Recent synchrotron X-ray scattering studies on the thermotropic liquid crystal lubricant 8CB under shear flow have shown that for high shear rates the smectic layers are oriented perpendicular to the orientation assumed by conventional solid layered lubricants. This result invalidates existing theories of the lubrication mechanism of these materials. We show that the new orientation is the result of flow deformation of thermal fluctuations. This same mechanism is found to create a “normal stress” lift-force which we propose as the new lubrication mechanism.

Direct measurements of slippage of a polymer melt within 100 nm of a solid surface, under simple shear, are reported. In the case of a high molecular weight polydimethylsiloxane (PDMS) interacting weakly with the solid surface, a sharp transition between weak and strong slippage is observed as a function of shear stress. For strong polymer-surface interactions, slippage is highly reduced, and no transition is observed. The results are compared to a theoretical model recently proposed by Brochard and de Gennes.

We study numerically directed polymers in random potential fields for one-dimensional and fractal substrates. For fractal substrates the time evolution of the mean transverse fluctuations depends besides on the randomness of the potential also on the fractal nature of the substrate. The two effects enter in a subordinated way, i.e. the corresponding characteristic exponents due to the potential and the substrate combine multiplicatively. For a one-dimensional substrate the propagator P(x, t), the probability distribution of the transverse displacement x(t), follows the scaling form P(x, t) ∼ 〈x2(t)〉-1/2f (ξ), where ξ is the scaling variable ξ = x/〈x2(t)〉1/2. The numerical results support the scaling function f (ξ) ∼ exp (-cξδ) with δ > 2 which indicates an “enhanced” Gaussian behavior. These results are compared with those of a related “toy model”.

Though the dynamics of bulk phase separation during spinodal decomposition is fairly well understood nowadays, recent experiments have shown that the same process occuring near a wall introduces some surprisingly fast domain growth kinetics. Studies of both polymer mixtures and simple binary fluids at critical composition quenched into the unstable regime reveal the presence of a fast mode associated with domains coarsening at the wall. The scattering data indicates that while the bulk domains grow in time as t1/3, as dictated by usual diffusion driven dynamics, the surface domains coarsen with exponents ranging from about 1.1 to 1.5 depending on the quench depth. We construct a model in which the average size of domains in a binary mixture undergoing spinodal decomposition near a wall can achieve growth exponents much larger than 1/3. The accelerated growth is associated with the domains of the non-wetting phase coarsening anisotropically against a wall coated with the wetting phase.The faster growth evolves from coupling Lifshitz-Slyozov type coarsening for critical volume fractions, modified to include the geometric constraint of growth near a wall, the process of domain coalescence. Included are some predictions for experimental tests of these ideas.

We analyze interfacial behavior in binary blends of polymers with different flexibilities. Our starting point is a free energy functional calculated from a microscopic model of wormlike chains. We show how this functional can be used to calculate interfacial properties within the Cahn-Hilliard approach. Here we present preliminary results applicable to chains that are globally coil-like. At the interface between two isotropic phases, each rich in a different polymer, we find that the polymers tend to be extended parallel to the interface. Thus their shape is oblate rather than spherical at the interface.

Because of both their practical importance and their fundamental properties [1], polymers at interfaces have been under constant study. In this paper, we will consider the adsorbtion of linear chains on flat surfaces. Theoretical interest in this question was renewed recently by the introduction of scaling methods by de Gennes [2], Pincus [3], Binder, Kremer and Eisenriegler [4]. This allowed a new approach to the problem. New experimental methods such as neutron reflectivity [5] and small angle neutron scattering [6] also provided a much more detailed information than was previously available. This combined progress in theory and technique improved tremendously our understanding of the adsorption of linear chains on a flat impenetrable surface when the surface attraction is not too large. Many important questions however remain open. These are related both to the quality of the surface and to the nature of the solvent. Actual surfaces are never completely flat or homogeneous. Work is in progress to take into account surface roughness [7,8] and chemical imperfections [9,10]. Similarly, the quality of the solvent was considered in many directions. These include surface driven phase separation [11], or the effect of a mixed solvent on a copolymer. A somewhat different solvent effect will be considered here and is related to the length of the solvent molecules. Previous studies were concerned with solutions in a simple solvent [12]. It is possible both theoretically and experimentally to consider long polymers, made of N monomers, dissolved in shorter chains, with P units, of similar nature and slightly different interaction with the surface. The interest in this question is twofold. First by changing P, it is possible to have either adsorbed or desorbed probe chains as we shall see. Second, a generalization of this problem is to consider a mixture of both long and short chains in a simple solvent. The question then is the displacement of an adsorbed polymer by another one. In the following, we will only consider the equilibrium properties for the first case, in the absence of any simple solvent. As we shall see, this question is already very rich.

