Complex Matter and Biophysics Seminar: Fall 2008

From WolfWikis

Fall 2008 Schedule

Tuesdays 4:00 to 5:00pm, 315 Riddick unless otherwise noted.

Tea/coffee/cookies will be served starting at 3:30pm in 415 Riddick.

Sept 16: Shuang Fang Lim

NCSU Physics

Upconverting Nanocrystals for Bioimaging

ABSTRACT: We present or work on upconverting nanocrystals for biological imaging. These rare-earth doped materials have the ability to convert infrared light into visible light through sequential absorption of multiple photons. We have synthesized such particles with diameters down to 6 nm, demonstrated their use in imaging C. elegans, studied their toxicity, and demonstrated functionalization by binding them to specific targets. We have also investigated how size and shape influence the optical properties.

Host: Robert Riehn

Sept 23: Dan Dougherty

NCSU Physics

The Dipole Fluid Phase of C60 at Organic Interfaces

ABSTRACT: Thin film junctions between different organic materials have recently emerged as systems of pressing technological interest due to the commercialization of organic LED-based displays and the need to develop low cost methods of solar energy utilization [1]. The performance of these organic heterojunction devices depends significantly on the details of the nanometer scale interface morphology. However, the driving forces for organic interface morphology are poorly understood as a result of the often subtle interplay between a hierarchy of competing intermolecular interactions. As a result, there is substantial need for research efforts directed toward the detailed phenomenology and modeling of these interfaces.

I will describe scanning tunneling microscopy (STM) experiments that have uncovered a new and generic structural motif for the initial stages of growth of “buckyball” (C60) films on top of several other organic films. In this “dipole fluid” phase, C60 molecules form single molecule wide chains that meander essentially randomly in the plane of the substrate organic film. Reasoning from the substrate independence of this phenomenon, we model the tendency toward chain formation with a variant of the 2D Ising (lattice gas) model [2] that includes long range electrostatic repulsions between induced dipoles on the C60 molecules. This interpretation points out the influence on structure development of the unique dielectric environment of an organic heterojunction.

[1] S.R. Forrest, Nature 428, 911 (2004).

[2] Stoycheva and Singer, Phys. Rev. B 64, 016118 (2001).

Host: Karen Daniels

Oct 14: Jean-Luc Thiffeault

University of Wisconsin, Madison

Stirring and Mixing: Topology, Optimization, and those Pesky Walls

I review various aspects of current research, both experimental and theoretical, on stirring and mixing in fluids. Three main threads are followed: 1) How topological features influence mixing effectiveness; 2) How this leads to novel optimization methods; and 3) How one has to be mindful of wall effects, which can dramatically slow down mixing.

Host: Karen Daniels

Oct 21: Bradley Marts

Dept of Chemistry, Florida State University

Three dimensional waves in excitable systems: Dynamic instabilities in a chemical reaction

Abstract: Excitable media are relevant in a number of biological systems such as neural networks, the pancreas, and cardiac muscle. The underlying dynamics of these complex systems can be explored by studying well controlled model chemical systems. A three dimensional Belousov-Zhabotinsky reaction has been used to demonstrate the behavior of scroll waves like those found in cardiac tissue and believed to play a role in arrhythmia. Qualitative phenomenon such as negative filament tension and scroll wave anchoring have been demonstrated as well as quantitative measurements of the underlying nonlinear diffusion equation for the filament twist.

Host: Karen Daniels

Nov 4: Michael Lublow

Division of Solar Energy, Helmholtz Zentrum Berlin for Materials and Energy

Optical real-time control of silicon nanotopography formation for photocatalytic and photovoltaic applications

Surface and interface topographies at silicon electrodes are assessed by in situ Brewster-angle reflectometry during conditioning in diluted ammonium fluoride solutions. Subsequent Pt-electrodeposition results in the formation of local Schottky-barriers which are important for carrier collection in photovoltaic and photocatalytic applications. In the limit of ultra-thin films, charge flow and reflectance are related by a linear relationship facilitating thus the interpretation of complex optical multi-layer data. Assessment and control of the conditioning process is thereby improved and allows, e.g., the employment of selective surface oxidation steps which transform porous silicon layers into single nanostructures or nano-networks. After introducing this engineering approach, the photovoltaic/photocatalytic device performance in dependence of the silicon interface morphology will be discussed by selected results.

Host: Harald Ade

Nov 11: Laura Wessels

Weninger Group, NCSU Physics

Fusion between a content labelled liposome and an enveloped virus particle

Membrane fusion is critical during enveloped virus entry into cells for release of the viral genome to the cell. We have developed a fluorescence assay to observe individual virus particles fusing with immobilized liposomes. Dye encapsulated inside a liposome will be released into the virus particle's interior through a fusion pore that is created between the liposome's bilayer and the viral envelope. We used Total Internal Reflection Microscopy (TIRFM) to observe fusion between a liposome with calcein in the intravescular buffer and an influenza particle. A sudden buffer exchange to acidic pH is used to trigger the fusion event. TIRFM allows a time resolution of 67 frames/sec. We plan to use confocal microscopy to improve the time resolution of our measurements of the opening of the fusion pore.

Nov 18: Iyam Lynch

Krim Group, NCSU Physics

A quartz crystal microbalance (QCM) study of the dynamics and frictional properties of physisorbed latex microspheres on a Au Surface.

In this study we have analyzed the behavior of 5μm diameter polystyrene spheres physisorbed on the electrode of a quartz crystal microbalance (QCM) in orientations normal and parallel to gravity. By varying the driving voltage of the QCM, the samples exhibit a frequency response with respect to the crystal amplitude (“decoupling curve”). This decoupling curve gives information about the motion of the spheres on the surface and leads to the calculation of the particles slip time which gives rise to the frictional force between the particles and surface. Optical observations show that the particle motion is dependent on the oscillation direction of the QCM and gravity to a lesser extent. Quantitative comparisons of the friction force and the optically observed sliding motion have been performed and will be reported on.

Dec 2: Frederic Lechenault

Daniels group, NCSU Physics

Equilibration of two granular subsystems: A probe for athermal entropy

Abstract: We experimentally investigate the statistical features of the stationary states reached by two idealized granular liquids able to exchange volume. The system consists in two binary mixtures of the same number of soft disks, hence covering the same area, but with different surface properties. The disks sit on a horizontal air table, which provides ultra low friction at the cell bottom, and are separated by a mobile wall. Energy is injected in the system by means of an array of randomly activated coil bumpers standing as the edges of the cell. Due to the energy injection, the system acts like a slow liquid and eventually jams at higher packing fraction. We characterize the macroscopic states by studying the motion of the piston. We find that its average position is different from one half, and a non monotonic function of the overall packing fraction, which reveals the crucial role played by the surface properties in the corresponding density of states. We then study the bulk statistics of the packing fraction and the dynamics in each subsystem. We find that the measured quantities do not equilibrate, and become dramatically different as the overall packing fraction is increased beyond the onset of supercooling. However, the local fluctuations of the packing fraction are uniquely determined by its average, and hence independent of the interaction between disks. We then focus on the mixing properties of such an assembly. We characterize mixing by computing the topological entropy of the braids formed by the stationary tra jectories of the grains at each pressure. This quantity is shown to be well defined, very sensitive to onset of supercooling, reflecting the dynamical arrest of the assembly, and to equilibrate in the two subsystems.