Protein fibers (e.g., actin filaments, microtubules, and sickle cell hemoglobin polymers), formed by reversible association of quasispherical protein monomers, contain substantial solvent. At high concentrations, solutions of these fibers become non-ideal due to interactions between the elongated particles. One manifestation of this non-ideality is the spontaneous alignment of the fibers. Model calculations, involving simple intra- and inter aggregate interactions leading to polymerization and alignment, agree well with osmotic pressure measurements of sickle cell hemoglobin solutions in the region of the phase transition. However, at higher concentrations the theory cannot fit the experimental data unless solvent is gradually squeezed out of the fiber under osmotic stress. We find that a linear potential for the fiber dehydration gives osmotic pressure results consistent with the experimental data. Under these conditions, substantial fiber dehydration occurs with increasing protein concentrations beyond the alignment transition. This indicates that particle interactions sufficient to cause alignment are also sufficient to squeeze significant amounts of solvent out of the sickle cell hemoglobin fibers.

The thermal transport across a thick (0.66 cm) liquid crystal cell has been measured versus applied ac voltage and frequency. These measurements are correlated with the optically observed onset of flow and turbulence in cells as identical as practicable to those used for the thermal transport measurements. In addition, the measurements are compared with reported observations in thin cells. The thermal transport across the liquid crystal is characterized by an effective thermal conductivity Kf. It was found that Kf increases with increasing frequency, at constant voltage, to a maximum enhancement at about 40 Hz at room temperature. Optical observations on thick cells indicate that dynamic columnar domains of flowing liquid crystal are the primary mode of heat transport, as determined by correlating the structure and characteristic lifetime of such domains as a function of voltage and frequency. Optical observations at low voltages suggest that Williams Domains do not exist in these thick cells, and that all observed responses are functions of electric field strength, not applied voltage (as in thin Williams Domain cells).

Four solutions to the Leslie-Ericksen equations for nematic liquid crystals are obtained for pressure-driven radial out-flow between concentric parallel disks and homeotropic director wall anchoring. At low pressure drops the in-plane mode is stable; the centerline director orientation is normal to the flow direction. For each radial distance from the entrance hole there is a critical pressure drop at which the elongational torque twist the director out-ofthe- plane of flow ; left and right rotations are possible. The transition results in the appearance of secondary flows in the azimuthal direction. The radius of the boundary between in-plane and out-of-the-plane modes increases with increasing pressure drops. At higher pressure drops a twist wall develops at the centerline. There is a second critical pressure drop at which the wall collapses with the nucleation and growth of a twist disclination loop. The resulting stable director field is in-plane and flow-aligned throughout the cell.

Khokhlov and Semenov (KS) have proposed a theory for long persistent chains that quite accurately describes the liquid crystalline behavior of dilute solutions of long homogeneous bend-elastic macromolecules. By replacing their second virial description of excluded volume effects with Lee's generalization of the Carnahan-Starling equation, results are obtained which are valid over a wider range of polymer concentrations. Comparison of the calculated order parameter and equation of state with experimental data for Poly(γ-benzyl-L-glutamate) yields good agreement for large polymer axial ratios, where rigid particle theories deviate strongly from experimental results. In addition, we show how this approach to persistent flexibility can be connected to a recent model for translational order in lyotropic liquid crystals.

A nonperturbative approach to the density-functional description of phase transitions in fluids composed of anisotropic particles is presented. The theory is exact to second order in functional perturbation theory, and at higher orders satisfies all sum rules derived from density derivatives of the two-point direct correlations. We have applied this theory to orientational and translational freezing of hard ellipsoids, and we find that the description of the structure of crystalline phases is improved in comparison to the commonly used second-order theory. For the case of orientational freezing, it is found that the higher-order contributions do not modify the predictions of the second-order theory, and that accurate liquid structure appears to be the key factor leading to improved description of the nematic phase.

Recently, modulated phases of insoluble monolayers of fatty acids and phospholipids spread on the water/air interface have been observed by fluorescence microscopy experiments. We propose a theoretical explanation of this observation by including electrostatic (dipolar) interactions in the total free energy calculation for the monolayer. Dipoles can originate from two sources: neutral amphiphiles have a permanent dipole and charged amphiphiles have an induced one. Modulated phases are found to be stable in two different limits: close to the liquid-gas transition and at low temperatures. Several phases with stripe and hexagonal symmetry are predicted and the phase transitions between them are calculated.

