Complex Matter and Biophysics Seminar: Fall 2007

From WolfWikis

Fall 2007 Schedule

Tuesdays 1:30 to 2:30pm, 314 Riddick unless otherwise noted.

Seminars will be followed by tea/coffee/cookies, while journal clubs will tea/coffee/cookies alongside discussion.

Sept 18

EXTERNAL SPEAKER: Laura Miller, UNC Applied Mathematics

Fluid Dynamics of Embryonic Heart Development

ABSTRACT: In the first part of this talk, I will discuss the transition to vortical flow that occurs in the developing heart tube between 3 and 4 weeks post fertilization. This transition is significant because it affects the spatial and temporal distribution of shear stress along the cardiac wall. Recent in vivo studies in zebrafish suggest that shear stress is critical to the proper development of the heart chambers and the valves. Numerical simulations using the immersed boundary method were performed to determine how the transition to vortical flow depends on geometry and Reynolds number. The spatial distributions of shear and pressure for each simulation will also be shown.

In the second part of the talk, I will discuss how cardiac endothelial cells might sense shear stress through the endothelial surface layer. Numerical simulations and physical models will be used to investigate how spatial distributions of shear depend on heterogeneities in the surface layer itself.

Host: Karen Daniels

Oct 2

Journal club/schedule planning

Please bring article/speaker suggestions, or email them to Karen Daniels ahead of time.

Oct 23

Wor Thongthai (Weninger Group)

Photoinactivation of dye labeled Sindbis Virus

Abstract:

Membrane fusion with the exterior surface of a cell has long been accepted as one of the possible entry mechanisms for virus. We have observed membrane fusion between Sindbis virus (SV) and liposomes formed from purified lipids using a fluorescence dye assay. We verified that low-pH exposure is required to trigger fusion and demonstrated that fluorescence dye labeling has very little effect on fusion capability. We also show that while photobleaching R18/DiI attached to SV has no effect on fusion capability photobleaching R18 led to dramatic reduction in SV infectivity. A model explaining these results will be discussed.

Oct 30

EXTERNAL SPEAKER: Raphael Blumenfeld, Imperial College & Cambridge University

Modelling stresses in isostatic systems and applications to granular materials

Abstract: A key concept to the understanding of stress transmission in compacted granular materials is the isostatic State. A simple experiment demonstrating the relevance of this state is described. The simplest isostatic granular materials are statically determinate, which decouples stress from strain and makes elasticity theory ineffective.

An isostaticity theory is presented for stress transmission in two-dimensional such materials. The stress field equations are described and exact and numerical solutions are found. The theory explains the frequently observed force chains and makes it possible to predict their trajectories. Moreover, it explains the rich behavior of force chain networks including phenomena such as leakage from chains and branching.

The experiment further suggests that the isostatic state behaves as a critical point, wherein a particular correlation length diverges. This observations suggests a theoretical handle on real granular materials. In particular, it is argued that granular materials should be regarded as two-phase composites. It is concluded that granular stresses should be modelled by a new stato-elastic theory.

Host: Karen Daniels

Nov 13

Cheng Wang (Ade Group)

Polymer LED interfaces studied with resonant soft x-ray reflectivity

Abstract:

Multilayered polymer structures produced by alternate spin casting from polar and non-polar solvents are promising for polymer light emitting diodes (PLEDs). The structure of the interface between the active layers most likely significantly affects the device efficiency, yet little is known about the interfacial structure and how it affects properties of such devices. Recently, it has been shown that Resonant Soft X-ray Reflectivity (RSoXR) is an excellent tool to study polymeric thin films without the need for deuteration. RSoXR can enhance the sensitivity to a particular interface by using specific photon energies. We have used RSoXR and characterized the interfacial width of bilayers of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly[9,9-bis(6’-N,N,N,-trimethylammoniumhexyl)fluorene-co-alt-1,4 phenylene bromide] (PFNBr), materials relevant to PLED devices. MEH-PPV is a neutral conjugated polymer spun-cast from toluene (non-polar solvent) and PFNBr is a charged conjugated polymer (conjugated polyelectrolyte) spun-cast from methanol (polar solvent).

Nov 27

Derrick Stevens (Clarke/Bochinski Group)

Dimensional analysis of percolation and conductivity in nanocomposites

From microscopic, conductive nanocomposites, to macroscopic systems, forest fires, percolation theory is useful in describing the random population of a sample space by some finite object. In this talk, the relation of percolation theory to conductivity in nanocmposites will be presented alongside our experimental data. Polyethelyne Oxide/multiwalled carbon nanotube (or silver nanoparticle) composites were elctrospun into highly porous nanofiber mats. To better understand the percolation process within this unique morphology, Monte Carlo simulations were developed to conduct a dimensional analysis of percolation theory. The results of this analysis and their application to percolating systems will be discussed. The effect of particle aspect ratio is also presented.