The effect of even dilute silica networks on the critical phenomena of binary liquid mixtures is profound. The network preferentially adsorbs one component, preventing a portion of the mixture from participating in critical fluctuations. Fluctuations in the remaining mixture are found to decay with a non-exponential correlation function near the consolute point. A correlation function consisting of the sum of an exponential decay and a non-exponential term of either an activated or stretched exponential form fits the data well. In the presence of the silica network, the mixtures are observed to phase separate near the critical temperature of the pure system, but while still in the one-phase region of the pure system.

We have investigated temperature-induced aggregation of silica colloids in a binary mixture of 2,6-lutidine and water. Aggregation occurs in the one-phase region of the binary mixture and can be reversed by cooling the sample.

Using low-angle light scattering, we have determined the growth kinetics and aggregate structure for this system. We find that the time dependent scattered intensity is well described by I(q, t) = ¯(t)/[1 + (q¯R(t))2/10]2\, where the weight-average cluster mass ¯I(t) ⋧ t, and the average cluster radius ¯(t) -çt1/3.

Our results indicate that this experimental system generates compact clusters, ¯ ⋧ ¯ 3, and that the rate of aggregation is diffusion limited. The dense, non-fractal, structure we observe suggests that the bonds between the silica spheres are not rigid, and supports the hypothesis that aggregation is induced by the adsorption of lutidine on the silica surface.

Phase changes in Lennard-Jones (IJ) and Stockmayer fluids confined between two parallel fcc (100) planes of rigidly fixed IJ atoms are studied by means of mixed isostress-isostrain canonical and isostrain grand-canonical Monte Carlo methods, and a microcanonical ensemble molecular dynamics technique. A nonequilibrium continuum thermodynamic derivation of the constitutive laws for the basic slit-pore model is developed and an equilibrium statistical thermodynamics anolog is reviewed. Independent constitutive parameters are chosen from the list T, μ, Ēv or their thermodynamic duals μ, N, Tij where T is absolute temperature, μ is the chemical potential, Ēv is the infinitesimal strain tensor, N is the particle number, μ is the entropy and Tij is the Piola stress tensor.

Relaxation in monolayer films is studied via mean square displacement of an atom. The vicinal atoms are found to diffuse anomolously slow in accordance with a power-law relation.

The main goal of our novel NMR experiments on confined liquids was to determine the effects of pressure on the dynamics of liquids in the surface layer by using the twostate, fast exchange model and compare them to the pressure effects observed for bulk liquids. With this goal in mind, the deuteron NMR spin-lattice relaxation times, T1, in liquid pyridine-d5, nitrobenzene-d5, and methylcyclohexape-d confined to sol-gel porous silica glasses with pore radii in the range from 18Å to 49˚A were measured as a function of pressure up to 5 kbar at 300 K. In another set of high resolution natural abundance 13C NMR experiments, the 13C relaxation behavior of each carbon in 2- ethylhexyl benzoate model lubricant was measured as a function of pressure in porous silica glasses. In fact, the described experimental approach which allows investigation of the effects of pressure on the dynamic behavior of surface-layer liquids, may provide a new tool in studies of model liquid lubricants at extreme conditions of pressure and temperature. In addition selected results of a recent study of acetonitrile-d3 are reviewed. The 2H and 14N spin-lattice relaxation times of acetonitrile-d3 in porous silica glasses were measured in order to study the confinement effects on the anisotropic reorientation characterized by rotational diffusion constants, D⊥ and D‖, of this symmetric-top molecule.

This work shows how the geometrical confinements enhances the nuclear relaxation of a non wetting liquid in a model porous systems. Application of the proposed theory is made to interpret the size and frequency dependences of the 1H relaxation of methylcyclohexane liquid in sol-gel porous silica glasses with narrow pore-size distribution.

Useful information on interactions and motion of guest molecules in sodalite cavities is obtained by estimating the spin-lattice and spin-echo decay rates of encapsulated Br− ions. An extremely fast spin-lattice relaxation rate is observed for 81Br nuclei, adjacent to Br−-empty cages which contain rapidly tumbling water molecules. We detect a further significant different T1 of the 81Br in Na8Br2- sodalite and AgsBr2-sodalite, respectively, which probably indicates a less restricted motion within the cavities of the Ag+-exchanged material. Spin-echo decay experiments additionally indicate a difference between the dynamic properties of the Br− anion in the encapsulated Na4Br and Ag4Br clusters, respectively.

Comparative differential scanning calorimetry (DSC) measurements were made at 200 K < T < 310 K on geometrically restricted cyclohexane and n-decane in nude-, trimethyl derivatized-, and hexyl derivatized-porous (Rp = 4 nm) silica with a view to determine how the surface structure of the confining media affects the thermodynamic behavior of the restricted fluid. Our results suggest that, irrespective of the fact that both trimethyl derivatized- and hexyl derivatized-silica have methyl terminal groups, the freezing or melting transition of cyclohexane is much more depressed in trimethyl derivatized-silica than in hexyl derivatized- or nude-silica. This is not the case for n-decane where the depression in the melting transition is consistent with the fact that the effective pore radius of the hexyl derivatized-silica is smaller than the trimethyl derivatized- or nude-silica.

