Direct imaging of dynamical heterogeneities near the colloid-gel transition.

We observe the microscopic dynamics of a suspension of colloids with attractive interaction by confocal fluorescence microscopy to provide a deeper understanding of the relationship between local structure and dynamics near the gel transition. We study the distinct and self-parts of the van Hove density-density correlation function applied to our experimental data. Separable fast and slow populations emerge in the self-part, while the distinct part shows a pronounced signature of dynamic heterogeneities close to the gel transition, dominated by the fast particles. The slow population close to the gel transition shares features with an attraction-driven colloidal glass, including a plateau in the mean squared displacement that provides an estimate for the dynamical localization length.

Biaxial deformation of a polymer under shear: NMR test of the Doi-Edwards model with convected constraint release.

2H NMR quadrupole interaction spectroscopy has been used to measure the deformation of a 670 kD poly(dimethylsiloxane) melt under shear in a Couette cell. The signals were acquired from a per deuterated benzene probe molecule which provides a motionally averaged sampling of the entire segmental ensemble. We have measured the dependence on shear rate of the S(XX) (velocity), S(YY) (velocity gradient), S(ZZ) (vorticity), and S(XY) (shear) elements of the segmental alignment tensor, as well as the angular dependence of the deuterium quadrupole splitting at fixed shear rate. We show that the data agree quite well with the Doi-Edwards theory but significantly better when convected constraint release effects are included. These fits return a value for the tube disengagement time of 100 ms.

Selective storage of magnetization in strongly relaxing spin systems.

We have investigated the use of a "split-sinc" RF pulse to selectively store magnetization from a selected region of a sample, for later recall in imaging or spectroscopy experiments. The pulse sequence is based on an original suggestion by Post et al. (West German Patent No. P3209263.6, 13 March 1982), later implemented by Aue et al. (J. Magn. Reson. 56, 350 (1984)). We have carried out detailed numerical calculations using the Bloch equations and show that this particular sequence is robust in the face of strong transverse relaxation, and we demonstrate its application to imaging of polymer samples in shearing and extensional flow cells.