Probing structural relaxation in complex fluids by critical fluctuations

Complex fluids, such as polymer solutions and blends, colloids and gels, are of growing interest in fundamental and applied soft-condensed-matter science. A common feature of all such systems is the presence of a mesoscopic structural length scale intermediate between atomic and macroscopic scales. This mesoscopic structure of complex fluids is often fragile and sensitive to external perturbations. Complex fluids are frequently viscoelastic (showing a combination of viscous and elastic behaviour) with their dynamic response depending on the time and length scales. Recently, non-invasive methods to infer the rheological response of complex fluids have gained popularity through the technique of microrheology, where the diffusion of probe spheres in a viscoelastic fluid is monitored with the aid of light scattering or microscopy. Here we propose an alternative to traditional microrheology that does not require doping of probe particles in the fluid (which can sometimes drastically alter the molecular environment). Instead, our proposed method makes use of the phenomenon of "avoided crossing" between modes associated with the structural relaxation and critical fluctuations that are spontaneously generated in the system.Comment: 4 pages, 4 figure

Competition of Mesoscales and Crossover to Tricriticality in Polymer Solutions

We show that the approach to asymptotic fluctuation-induced critical behavior in polymer solutions is governed by a competition between a correlation length diverging at the critical point and an additional mesoscopic length-scale, the radius of gyration. Accurate light-scattering experiments on polystyrene solutions in cyclohexane with polymer molecular weights ranging from 200,000 up to 11.4 million clearly demonstrate a crossover between two universal regimes: a regime with Ising asymptotic critical behavior, where the correlation length prevails, and a regime with tricritical theta-point behavior determined by a mesoscopic polymer-chain length.Comment: 4 pages in RevTeX with 4 figure

Early postoperative MRI overestimates residual tumour after resection of gliomas with no or minimal enhancement

Standards for residual tumour measurement after resection of gliomas with no or minimal enhancement have not yet been established. In this study residual volumes on early and late postoperative T2-/FLAIR-weighted MRI are compared. A retrospective cohort included 58 consecutive glioma patients with no or minimal preoperative gadolinium enhancement. Inclusion criteria were first-time resection between 2007 and 2009 with a T2-/FLAIR-based target volume and availability of preoperative, early (<48 h) and late (1-7 months) postoperative MRI. The volumes of non-enhancing T2/FLAIR tissue and diffusion restriction areas were measured. Residual tumour volumes were 22% smaller on late postoperative compared with early postoperative T2-weighted MRI and 49% smaller for FLAIR-weighted imaging. Postoperative restricted diffusion volume correlated with the difference between early and late postoperative FLAIR volumes and with the difference between T2 and FLAIR volumes on early postoperative MRI. We observed a systematic and substantial overestimation of residual non-enhancing volume on MRI within 48 h of resection compared with months postoperatively, in particular for FLAIR imaging. Resection-induced ischaemia contributes to this overestimation, as may other operative effects. This indicates that early postoperative MRI is less reliable to determine the extent of non-enhancing residual glioma and restricted diffusion volumes are imperativ

Functional anatomy and differential time courses of neural processing for explicit, inferred, and illusory contours. An event-related fMRI study.

The perception of shape does not necessarily require viewing an explicit outline figure. Using event-related functional magnetic resonance imaging we examined the time courses of neural activations provoked by shapes defined by (1) lines, (2) illusory contour inducers, and (3) reversed inducers. SPM99 was used to analyze the common and differential neural responses associated with the stimuli and their temporal derivatives. Illusory figures versus reversed inducers activated extrastriate cortex. Reversed inducers versus illusory figures activated the right parietal cortex. For both illusory and line contours versus reversed inducers, analysis of the temporal derivatives showed earlier activations in extrastriate and left parietal cortex and for line contours also in the extrastriate cortex bilaterally and in the right parietal cortex; these earlier activations were mirrored by differences in reaction times with subjects responding more slowly to shapes defined by reversed inducers. The results show substantial bottom-up effects (in occipital cortex) in the recognition of illusory and explicit shapes. By contrast, in stimuli where the shape must be inferred, there is greater reliance on right parietal cortex, consistent with increased attentional demands and top-down processing. The temporal derivatives provide useful information on the differential timing of the associated hemodynamic responses in occipital, parietal, and motor cortex