Two types of model pore have been considered as sorbents of methane at 300 K: the first is a straight cylinder with atomically rough walls characteristic of an amorphous solid. Three cylinder radii plus the reference flat surface were taken for study. The second is a parallel-walled slit pore made up of graphite basal planes that had been modified by the insertion of sulfide atoms. Here, two values of sulfur loading were taken together with the reference pure graphite pore. Two values of the wall spacing were assumed for each sulfur loading. Thermodynamic and structural properties are presented elsewhere; here, the dependence of diffusion constants as a function of the monolayer density is presented for methane in the various model pores. It is shown that these diffusivities depend primarily on the methane-methane collisions in the monolayer films. There is a slight dependence upon the nature of the solid surface which is interpreted in terms of the hindrances produced by soft obstacles to molecular motion parallel to the surfaces.

We report Grand Canonical Monte Carlo (GCMC) simulation studies of the adsorption and heat of adsorption of simple inert gases in two model microporous materials: aluminophosphates (AℓPO4-5, AℓPO4-8 and VPI-5) and alumina-pillared clays. The intermolecular potentials are spherical Lennard-Jones for both the fluid-fluid and fluid-solid interactions; both structured and structureless walls are considered. For argon in VPI-5 and AℓPO4-5 we find qualitative agreement with experiment, but the predicted maximum adsorption is about 20% higher than that obtained experimentally; possible reasons for this discrepancy are discussed. For the pillared clays we find a first order phase transition below some critical pillar density. This finding seems to be in qualitative agreement with existing experimental data.

A finite lattice of adsorption sites, as shown by Monte Carlo simulation, is used to develop a simple hopping model of small molecules within the alpha cage of zeolite NaA. A two body attractive energetic interaction is employed for occupied pairs of nearest neighbor sites. A many body repulsive interaction term accounts for the crowding associated with site saturation. This term becomes important when the site-site spacing is less than the van der Waals diameter of the adsorbate. The dynamic Monte Carlo method is used to evaluate site to site hopping frequencies as a function of loading based on this potential energy function. As the sorbate-sorbate attractive interaction is increased (or, equivalently, as the temperature is reduced), mobility minima occur at certain lattice occupancies which may be explained by the formation of energetically favorable clusters on the cubocathedral lattice. In other words, molecular crowding can cause sorbate mobility to suffer minima as loading is increased. This prediction is in agreement with recent Xe NMR measurements.

Most surfaces are rough. In many cases the roughness is self-affine, with a width w that scales with the observation length L as w ∼ LC. We consider the medium confined between two such plates, and discuss how the dependence of various quantities on the separation of plates is modified by their roughness. Specific examples include fluctuation–induced forces, shifts in critical temperatures, and capacitance.

Diffusion dynamics for fluids in sandstone rocks were studied by CPMG and pulsed field gradient (PFG) NMR measurements. Stretched exponential decays of the transverse magnetization were observed while varying the interpulse spacing, τ, in the CPMG experiment which sets the time for diffusion. A crossover from free to restricted diffusion is evident in the dependence of both T2 and the stretch exponent, β2, on the diffusion time τ. The T2(τ) data is fit to a model which interpolates between free and restricted diffusion and has only one free adjustable length scale despite the fact that the sandstone rocks are characterized by a pore size distribution. This NMR derived length scale correlates with a characteristic length derived from Hg injection experiments at the percolation threshold. We compare these results from the CPMG experiment with stimulated echo PFG-NMR diffusion measurements. This provides a complementary method of probing the diffusion dynamics since the longitudinal magnetization is insensitive to the dephasing effects of diffusion in internal field gradients.

Fractal fingering is a well known phenomenon, ususally visualised using photography. Previous experiments have shown that the patterns can be visualised using NMR imaging, and in this paper we show that the fractal dimension of such a pattern can be easily calculated.

Water infiltrated nanoporous glasses have a damping peak at about 1 Hz (4.6 nm pore size) and 6 Hz (8.8 nm pore size) at room temperature; their relaxation strengths are consistent with the damping mechanism in which water flows from the regions of compression to those of tension. However, the damping peaks disappear when water content is below 60% for the 4.6 urn pore size glass and 40% for the 8.8 urn pore size glass. These water contents correspond to about a 0.85 and 1.0 urn thick interfacial water layer on the internal surfaces of the nanoporous glasses. The adsorbed layer contributes only to the damping background. Finally, dependencies of relaxation time on sample thickness, bulk modulus and viscosity agree well with the predictions of the model.