– It has been realised that noise plays an important rôle in cellular processes where fluctuation induced number fluctuations of certain messenger molecules become non–negligible, due to the small total number of these molecules within one cell. In the following, it is argued that spatial fluctuations of such molecules and their impact on genetic switches should be considered as well.

We investigate diffusion on newly synthesized molecules with dendrimer structures. We model these structures with geometrical Cayley trees. We focus on diffusion properties, such as the excursion distance, the mean square displacement of the diffusing particles, and the area probed, as given by the walk parameter SN, the number of the distinct sites visited, on different coordination number, z, and different generation order g of a dendrimer structure. We simulate the trapping kinetics curves for randomly distributed traps on these structures, and compare the finite and the infinite system cases, and also with the cases of regular dimensionality lattices. For small dendrimer structures, SN approaches the overall number of the dendrimer nodes, while for large trees it grows linearly with time. The average displacement R also grows linearly with time. We find that the random walk on Cayley trees, due to the nature ot these structures, is indeed a type of a “biased” walk. Finally we find that the finite-size effects are particularly important in these structures.

Two-photon time-resolved fluorescence anisotropy methods were used to study the dynamical environment when fluorescent-labelled DNA oligomers (labelled with FAM, 6-fluorescein-6-carboxamido hexanoate) formed surface complexes with quaternized polyvinylpyridine (QPVP) cationic layers on a glass surface. We compared the anisotropy decay of DNA in bulk aqueous solution, DNA adsorbed onto QPVP, and QPVP-DNA-QPVP sandwich structures. When DNA was adsorbed onto QPVP, its anisotropy decay was dramatically retarded compared to the bulk, which means it had very slow rotational motion on the surface. Motions slowed down with increasing salt concentration up to a level of 0.1 M NaCl, but mobility began to increase at still higher salt concentration owing to detachment from the surface-immobilizing QPVP layers.

Electron escape over a one-dimensional potential barrier is treated with a Monte Carlo method that incorporates simple models for the electron-phonon interaction. The consequences of these models are considered here through the calculation of the escaping electron velocity distribution and the electron energy distribution before escape. Effective temperatures are derived from both distributions. The numerical results are compared with those from the classical model of thermionic emission.

The systematic variation in line width of ferromagnetic resonance with the Fe content was observed at X band (9.78GHz) in (Ni0.5Zn0.5)1−xFe2+xO4 (-0.2 ≤ x ≤ 0.2). The line width of the stoichiometric composition (x = 0) showed minimum value, 50 Oe. In contrast, the line width of the non-stoichiometric compositions sharply increased to 210 Oe with increasing nonstoichiometry (x). The mechanism for this line width broadening was investigated using thermoelectric power and electrical resistivity, since the contribution of anisotropy and porosity to the line width was negligible in this compositional region. In Fe excess region, Fe2+ ion concentration increased with increasing Fe content, resulting in line width broadening due to relaxation. But, it was suggested that Ni3+ and Fe2+ ions coexist in Fe deficient region. Therefore the increase of line width in nickel-zinc ferrites originated from the Fe2+ /Fe3+ magnetic relaxation in Fe excess region, and the Fe2+/Fe3+, Ni2+/Ni3+ magnetic relaxation in Fe deficient region.

We have used both conventional small-angle neutron scattering and contrast variation techniques to characterize the polymer conformations of two non-ionic water soluble polymers: poly(ethylene oxide) and poly(N-vinyl-2-pyrollidone). The second of these is able to kinetically suppress hydrate crystallization, and an objective of these studies is to obtain a understanding of this inhibition mechanism. The dilute-solution polymer conformation in a hydrate-forming tetrahydrofuran/water fluid shows only a small difference between the polymers. The single- chain characteristics are unperturbed, but the hydrate inhibitor polymer seems to show an enhanced tendency to form aggregates in solution. This is evidenced by excess low-q scattering following a q-2.5 power law. Much more evident is the strong perturbation in the conformation of the inhibitor polymer upon crystallization of the hydrate. We utilize contrast variation methods to resolve the scattering of the polymer on the hydrate surface. Unlike expectations from polymer scaling laws, the resulting layer is considerably thicker (550) than the single- chain radius of gyration, 80. Also surprising, only 2 percent of the available crystal surface is covered. Within the covered areas, the polymer concentration is significantly enhanced over that in the surrounding solution, about 2.5c* (where c* is the overlap concentration). This suggests a coverage of clumps of polymer widely separated from one another. Consideration of the concentration and implied spacing of such clumps on the hydrate surface suggests that they can effectively inhibit crystal growth.

