We present the results of an X-ray scattering study on partially confined smectic-A (layered) LC films. This partial confinement is achieved by placing the LC material inside the open grooves of a glass grating. Samples prepared in this manner are confined in the direction perpendicular to the gratings. Samples in which the LC is contained entirely inside the grooves develop an induced molecular tilt, which results in a compression of the smectic layers. The molecular tilt varies as the amount of the LC material increases, eventually forming a thin overlayer film above the gratings. As the thickness varies, a second region develops in the films. The layer spacing in this region is close to the bulk layer spacing. This structural evolution is coupled to a variation in the nematic-to-smectic-A phase transition temperature of the samples from the bulk. In addition, the nature of the phase transition is driven first order. The effects of partial confinement on a sm-C* LC film is discussed briefly.

We present the results of photon correlation spectroscopy investigations of the influence of confinement, interface, porous matrix structure, pore size and shape on the dynamic behavior of nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses) and parallel cylindrical pores (Anopore membranes). Investigations of LC in cylindrical pores together with studies in random porous matrices, makes it possible to separate the role of random structure and domain formation from the contributions due to existence of LC - solid pore wall interface and pure finite size effect in relaxation of order parameter or director fluctuations. In the temperature range below nematic - isotropie phase transition temperature we observed two overlapping relaxational processes which are satisfactorily described by the decay function f(q,t) = a·exp(–t/τ1) + (1–a)·exp(–xz), where x = ln(t/τ0)/ln(τ2/τ0) and τ0 = 10−8s. For LC in 100 Å random pores the second term describing the slow process dominates, whereas for 200 Å and 2000 Å cylindrical pores as well as 1000 Å random pores the contribution from the first term (fast process) is more visible. Since the slow relaxational process which does not exist in the bulk LC and broad spectrum of relaxation times (10−6 - 10)s appear not only for LC in random pores but in cylindrical as well, we conclude that differences in dynamical behavior of confined LC from that in the bulk are mainly due to the existence of the interface.

This paper reports experimental data on both the solid-liquid and liquid-vapour phase transitions of an organic fluid confined in the pore network of porous silicon, by using respectively differential scanning calorimetry and X-ray diffraction. Due to the nanometric pore sizes of this material, the surface effects have a strong influence, shifting the transition parameters (lower melting temperature and lower condensation vapour pressure respectively). In particular, the effect of chemical dissolution on the pore size distribution of porous silicon layers has been investigated.

We present a theory of nuclear spin relaxation and experiments to probe the microdynamics of polar solvents at the surface of calibrated microporous glass beads. The theory is tested using the magnetic field dependence of the proton spin-lattice relaxation rate recorded with a field-switched magnetic relaxation dispersion spectrometer.

Using dielectric spectroscopy in the frequency range 0.1 Hz-1.5 GHz, we investigated the influence of confinement on the dynamic properties of polar nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores as well as in parallel cylindrical pores with different, pores sizes. The confinement has a strong influence on the dielectric properties of LC which resulted in the appearance of a low frequency relaxational process (f ≤ 10 KHz) absent in bulk and a strong modification of modes due to the molecular rotation around short axis and librational motion. The differences between bulk and confined behavior are: (a) - the dielectrically active modes in confined LC are not frozen even at temperatures about 20 degrees below the bulk crystallization temperature; (b) - in the temperature range corresponding to the anisotropie phase in pores, lnτ, where τ is the relaxation time corresponding to the molecular rotation around short axis, is not a linear function of 1/T and there is an evidence for smectic type order formation at sufficiently low T; (c) - the retardation factor g = τ/τis is ≃ 1.5, where as the typical value of g in bulk nematic liquid crystals is ≃ 4; (d) - smooth and small changes in T at phase transition in pores suggest that the “isotropie” phase of LC in pores is not bulk like isotropie phase with complete disorder in molecular orientations, and some degree of orientation order still persists.

We have investigated the spreading behavior of solvent droplets on a bulk water support using solvents with different vapor pressures and spreading coefficients. Instead of seeding the surface with tracer particles, as is usually done to track moving fronts, we employ laser shadowgraphy to visualize the entire surface of the spreading film including the leading edge. For non-volatile systems it has previously been shown that the leading edge advances in time as t3/4. We find that volatile systems with positive initial spreading coefficients exhibit two spreading fronts, both of which demonstrate power law growth but with exponents closer to 1/2. Surprisingly, differences in the liquid vapor pressure or the spreading coefficient seem only to effect the speed of advance but not the value of the exponent. We are presently investigating the behavior of the subsurface flow to determine the mechanism leading to the smaller spreading exponent.

