Session F2: Symposium on Soft Matter III

Saturday, October 24, 2009

We present a theory for calculating the fractal dimension of Diffusion Limited Cluster Aggregates (DLCA) based on cluster shape preservation. The shape is described by a d-dimensional Golden Mean, which is the ratio of consecutive d-dimensional Fibonacci numbers. For d =2 the canonical Fibonacci series is found with the Golden Mean value known since antiquity, phi = 1.618{\ldots} to yield a fractal dimension of 1.44, in agreement with simulations and experiment. Generalizations to other dimensions are equally successful. Recent computer simulations also yield accurate values for the fractal aggregate prefactor, thus completing the theory.   

Saturday, October 24, 2009

The electric dipole moments of globular proteins, determined experimentally by dielectric relaxation spectroscopy, contribute to both protein function and structure. Numerical computations of dipole moments can be obtained from structures in the Protein Data Bank. However, previous computations in literature have agreed with experimental results for only a limited number of proteins. This paper presents a method to compute the pH-dependent dipole moment. The protein molecule is considered as an array of electrical point charges in aqueous solution. The dipole moment is calculated as the vector sum of two components: (1)the core dipole moment which emerges from the unequal sharing of electrons in covalent bonds; (2)the surface charge dipole moment resulting from pH-dependent side chain partial charges. pKa shifts for each side chain amino acid are determined by the H++ server employing the Poisson-Boltzmann equation. The net charge and dipole moment over a range of pH are calculated. The Oncley equation is used to predict the dielectric increment at arbitrary pH, temperature, and protein concentration.   

Saturday, October 24, 2009

We explore liquid crystal order in a cell with a ``dirty'' substrate imposing a random surface pinning. Modeling such systems by a random-field xy-model with {\em surface} heterogeneity, we find that orientational order in the three-dimensional system is marginally unstable to such surface pinning. We compute the Larkin length scale, and the corresponding surface and bulk distortions. On longer scales we calculate correlation functions using the functional renormalization group and matching methods, finding a universal logarithmic and double-logarithmic roughness in two and three dimensions, respectively. For a finite thickness cell, we explore the interplay of homogeneous-heterogeneous substrate pair and detail crossovers as a function of disorder strength and cell thickness.   

Saturday, October 24, 2009

Photo-active materials are widely applied in optical data storage,wave guides,refractive index manipulation, and photo-alignment of liquid crystals. Many of these materials involve an azobenzene unit, which undergoes trans-cis photoisomerization. In this work, a methyl red derivative (dMR) monolayer was studied to understand its high light sensitivity and power-law kinetics when responding to polarized light. We simulated a variety of monolayer molecular fields by dilute dMR solutions in different organic solvents. We studied the spectral and time-dependent characteristics of the absorbance in these environments and in monolayer films. The cis-trans thermal relaxation demonstrates an unexpectedly complex dependence on molecular environments that may be related to the monolayer behavior.   

Saturday, October 24, 2009

Organic semiconductors such as MEH-PPV are carbon based conjugated polymers and, as a result of their low nuclei mass, charge carriers within these materials have very small spin-orbit coupling. With this absence of spin-orbit coupling, strong spin selection rules are imposed on their optical and electrical transitions. As these materials are being utilized for light harvesting and device applications, the nature of the spin-dependent transitions is of significant interest. In this talk I will discuss the details of a novel magnetic resonance technique that is sensitive to these spin-dependent transitions, namely pulsed electrically detected magnetic resonance. We show that, with the application of spin-resonant magnetic field pulses, the device current can be modulated by the spin-Rabi nutation of charge carrier pairs between triplet and singlet configurations in both solar cell and light emitting diodes. With this tool we can address questions regarding efficiency, spin coherence times, recombination and generation rates, all of which are paramount to understanding the overall effectiveness of these unique materials.