Georg Maret

Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation

Abstract Suspensions of rod-like cellulose crystallites of axial ratio ≈ 20–40, prepared by acid hydrolysis of natural cellulose fibres with sulphuric acid, give stable ordered fluids that display well-formed textures and disclinations characteristic of chiral nematic liquid crystalline phases. The critical volume fraction for phase separation of salt-free suspensions is typically 0.03, with a relatively narrow biphasic region. Because of the negative diamagnetic susceptibility of cellulose, the ordered phase becomes oriented in a magnetic field with its chiral nematic axis parallel to the applied field.

Imaging of dynamic heterogeneities in multiple-scattering media

A new method of visualizing objects with distinct internal dynamics of the constituent scattering particles embedded in a liquid multiple-scattering medium is presented. We report dynamic multiple-light-scattering experiments and a theoretical model, based on diffusing photon-density waves for concentrated colloidal suspensions in Brownian motion, as a background medium into which is inserted a capillary containing (i) the same suspension under flow, or (ii) suspensions of different particle sizes in Brownian motion. These model objects, with purely dynamic but no static scattering contrast, can be visualized by space-resolved measurements of the time autocorrelation function g2(t) of the scattered light intensity at the sample surface. Maximum contrast occurs at a parameter-dependent finite correlation time t. The physical origin of this effect is outlined. Our data are in excellent quantitative agreement with the model, with no adjustable parameter.

Diffusing-wave spectroscopy from head-like tissue phantoms: influence of a non-scattering layer

We investigate the influence of a non-scattering layer on the temporal field autocorrelation function of multiple scattered light from a multilayer turbid medium such as the human head. Data from Monte Carlo simulations show very good agreement with the predictions of the correlation-diffusion equation with boundary conditions taking into account non-diffusive light transport within the non-scattering layer. Field autocorrelation functions measured at the surface of a multilayer phantom including a non-scattering layer agree well with theory and simulations when the source-receiver distance is significantly larger than the depth and the thickness of the non-scattering layer. Our results show that for source-receiver distances large enough to probe the dynamics in the human cortex, the cortical diffusion coefficient obtained by analyzing field autocorrelation functions neglecting the presence of the non-scattering cerebrospinal fluid layer is underestimated by about~

Non-thermal microwave effects on protein dynamics? An X-ray diffraction study on tetragonal lysozyme crystals

40,%

Static imaging of dynamic fluctuations in multiple light scattering media

in situations representative of the human head.

Synthesis and characterization of hollow and non-hollow monodisperse colloidal TiO2 particles

X-ray diffraction (XRD) was used to investigate the structural and dynamical effects of microwave fields on tetragonal single crystals of hen egg-white lysozyme at a resolution of 2.0 A. Using a modified slab-line waveguide allows on-line XRD to be carried out while the protein crystal is exposed to well defined microwave fields. High microwave power levels mainly lead to increased, but largely recoverable, lattice defects owing to the evaporation of crystal water. At lower microwave power levels, the presence of the microwave field results in localized reproducible changes in the mean-square displacements (B factors). At particular sites, it is found that the B factors even decrease with increasing microwave power. Most of these effects can be explained by a comparison of the data obtained under microwave irradiation with data obtained at elevated temperature which simulate heating owing to microwave absorption by unbound crystal water. The data show no indication of large microwave-driven displacements of structural subunits in the protein that would be expected if microwaves were to be absorbed resonantly by protein vibrations. Rather, the observed changes in the atomic mean-square displacements suggest that if microwaves couple non-thermally to globular proteins at hydration levels at which they still function, their effect on protein dynamics and structure is very small.

A broadband waveguide for protein crystallography under intense microwave fields

Multiple light scattering experiments in backscattering geometry are performed on highly viscous and solid turbid media which contain macroscopic inclusions of a low viscosity colloidal suspension. We demonstrate that the intensity probability distribution P(I) of the multiple scattering speckle patterns can be used to image an inclusion even if its photon transport mean free path matches the value of the surrounding medium. The main difference between solid and liquid samples is discussed.

Measurements of human motor and visual activities with diffusing-wave spectroscopy

Abstract Monodisperse spherical hollow and non‐hollow titania particles of variable sizes are produced in a sol–gel synthesis from Ti(EtO)4 in ethanol. Hollow spherical particles of rutile were obtained by coating colloidal polystyrene beads with a titanium oxide hydrate layer and subsequently calcination at elevated temperatures in oxygen atmosphere. The non‐hollow titania particles were produced in the presence of salt or polymer solution. The influence of different salt ions or polymer molecules on the size and on the size distribution of the non‐hollow particles was investigated. Nitrogen absorption measurements revealed that the addition of polymers yields porous titania colloids.

Imaging of Dynamic Heterogeneities in Multiple Light Scattering

We present a slab-line waveguide whose geometry is optimized for wide-angle x-ray diffraction (XRD) experiments on protein crystals during irradiation with intense microwave fields. Characterization of the waveguide transmission and reflectivity (using time-domain reflectometry) and of the electric field distribution inside the waveguide (using finite-difference time-domain calculations) shows that the present device has a broad bandwidth from below 0.5u2002tou200218u2002GHz, allowing one to perform frequency-dependent XRD studies with a well-defined transverse mode structure and negligible reflection losses. As shown with a specific example, our device provides a simple way to couple microwave irradiation experiments with high-resolution x-ray diffraction measurements from millimeter-size crystalline samples. The present design might prove useful for systematic studies of microwave effects on protein structure and dynamics.

Orientation of cellulose microcrystals by strong magnetic fields

Diffusion coefficients in the human sensorimotor and visual cortices were measured using diffusing-wave spectroscopy. Motor and visual activation leads to increases of the diffusion coefficients in the respective cortical areas over the values at rest.