Molecular dynamics simulations have been performed to study friction in Langmuir-Blodgett monolayers of perfluorocarboxylic acid, semifluoro acid and hydrocarboxylic acid on SiO2. The frictional coefficient of perfluorocarboxylic acid is about three times as large as that of hydrocarboxylic acid, while the frictional coefficient of semifluorocarboxylic acid is about two times as large as that of hydrocarboxylic acid. The qualitative aspects of these simulation results are consistent with known experimental results. In order to interpret the difference in the frictional coefficient, a series of simulations have been carried out by changing molecular potential parameters. The simulation results suggest that the 1,4-van der Waals interaction is the main cause of the larger frictional force for perfluorocarboxylic acid than that for hydrocarboxylic acid. Further frictional coefficients of semifluorocarboxylic acid are found to change by the fluorination ratio. The results also show that frictional force is roughly proportional to the excess r.m.s. fluctuation of the potential energy under shear from the equilibrium. The relation between the frictional force and the energy needed for molecular deformation under shear conditions is also discussed.

The chemical reactivity of catalytic surfaces often arises from a unique crystal or electronic structure confined to the first few atomic layers. Scanning Tunneling Microscopy (STM) is particularly well suited to studying the spatially confined structural properties of such systems. In this paper, I report recent advances in the characterization of colloidal layered transition metal chalcogenides using STM. These particles mimic the surface (layer edge) properties of bulk catalytic materials. It is found that the materials adopt distinct equilibrium chemical and electronic structures as compared to the bulk. The possible significance of these novel structures in regard to the bulk catalytic functionality of the parent material will be briefly discussed.

We studied the equilibrium architecture of polymer layers strongly adsorbed from the melt. Immobilized layers of poly-(methyl methacrylate) (PMMA) were produced by the following method: 1) The polymer was spin-coated onto silanol bearing surfaces of single crystal and fused quartz, and annealed at melt conditions, 2) The annealed layer was quenched to room temperature (below the glass transition temperature) in order to “freeze in” the melt structure near the substrate, 3) Unbound material was leached away in good solvent (benzene) to leave a residual, strongly-adsorbed layer. The architecture of this layer was studied by neutron reflection. Data on dried adsorbed layers indicates a dense PMMA film whose thickness gradually increases with annealing time in the melt from a minimal value. Evidently, annealing gradually relaxes a rather flat non-equilibrium structure produced by spin-coating. The thicknesses, h, in a series of dry layers annealed long enough to achieve equilibrium conditions in the melt scale as h ∼ N1/2. Data on swollen layers suggest a dilute, extended layer, but the preliminary results cannot give a definitive confirmation of the brush structure predicted by Guiselin.11

We investigate the anomalous kinetics in one dimension of a diffusion limited catalytic trapping reaction, A + T → T, by measuring the oxidation of glucose. The reaction is carried out in a thin capillary tube, and the depletion of oxygen in the vicinity of the reaction front is monitored by the fluorescence of a Ru(II) dye. Theoretical results and simulations have predicted an asymptotic t1/2 dependence for the rate coefficient. We observe a depedence on t0.56, with what appears to be an asymptotic behavior approaching t1/2.

We report results of an investigation of magnetic-field dependent critical currents in artificially disordered superconducting Al wire networks well below Tc. Aperiodic but reproducible variations with field are found, in sharp contrast with the periodic oscillations observed close to Tc. Critical currents also exhibited strong variations on cycling the current at low fields reflecting variations in frozen-in metastable vortex patterns. Both the variations in field and on cycling vanished above a critical field set by the disorder strength.

Within lubrication theory, we investigate the hydrodynamic stability of a thin surfactant coated liquid film spreading strictly by Marangoni stresses. These stresses are generated along the air-liquid interface because of local variations in surfactant concentration. The evolution equations governing the unperturbed film thickness and surface surfactant concentration admit simple self-similar solutions for rectilinear geometry and global conservation of insoluble surfactant. A linear stability analysis of these self-similar flows within a quasi steady-state approximation (QSSA) yields an eigenvalue problem for a single third-order nonlinear differential equation. The analysis indicates that a thin film driven purely by Marangoni stresses is linearly stable to small perturbations of all wavenumbers. The insights gained from this calculation suggest a flow mechanism that can potentially destabilize the spreading process.

We performed Monte Carlo simulations of diffusion processes on baguette-like lattices, which have very small width and height, but long length, in effect providing quasi-one dimensional systems. This is done by investigating random walk properties of single particles, and also the well known model bimolecular reactions A + A and A + B. We monitor the number of distinct sites visited, as a function of time, and also the survival probability in the presence of static traps. For the reaction systems we monitor, as usual, the decay of the particle density. The expected one-dimensional behavior is recovered, in the long time limit, for all cases studied. Our interest here is in the crossover time, from 3- (or 2-)dimensionaI behavior (early time) to one-dimensional (long time). We find that this crossover time scales with respect to the baguette's short dimension. However, this scaling deviates significantly from a mean square displacement law, and it is specific to both tube dimensionality and reaction type (e.g. A + A or A + B). Specifically, instead of an expected power of 2, the exponents range between 1 and 4. The densities of the A + B reactions at the dimensional crossover are compared to the densities at the segregation crossover in regular lattices at all three dimesnions. As expected, the time evolution of the A + A reaction parallels the behavior of the average number of distinct sites visited.