The microdynamics of water and oil in macroporous media with SiO2 or CaCO3 surfaces has been probed at various temperatures by magnetic field-cycling measurements of the spin-lattice relaxation rates. These measurements and an original theory of surface diffusion allowed us to obtain surface dynamical parameters, such as a coefficient of surface affinity of the liquid molecules and the surface diffusion coefficient. The water surface diffusion coefficients are compared to the volume self-diffusion coefficients of water in pores, measured by PGSE method, the latter values being more than an order of magnitude higher than the surface ones. Complementary information on the nature of the solid-liquid interface was given by NMR chemical shift experiments at high magnetic field.

Shear melting of a thin layer trapped between two crystalline solids is considered in the frame ofü the Landau's theory of phase transitions. Kinetics of melting and solidifying by static and periodic loading is analyzed.

Several workers have argued that should be negligible static friction per unit area in the macroscopic solid limit for interfaces between both incommensurate and disordered atomically flat weakly interacting solids. In contrast, the present work argues that in the low load limit, when one considers disorder on the multiasperity scale, the asperties act independently to a good approximation. This can account for the virtual universal occurence of static friction.

Polymer dispersed liquid crystals (PDLC) are materials formed by nematic liquid crystals droplets with radii of a few hundred Å embedded in a polymer matrix. We discuss the use of relaxation methods for the study of the response of the director of a PDLC under the switching of an external electric field. We simulate the confining system by considering different boundary conditions at the droplet surface.

We study the dynamics of sol-gel clusters. We find that the dynamic structure factor decays in time as a stretched exponential, and the viscoelastic modulus is an algebraic function of time. The difference beween screened and non-screened systems in the context of cluster dynamics is discussed.

Wettability is a manifestation of rock-fluid interactions associated with fluid distribution in porous media. Conventional wettability evaluation is performed by a sequence of spontaneous and forced displacements of different fluids into a porous sample, a method which is costly and time consuming. A new attractive approach is to estimate this quantity from dielectric measurements, since they can be done rapidly and economically.

The dielectric frequency response of several rock samples of known wettability condition was studied in the range from 10 Hz to 100 MHz. Samples were saturated with brine and oil. The results confirm the strong influence of wetting condition on dielectric response. Water wet samples have significantly higher values of and (real and imaginary parts of generalized complex permitivity) than oil wet samples. In particular, the high frequency behavior of is most affected. Different regimes are identified as a function of frequency. They correspond to zones where different polarization effects are manifested. We quantify this effect and find a correlation with the modified Amott wettability index. Based on these findings, we propose an experimental protocol for the indirect measurement of wettability at laboratory scale.

We have studied adsorption of argon in a mesoporous silica Controlled Porous Glass (CPG) means of Grand Canonical Monte-Carlo (GCMC) simulation. A numerical sample of the CPG adsorbent has been obtained by using an off-lattice reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered porous materials. The off-lattice functional of (100 m2/g)-Vycor is applied to a simulation box containing silicon and oxygen atoms of cubic cristoballite with an homothetic reduction of factor 2.5 so to obtain 30Å-CPG sample. A realistic surface chemistry is then obtained by saturating all oxygen dangling bonds with hydrogen. The Ar, Kr and Xe adsorption/desorption isotherms are calculated at different temperatures. At sufficiently low temperature, they exhibit a capillary condensation transition with a finite slope by contrast to that theoretically predicted for simple pore geometriessuch as slits and cylinders. In the low pressure and temperature domain, we have identified different adsorption scenarios, which can be interpreted on the basis of a Zisman-type of criterium for wetting. We demonstrate that the BET surface area is strongly related to this criterion. At higher pressure, we demonstrate that the pore size distribution obtained by using the standard BJH analysis applied to both adsorption and desorption data qualitatively reproduces the main features of the chord length distribution.

