Molecular orientation and dynamics of flexible polymers in strongly deforming flow fields

A method of spatially resolved magnetic resonance spectroscopy has been developed to allow studies of order and dynamics in complex fluids having transverse relaxation times on the order of tens of milliseconds, studies which were otherwise not possible using existing techniques. The model of Doi and Edwards is a microscopic description for stress transmission in concentrated polymer solutions and melts under deformation. Central to the Doi-Edwards model is the dependence of the stress on bond orientational order of the chain segments. Different elements of the segmental alignment tensor for a polymer melt under strong shearing flow are measured here using localized deuterium NMR spectroscopy on a 61 OK molecular weight poly (dimethyl siloxane) melt in a concentric cylinder Couette rheometric cell. This approach provides a new means of testing the Doi-Edwards model and its refinements, in the important regime far from equilibrium where the entangled polymers exhibit nonlinear viscoelastic behaviour. -- The same rheo-NMR methodology is also used to test predictions of the model of Leslie and Ericksen which describes director dynamics in semi-flexible rod-like polymers subjected to viscous stresses. Director dynamics are studied in a lyotropic liquid crystal polymer PBLG (300K) in a highly ordered, nematic phase in a planar extensional flow around a stagnation point. In addition, bulk 2H NMR studies are carried out on PBLG under shear, in concentric cylinder Couette and cone and plate rheometric cells. Magnetic alignment (equivalent to the dynamic Freedericksz transition) is investigated in all three cells following deformation. Values are obtained for the Leslie viscosity coefficients a2 and a3, scaled by the diamagnetic susceptibility. Possible development of mesoscale structure under shear is discussed

A symmetrical method to obtain shear moduli from microrheology

Passive microrheology typically deduces shear elastic loss and storage moduli from displacement time series or mean-squared displacement (MSD) of thermally fluctuating probe particles in equilibrium materials. Common data analysis methods use either Kramers-Kronig (KK) transformations or functional fitting to calculate frequency-dependent loss and storage moduli. We propose a new analysis method for passive microrheology that avoids the limitations of both of these approaches. In this method, we determine both real and imaginary components of the complex, frequency-dependent response function $\chi(\omega) = \chi^{\prime}(\omega)+i\chi^{\prime\prime}(\omega)$ as direct integral transforms of the MSD of thermal particle motion. This procedure significantly improves the high-frequency fidelity of $\chi(\omega)$ relative to the use of KK transformation, which has been shown to lead to artifacts in $\chi^{\prime}(\omega)$. We test our method on both model data and experimental data. Experiments were performed on solutions of worm-like micelles and dilute collagen solutions. While the present method agrees well with established KK-based methods at low frequencies, we demonstrate significant improvement at high frequencies using our symmetric analysis method, up to almost the fundamental Nyquist limit.Comment: 8 pages, 4 figure

²H NMR studies of the effect of the DPPC/DPPG ratio on bilayer properties in the presence of Ca²⁺.

The presence of Ca²⁺ ions in the aqueous medium is known to influence the physical properties of bilayer membranes containing charged phospholipids such as dipalmitoylphosphatidylglycerol (DPPG). 2H NMR has been used to study the effect of Ca²⁺ on the order, dynamics and phase behaviour of bilayers containing mixtures of the anionic DPPG and a neutral lipid with identical acyl chains, dipalmitoylphosphatidylcholine (DPPC), dispersed in aqueous solutions. The effect of bilayer surface charge in this system was investigated by varying the DPPG/DPPC ratio in the presence of excess Ca²⁺. It was found that Ca²⁺ alters the temperature and width of the liquid crystal to gel bilayer transition and the quadrupole echo relaxation times of chain deuterons in a way which depends on the proportion of negatively charged lipid in the bilayer. The observed effects are consistent with a Ca²⁺-induced reduction in area per lipid of liquid crystal bilayers containing DPPG. The results do not support a preferential association of Ca²⁺ with the DPPG headgroup or segregation into DPPG or DPPC domains. The introduction of a negatively charged lipid component into DPPC membranes may alter the hydrogen bonding or modify the water layer, or both, resulting in changes in the adiabatic bilayer motions

A random walk description of the heterogeneous glassy dynamics of attracting colloids

We study the heterogeneous dynamics of attractive colloidal particles close to the gel transition using confocal microscopy experiments combined with a theoretical statistical analysis. We focus on single particle dynamics and show that the self part of the van Hove distribution function is not the Gaussian expected for a Fickian process, but that it reflects instead the existence, at any given time, of colloids with widely different mobilities. Our confocal microscopy measurements can be described well by a simple analytical model based on a conventional continuous time random walk picture, as already found in several other glassy materials. In particular, the theory successfully accounts for the presence of broad tails in the van Hove distributions that exhibit exponential, rather than Gaussian, decay at large distance.Comment: 13 pages, 5 figs. Submitted to special issue "Classical and Quantum Glasses" of J. Phys.: Condens. Matter; v2: response to refere

Accurate detection and complete tracking of large populations of features in three dimensions

Localization and tracking of colloidal particles in microscopy images generates the raw data necessary to understand both the dynamics and the mechanical properties of colloidal model systems. Yet, despite the obvious importance of analyzing particle movement in three dimensions (3D), accurate sub-pixel localization of the particles in 3D has received little attention so far. Tracking has been limited by the choice of whether to track all particles in a low-density system, or whether to neglect the most mobile fraction of particles in a dense system. Moreover, assertions are frequently made on the accuracies of methods for locating particles in colloid physics and in biology, and the field of particle locating and tracking can be well-served by quantitative comparison of relative performances. We show that by iterating sub-pixel localization in three dimensions, the centers of particles can be more accurately located in three-dimensions (3D) than with all previous methods by at least half an order of magnitude. In addition, we show that implementing a multi-pass deflation approach, greater fidelity can be achieved in reconstruction of trajectories, once particle positions are known. In general, all future work must defend the accuracy of the particle tracks to be considered reliable. Specifically, other researchers must use the methods presented here (or an alternative whose accuracy can be substantianted) in order for the entire investigation to be considered legitimate, if the basis of the physical argument (in colloids, biology, or any other application) depends on quantitative accuracy of particle positions. We compare our algorithms to other recent and related advances in location/tracking in colloids and in biology, and discuss the relative strengths and weaknesses of all the algorithms in various situations. We carry out performance tests directly comparing the accuracy of our and other 3D methods with simulated data for both location and tracking, and in providing relative performance data, we assess just how accurately software can locate particles. We discuss how our methods, now applied to colloids, could improve the location and tracking of features such as quantum dots in cells