A system of competing elementary reactions is investigated experimentally usingthe reaction of xylenol orange with Cr3+ in aqueous solution. The two reagents areinitially separated in a long, thin capillary tube and meet in the center, forming a reactionfront (s). The geometry of the reactor and the initial separation of the reagents makes the system effectively one-dimensional. Aqueous Cr3+ solution has a very rich chemistryand provides two different chemical Cr3+ reactants which compete to react with xylenolorange. Rich spatio-temporal patterns are observed experimentally and are explained by a reaction-diffusion model. Results from exact enumeration simulations predict that when the concentrations of the competing species are very different and the microscopic rate constants of the competing species are such that the majority species reaction rate is much faster than the reaction rate of the minority species, the reaction front splits into two distinct regions. The spatio-temporal patterns generated by theory and experiment agree quantitatively. Also in agreement with the theory are the experimental early time and asymptotic time global rate behaviors, which exhibit multiple crossovers.

A focused laser beam acts as both a “phototrap”, bleaching fluorophore molecules which diffuse into the beam path, and as a confocal probe, detecting the excited, unbleached fluorophore molecules still present in the trap. With this focused laser beam, we observe anomalous asymptotic rate laws similar to those predicted for a diffusion-controlled elementary trapping reaction, A + T → T, in one and two dimensions. One dimensional diffusion-limited trapping kinetics are approached in capillaries with 10 μm diameters while two dimensional diffusion limited trapping kinetics are observed with unstirred samples having a quasi 2-D geometry. In the presence of stirring, the 2-D samplesexhibit the classical, constant trapping rate over time.

Femtosecond optical-heterodyne detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) has been used to study the reorientational dynamics of CS2 in microporous glasses over a wide range of temperatures. Microconfinement is shown to affect the behavior of the liquid at all the temperature studied. The results are interpreted in terms of a two-state model of the confined liquid.

This work is a molecular-dynamics simulation study of the influence of chain branching on the molecular configuration in confined tetradecane thin films. Simulations of layered interfacial films of n- and i-tetradecane on Pt(111) show, in contrast to experiments, that a side methyl group does not impart sufficient asymmetry to alter the solvation force law from oscillatory to non-oscillatory. Based on previous experimental findings, a novel vertical structure resembling a self-assembled monolayer, is proposed for confined, long-chain iso-alkanes. Simulations show that i-tetradecane in this structure is stableover the time scales that can be probed by molecular dynamics and that vertical films have a lower energy per molecule than layered films. With this structure, many experimental features, including the non-oscillatory solvation forces, of confined iso-alkanes are explainable andwill be discussed in this paper.

The quantum mechanical escape rate is calculated for an electron in a one-dimensional potential well. First-order time-dependent perturbation theory is used for the bound-to-bound and the bound-to-free transitions. The bound-to-free transition probability decays exponentially with bound energy. The fraction of one-electron systems in a bound state decays exponentially with time. The characteristic time constant grows exponentially with an increasein the depth of the potential well.

The diffusion-limited annihilation reactions A+A→products and A+B→products havebeen subject to theoretical, numerical, and experimental study over the past decade. “Anomalous” behaviors have been reported for both reactions, but with different upper critical dimensions (d=2 and d=4 respectively). The critical dimensions and the scaling of the exponents for the generalized reaction nA+mB→products depends on n and m. We present Monte Carlo simulations for two different types of third-order reactions, A+A+A→0 and A+2B→0, on both one and two dimensional lattices. Our numerical results show that for the two-dimensional case the behavior of the reaction follows mean-field theory while in one dimension the result is“anomalous”. Our numerical results are in good agreement with theory based on microscopic arguments and scaling analysis.

We have investigated n-octane droplets resting on the surface of sodium chloride solutions as a function of the salt concentration in a saturated, closed cell. For high salt concentration, the system approaches a wetting transition : the contact angles are very small (∼ 1°), the macroscopic droplet is unstable, and it breaks up spontaneously into microdroplets. The stable polydisperse population of microdroplets (5 μm < r < 250 μm) allowed us to analyze the dependence of the contact angle on droplet size. Because of the low contact angle values, accurate measurement ofcontact angles was obtained by interferometry. Moreover the accuracy of the classical method was significantly improved through the systematic use of three wavelengths. The relationship between the contact angle and the size droplet size indicated a positive line tension, τ, and the order of magnitude of τ was in good agreement with the theoretical prediction, τ, varies between (8.6 ± 0.9). 10−11 N and (1 ± 0.1).10−9 N and was dependent on the salt concentration. The positive sign of τ and its significant effect on droplet shape were related to the fact that the system was approaching the wetting transition.