Condensed Matter & Biological Physics Seminar Condensed Matter & Biological Physics Seminar| Date | Speaker | Affiliation | Title | Host |
|---|---|---|---|---|
| September 12th | ||||
| September 19th | Anatoly Kolomeisky | Rice University | Complex Dynamics of Polymer Translocation: Effect of folded configurations and charge distribution | Movileanu |
| We investigate the effect of folded configurations and charge distribution on polymer translocation dynamics using simple discrete-state stochastic models. For folded polymers the overall transport is viewed as a sequence of 2 events: motion of the folded segment through the channel followed by the linear part of the polymer. It is shown that there are two dynamic regimes depending on the strength of interaction between the polymer and the pore. Our theoretical calculations successfully applied for analysis of experimental translocations through solid-state nanopores. For translocation of inhomogenously charged polymers we consider a polymer with a single charged monomer. It is found that the position of the charged site that minimizes the translocation time is determined by entropic factors. The presence of polymer-pore interactions modify the optimal charge position. Our results provide insight into the effect of charge inhomogeneity on protein translocation through biological membranes. | ||||
| September 26th | Kristian Mueller-Nedebock | University of Stellenbosch, South Africa | Dealing with counter-ions in polyelectrolyte solutions | Marchetti |
| The watery environment of cells contains a number of charged macro-ions in an environment with salt that in the first instance contributes to screening of interactions between the macro-ions. In the past decade it has become evident that the small counter-ions play a more significant role than simply decreasing the range of interaction: the counter-ions can condense on the polyelectrolytes and fluctuate causing surprising effective interactions between the macro-ions. In order to understand the origins of these effective interactions and how the valence of the counter-ion influences such behaviour, carefully constructed models are useful. In this talk an overview of polyelectrolytes will be given, to be followed by a discussion of continuum and discrete theoretical treatments that reveal the roles of the counter-ions in such systems. In particular, a theory of network formation can be used in order to gain some insight into the phenomena associated with multi-valence and relevant polyelectrolyte chain conformational properties. | ||||
| October 3rd | Jay Tang | Brown University | Biomechanics and Micro-rheology of F-actin network | Marchetti |
| Protein filaments F-actin are the primary component of cytoskeleton, endowing eukaryotic cells their 3-D shape, mechanical strength and compliance. A dynamical actin network is also vital for force generation and migration of many types of cells. My laboratory of biological physics focuses on the biomechanics of actin networks reconstituted in test tubes or microscope slides, in order to capture the fundamental physics responsible for their similar properties in live cells. This talk will illustrate micro-rheological properties of F-actin dictated by thermodynamic phase transition, weak inter-filament association and depletion force. These aspects of fundamental physics contribute much to the biological functions of actin network, and in some cases, provide useful insights to certain physiological processes such as wound healing, leukocyte migration and phagocytosis. Our conviction is that researchers with sound training in soft condensed physics can make major contributions to biological research on cell motility and beyond. | ||||
| October 10th | Carolina Ilie | SUNY Oswego | Water Interactions with Crystalline Polymers with Large Dipoles | Schiff |
| We compare the interactions of water with the ferroelectric copolymer poly(vinylidene fluoride (PVDF)- trifluoroethylene (TrFE)) and poly(methylvinylidenecyanide) (PMCV) a strongly dipole ordered polymer. We propose that the microscopic scale, dipole interactions matter and affect the surface chemistry at these polymer surfaces, as does lattice strain caused by water absorption. Surface dipoles can affect the binding site of water species adsorbed at the surface and sterically hinder or enhance desorption of adsorbed and absorbed water. Perturbations of local surface dipoles can affect desorption of absorbed water. As we are dealing with polymers, the absorption of water is persistent, even with largely hydrophobic polymers, and with the absorption of water, polymer lattice strain plays an important role [1].
The electronic structure of the ferroelectric copolymer films of vinylidene fluoride with trifluoroethylene films is locally altered with incident UV radiation suggesting metastable excited states that may involve dipole reorientation [2]. Light polarization dependent photo-assisted thermal desorption helps demonstrate that water desorption from surface and bulk can be influenced by the formation of electronic metastable states [2]. Changes in local dipole orientation and the formation of long lived metastable states affect the strength of the coupling between the dipoles of water molecules and the dipoles of the copolymer poly(vinylidene fluoride - trifluoroethylene). These effects were not observed for water absorption and adsorption on poly(methylvinylidenecyanide). The water desorption from poly(methyl-vinylidenecyanide) is an intrinsically activated process by the strain in the polymer [1,3]. This tends to suggest that dipole rotation in the polymer substrate may play a key role.
References: [1] Carolina C. Ilie, P.A. Jacobson, I.N. Yakovkin, Luis G. Rosa, Matt Poulsen, D. Sahadeva Reddy, James M. Takacs, and P.A. Dowben , J. Phys. Chem. B 111 (2007) 7742-7746. [2] Luis G. Rosa, P.A. Jacobson, and P.A. Dowben, J. Phys. Chem. B 110 (2006) 7944-7950. [3] P. A. Dowben, Luis G. Rosa, C. C. Ilie, Zeitschrift für Physikalische Chemie 222 (2008) 755-778. |
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| October 17th | Britton Plourde | Syracuse University | Tailored superconducting channels for controlling vortex dynamics | Middleton |
| The dynamics of vortex flow in confined geometries can be explored with nanostructured weak-pinning channels of superconducting a-NbGe surrounded by strong-pinning NbN channel edges. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We have fabricated such weak-pinning channels with asymmetric sawtooth edges for controlling the motion of vortices. This design results in substantial asymmetries in the vortex dynamics in the channels, thus forming a ratchet for producing net vortex motion in response to an oscillatory drive. Using these weak-pinning channels, we are able to explore the influence of vortex interactions on the ratchet response by fabricating strips with different channel spacings and measuring these over a range of vortex densities. We have also investigated vortices flowing in a single circular ratchet channel arranged in a Corbino disk geometry, where the asymmetric response is even more pronounced compared with our measurements of straight vortex ratchet channels. | ||||
| October 24th | ||||
| October 31st | Keith A. Williams | University of Virginia | TBA | Movileanu |
| November 7th | William Irvine | Center for Soft Matter Research, New York University | TBA | Bowick |
| November 14th | Sriram Ramaswamy | Indian Institute of Science | TBA | Marchetti |
| November 21st | ||||
| November 28th | Thanksgiving | |||
| December 5th | Liviu Movileanu | Syracuse University | TBA | Middleton |
| December 12th | ||||
| December 19th | William D. Oliver | MIT, Lincoln Laboratory | TBA | Plourde |