. The simulations, when continued down to the scale of individual galaxies, however, have trouble in matching some of the observations. In particular, I will discuss the substructure problem, for which the observations and the simulations differ by one to two orders of magnitude in the context of the Milky Way. I will examine the structure of the gas in the disk of the Milky Way and show that previous difficulties in determining Milky Way structure can be explained with a simple kinematic fix. This makes it possible to show that the vertical structure of the outer disk of the Milky Way can be explained by the interaction of the Dark Matter in the Milky Way with the largest of its satellite galaxies: the Magellanic Clouds. Furthermore, an embarrassing asymmetry of the disk thickness can be explained with an eccentric dark matter halo. Finally, examination of the Fourier modes of the surface density of the outer disk leads to the prediction of an outlying, undiscovered, possibly dark, satellite galaxy.
Computational Models of Long Gamma-Ray Burst Central Engines
Ernazar Abdikamalov (LSU), April 8th
Long gamma-ray bursts are among the most energetic explosions in the Universe. There is strong observational and theoretical evidence linking them with the death of massive stars. However, the details of this relationship and the nature of the long gamma-ray burst central engine remain uncertain. We perform numerical simulations of rotating core-collapse in the context of the collapsar model for long gamma-ray bursts. We explore dynamics of collapsar formation and stability properties of accretion disks around black holes.
Tanja Godsenschwege (FAU), April 15th