We investigate the response of an embedded system subject to an external drive using a microscopic model. The shear is shown to excite “shearons”, which are collective modes of the embedded system with well defined spatial and temporal patterns that dominate the frictional properties of the driven system. We demonstrate that the slip relaxation in stick-slip motion and memory effects are well described in terms of the creation and/or annihilation of shearons.

Phenylacetylene (PA) dendrimer labeled with perylene (See Fig 1) is discovered to exhibit temperature-dependent emission spectra in certain organic solvents over the temperature range of 20-65°C. The monomer signal is increasing rapidly when temperature increases, while the excimer signal decreases slowly. Models of excimer formation and weakly associated pairs (M+M) dissociation dynamics are included, and the equilibrium constants at different temperatures are calculated. This behavior suggests potential applications in fluorescence-based thermometry.

The intra- and interchain structure of sodium poly(styrenesulphonate) when free and when confined in contrast matched porous Vycor has been investigated by SANS. When confined, a peak is observed whose intensity increases with molecular weight and the 1/q scattering region is extended compared to the bulk. We infer that the chains are sufficiently extended, under the influence of confinement, to highlight the large scale disordered structure of Vycor. The asymptotic behavior of the observed interchain structure factor is ≍ 1/q2 and ≍ 1/q for free and confined chains respectively.

X-ray specular and off-specular reflectivity studies have been carried out to study the density modulations in liquids confined between two smooth silicon mirrors. The special technique as well as the advantages of using high energy and high brilliance synchrotron x-ray beams for carrying out such experiments will be discussed. Results will be presented on the ordering of octamethyl-cyclotetrasiloxane (OMCTS) as a function of the confining pressure, where we find evidence of layering as the gap is decreased from macroscopic down to a few nanometers.

Two model studies are presented in order to elucidate the effect of confinement on glass forming fluids, attempting to study the effects of the interactions between the confining walls and the fluid particles. In the first model, short bead-spring chains (modelling a melt of flexible polymers) are put in between perfectly flat, structureless walls, on which repulsive potentials act. It is shown that chains near the walls move faster (in the direction parallel to the walls) than chains in the bulk. A significant decrease of the (mode-coupling) critical temperature with decreasing film thickness is found. In the second model, a binary Lennard-Jones liquid is confined in a thin film, whose surface has an amorphous structure similar to the liquid. Although, as expected, the static structural properties of the liquid are not affected by the confinement, relaxation times near the wall are much larger than in the bulk. Consequences for the interpretation of experiments are briefly discussed.

Thermotropic liquid crystals (5CB and 8CB) were confined in silica porous matrices (xerogels and xero-aerogels) with different pore sizes. The structure and dynamics of confined liquid crystals were studied by Raman spectroscopy and Nuclear Magnetic Resonance. In Raman, the frequency of the CN stretching peak is a good probe of the smectic_A-crystal phase transition. The CN peak is observed to split upon quenching. This suggests a coexistence of crystalline and supercooled liquid phases for confined LC which was not observed in the bulk. In NMR, strong differences in both chemical shifts and linewidths are observed in confinement with respect to the bulk. We present a model which analyses the changes in the spectra in terms of changes of the order parameter and molecular dynamics for the confined LC.

The no-slip boundary condition, believed to describe macroscopic flow of low-viscosity fluids, overestimates hydrodynamic forces starting at lengths corresponding to hundreds of molecular dimensions when water or tetradecane is placed between smooth nonwetting surfaces whose spacing varies dynamically. When hydrodynamic pressures exceed 0.1-1 atmospheres (this occurs at spacings that depend on the rate of spacing change), flow becomes easier than expected. Therefore solid-liquid surface interactions influence not just molecularly-thin confined liquids but also flow at larger length scales. This points the way to strategies for energy-saving during fluid transport and may be relevant to filtration, colloidal dynamics, and microfluidic devices, and shows a hitherto-unappreciated dependence of slip on velocity.