Simple method for the complete optical analysis of very thick and weakly absorbing films

In this paper a method for the determination of the thickness and spectral dependences of the optical constants characterizing very thick and weakly absorbing thin films located on nonabsorbing substrate is presented. Within the framework of this method the positions and values of the extrema in the spectral dependences of the reflectance and transmittance are utilized for evaluation the values of the optical parameters mentioned above. The main advantage of the method is that the values of the optical parameters can be determined by means of explicite formulae with relatively high accuracy. The method is utilized for the complete optical analysis of magnetic garnet films.     

On the Applicability of Plasma Assisted Chemical Micropatterning to Different Polymeric Biomaterials

A plasma process sequence has been developed to prepare chemical micropatterns on polymeric biomaterial surfaces. These patterns induce a guided localized cell layover at microscopic dimension. Two subsequent plasma steps are applied. In the first functionalization step a microwave ammonia plasma introduces amino groups to obtain areas for very good cell adhesion; the second passivation step combines pattern generation and creation of cell repelling areas. This downstream microwave hydrogen plasma process removes functional groups and changes the linkages of polymer chains at the outermost surfaces. Similar results have been obtained on different polymers including polystyrene (PS), polyhydroxyethylmethacrylate (PHEMA), polyetheretherketone (PEEK), polyethyleneterephthalate (PET) and polyethylenenaphthalate (PEN). Such a rather universal chemical structuring process could widen the availability of biomaterials with specific surface preparations.     

Coherent dynamics of meta-toluidine investigated by quasielastic neutron scattering

The coherent dynamics of a typical fragile glass former, meta-toluidine, was investigated at the molecular level using quasielastic neutron scattering, with time-of-flight and neutron spin echo spectrometers. It is well known that the static structure factor of meta-toluidine shows a prepeak originating from clustering of the molecules through hydrogen bonding between the amine groups. The dynamics of meta-toluidine was measured for several values of the wavevector transfer Q, which is equivalent to an inverse length scale, in a range encompassing the prepeak and the structure factor peak. Data were collected in the temperature range corresponding to the liquid and supercooled states, down to the glass transition. At least two dynamical processes were identified. This paper focuses on the slowest relaxation process in the system, the α-relaxation, which was found to scale with the macroscopic shear viscosity at all the investigated Q values. No evidence of "de Gennes" narrowing associated with the prepeak was observed, in contrast with what happens at the Q value corresponding to the interparticle distance. Moreover, using partially deuterated samples, the dynamics of the clusters was found to be correlated to the single-particle dynamics of the meta-toluidine molecules.     

A new interpretation of dielectric data in molecular glass formers

Literature dielectric data of glycerol, propylene carbonate, and ortho-terphenyl show that the measured dielectric relaxation is a decade faster than the Debye expectation but still a decade slower than the breakdown of the shear modulus. From a comparison of time scales, the dielectric relaxation seems to be due to a process which relaxes not only the molecular orientation but also the entropy, the short range order, and the density. On the basis of this finding, we propose an alternative to the Gemant-DiMarzio-Bishop extension of the Debye picture.     

Coupling of neural activity and fMRI-BOLD in the motion area MT

The fMRI-BOLD contrast is widely used to study the neural basis of sensory perception and cognition. This signal, however, reflects neural activity only indirectly, and the detailed mechanisms of neurovascular coupling and the neurophysiological correlates of the BOLD signal remain debated. Here we investigate the coupling of BOLD and electrophysiological signals in the motion area MT of the macaque monkey by simultaneously recording both signals. Our results demonstrate that a prominent neuronal response property of area MT, so-called motion opponency, can be used to induce dissociations of BOLD and neuronal firing. During the presentation of a stimulus optimally driving the local neurons, both field potentials [local field potentials (LFPs)] and spiking activity [multi-unit activity (MUA)] correlated with the BOLD signal. When introducing the motion opponency stimulus, however, correlations of MUA with BOLD were much reduced, and LFPs were a much better predictor of the BOLD signal than MUA. In addition, for a subset of recording sites we found positive BOLD and LFP responses in the presence of decreases in MUA, regardless of the stimulus used. Together, these results demonstrate that correlations between BOLD and MUA are dependent on the particular site and stimulus paradigm, and foster the notion that the fMRI-BOLD signal reflects local dendrosomatic processing and synaptic activity rather than principal neuron spiking responses.     

On the relaxation of shear and normal stresses of viscoelastic fluids following constant shear rate experiments

By means of a cone and plate rheometer the relaxation of the shear stress and the first normal stress difference in polymer liquids upon cessation of a constant shear rate were examined. The experiments were conducted mostly in a high shear rate region of relevance for the processing of these materials. The relaxation behavior at these shear rates can only be measured accurately under extremely precise specifications of the rheometer. To determine under which conditions the integral normal thrust is a convenient measure for the relaxing local first normal stress difference the radial distribution of the pressure in the shear gap was measured. The shape of relaxation of both the shear stress and the first normal stress difference could be closely approximated for the entire measured shear rate and time range by a two parameter statistical function. In the range of measured shear rates, one of the parameters, the standard deviationS, is equal for the shear and the normal stress, and is independent of the shear rate within the limit of experimental error. The second parameter, the mean relaxation timet _50, of the shear stress andt _50, of the first normal stress difference, can be calculated approximately from the viscosity function and only a single relaxation experiment.     

Red blood cell rheology using single controlled laser-induced cavitation bubbles

The deformability of red blood cells (RBCs) is an important property that allows the cells to squeeze through small capillary vessels and can be used as an indicator for disease. We present a microfluidic based technique to quantify the deformability of RBCs by stretching a collection of RBCs on a timescale of tens of microseconds in a microfluidic chamber. This confinement constrains the motion of the cell to the imaging plane of the microscope during a transient cavitation bubble event generated with a focused and pulsed laser. We record and analyze the shape recovery of the cells with a high-speed camera and obtain a power law in time, consistent with other dynamic rheological results of RBCs. The extracted exponents are used to characterize the elastic properties of the cells. We obtain statistically significant differences of the exponents between populations of untreated RBCs and RBCs treated with two different reagents: neuraminidase reduces the cell rigidity, while wheat germ agglutinin stiffens the cell confirming previous experiments. This cavitation based technique is a candidate for high-throughput screening of elastic cell properties because many cells can be probed simultaneously in situ, thus with no pre-treatment.     

Bounds on the coupling parameters in field theoretical models of nuclear physics

Neutrons of energies within a 50 eV interval at 1970 eV have been selected from reactor neutrons by means of resonance scattering on a target of⁶³Cu and subsequently by the 1970 eV resonance of a⁸⁰Se target. Insertion of stationary filters and the technique of difference measurements with a resonance filter resulted in a high selectivity, which allowed the determination of cross sections for quasi-monoenergetic neutrons for the elements: H, C, O, F, Na, Mg, Al, Si, P, Cl, K, Ca, Sc, Ti, V, Mn, Co, Ni, Cu, Ga, Pb and Bi. The precision cross sections of Pb and H contribute to investigations of fundamental neutron interactions. The measured σ(Pb)=11.198±0.003 b was recently used to deriveα _n=(0.8±1)10^?3 fm³ for the electric polarizability of the neutron. The neutron-proton cross sectionσ(¹ H)=20.13±0.03 b and data at 143 keV, 〈1.3〉 MeV, 〈2.1〉 MeV and from the literature provide a refined set of the scattering parameters for the shape-independent effective-range approximation of the neutron-proton interaction.