Structure and interfacial interactions of thin films of amylopectin

Structure and interfacial properties of micrometer-thick amylopectin films were investigated. The polysaccharide amylopectin forms the crystalline regions in native starch. By applying X-ray scattering at wide and small angle detection modes, and surface microscopy (phase interference microscopy and atomic force microscopy), we were able to probe the film structure and the surface morphology. The thin films were found to be amorphous with some degree of orientational order in the direction normal to the surface. A step-like surface morphology was observed, with a typical step-height of 100 nm and lateral dimensions of 100 mu m We investigated the interfacial interaction between amylopectin and polystyrene and found that the stability of polystyrene films on top of the amylopectin substrate depends on film thickness: thin films (adsorbed layers of a few nanometers) were stable due to short-range interactions, while thicker films (of a few hundred nanometers) dewetted due to van der Waals interactions

Static and dynamic light scattering experiments on semidilute solutions of polystyrene in cyclohexane between the Theta-temperature and the binodal curve

Static and dynamic light scattering experiments are carried out with solutions of a high molar mass polystyrene (M-w = 0.96 x 10(6) g mol(-1)) in cyclohexane at different concentrations of the polymer. At the Theta-temperature T-Theta the critical concentration of the system is located in the semidilute regime. The temperature is varied between T-Theta and the temperature of phase separation T-P of the solutions. At T-Theta the distribution of the amplitudes of the relaxation rates of the autocorrelation function of the electric field is bimodal (slow and fast relaxation rates) in the semidilute regime. It becomes monomodal approaching T-P. The contribution of the fast relaxation rates disappears and the weak q-dependence of the slow mode changes over to a q-dependence similar to that of the critical relaxation rate in binary liquid mixtures of low molar mass components close to the critical temperature T-c. On approaching the temperature of phase separation by decreasing the temperature, concentration fluctuations with long range correlations develop. The change of the q-dependence of the slow relaxation rate is rationalised using the transient gel model of Brochard and de Gennes. Here it is assumed that approaching T-P transient gels formed by interpenetrating polymer coils become part of the concentration fluctuations with long range correlations. The dynamics of the solutions in the vicinity of the binodal curve monitored by dynamic light scattering experiments is dominated by the dynamics of the concentration fluctuations with long range correlation

Development of brain infarct volume as assessed by magnetic resonance imaging (MRI): follow-up of diffusion-weighted MRI lesions.

PURPOSE: To investigate the development of ischemic brain lesions, as present in the acute stroke phase, by diffusion-weighted magnetic resonance imaging (DWI), and in the subacute and chronic phases until up to four months after stroke, in fluid-attenuated inversion recovery (FLAIR)- and T2-weighted (T2W) magnetic resonance (MR) images. MATERIALS AND METHODS: Twelve consecutive patients with their first middle cerebral artery (MCA) infarction were included. Lesion volumes were assessed on T2W images recorded with a turbo spin echo (TSE) and on images recorded with the FLAIR sequence on average on day 8 and after about four months. They were compared with acute lesion volumes in perfusion and DWI images taken within 24 hours of stroke onset. RESULTS: On day 8, lesion volumes in images obtained with FLAIR exceeded the acute infarct volumes in DWI. The chronic lesion volumes were almost identical in T2W and FLAIR images but significantly reduced compared with the acute DWI lesions. The lesion volumes assessed on DWI images correlated highly with the lesions in the images obtained with TSE or FLAIR, as did the lesions in the images obtained with FLAIR and TSE. The secondary lesion shrinkage was accompanied by ventricular enlargement and perilesional sulcal widening, as most clearly visible in the images obtained with FLAIR. CONCLUSION: Our results show that the acute DWI lesions are highly predictive for the infarct lesion in the chronic stage after stroke despite a dynamic lesion evolution most evident in MR images obtained with FLAIR