A lattice statistical mechanical method has recently been developed and used to predict structural and thermodynamic properties of fluid phases of surfactant monolayer films of uniform chain length. This mean-field approach is extended here to examine films of mixed chain length. As in the earlier work, a fluid-fluid (LC-LE) phase transition is predicted to occur at high surface densities. This transition is found to exhibit non-ideal mixing behavior, ranging from positive azeotropy to phase separation (in the LC phase) as the difference in chain lengths increases. In contrast, the phase diagram in the liquid-gas (LEG) transition region is predicted to exhibit almost ideal behavior, with a nearly linear bubble-point curve.

X-ray specular reflectivity is used to study the liquid-vapor interface of pure water and of fatty acid and polymer monolayers at that interface. For the pure water surface the reflectivity was measured for three different spectrometer resolutions and simultaneous fits with only one free parameter to all of the data are in excellent agreement with the prediction of capillary wave theory for the RMS surface roughness. Diffuse scattering away from the specular condition, at wavevectors corresponding to those of the capillary waves, yields intensities and line shapes in agreement with theory with no significant adjustable parameters. Reflectivity from separate monolayers of co-poly 1,2-butadiene/butyl alcohol (50% random substitution) and lignoceric acid (CH3(CH2)22COOH) at the water/vapor interface are interpreted to obtain profiles of the average electron density ρ(z) as a function of distance z along the surface normal. For the polymer monolayer we find the following: 1) a local maximum in the electron density approximately 10% larger than that of the bulk polymer and 2) the RMS roughness of the vapor/polymer interface agrees with capillary wave theory predictions for the lower surface pressures. For the highest surface pressure the RMS roughness exceeds the value predicted by the capillary wave model. Measurements of reflectivity from a lignoceric acid monolayer, as a function of surface pressure throughout an isotherm (near room temperature), reveal the following behavior: 1) the overall thickness of the monolayer increases with increasing pressure and 2) the head groups occupy a progressively larger region along the surface normal as the pressure increases, indicating that they rearrange normal to the interface.

Optical second harmonic generation (SHG) is applied to investigate the structure of the interfacial layer of soluble surfactants adsorbed to the air/water interface.

We have measured the composition profile between two polymer phases above and below their critical temperature for phase separation Tc, using the 2H(3He,4He)1H nuclear reaction in a direct profiling technique. Above Tc, we verify the “thtrmodynamic slowing down” effects on interdiffusion close to the critical point. Pelow Tc, we find that the width of the interface grows with time to a finite limiting value. The variation of this limiting value, as Tc, is approached from below is in good agreement with the predictions of mean-field theories. The time dependence of the initial interfacial broadening below Tc has been found to be significantly slower than t1/2.

We report a recent theoretical study of polymers moving on a random two-dimensional fluid surface. The metric of the liquid membrane is free to fluctuate and these new degrees of freedom couple to those of the polymers - as a result, the critical exponents describing the polymers are changed, but are nevertheless related to those in two dimensions. The relation to possible experimental studies on membranes with vanishing surface tension, which should search for new universal values of exponents u and 7, is discussed.

We present a coupled two-fluid model for hydrodynamic surface modes on concentrated polymer solutions and swollen polymer gels. This model is used to investigate such surface modes in the limit of strong coupling. We discuss the form of the modes as a function of polymer concentration and the effects of chain diffusion and surface tension on the nature of these modes.

X-ray interferometry was used to study monolayers of the protein yeast cytochrome c and a bimolecular cytochrome c/photosynthetic reaction center complex bound to self-assembled monolayers of 11-trichlorosilylundecyl thiol on inorganic substrates. Optical absorption spectroscopy and dissociation experiments show that the cytochrome c monolayer is covalently attached to the self-assembled monolayer via a disulfide bond and they are consistent with the cytochrome c and its bimolecular reaction center complex forming a close-packed array on the surface of the solid substrate. By sequentially forming these bio-organic monolayers on synthetic inorganic multilayers (“quantum wells”), we were able to determine the electron density profile of the protein layers to ∼13 Å resolution using x-ray interferometry and refinement techniques.

Domain wall fluctuations and instabilities in monomolecular amphiphilic films have been probed by fluorescence microscopy and spectral analysis based on techniques of digital image processing. Recent results address the analysis of the dynamics of domain wall motion.

Optical properties of fatty acid monolayers were measured as a function of surface density and under the influence of cations in the aqueous substrate. The monolayer behavior of pentadecanoic acid (C15) on an aqueous solution of HCl at pH=2.0 was studied. The change in the ellipsometric phase angle δΔ was mildly affected by increases in surface density at constant orientation but strongly influenced by the liquid-expanded/solid phase transition presumably due to a change in conformation.

The increase in the ellipsometric phase angle 6Δ with chain length C15, C16, C18, and C22 was attributed to an increase in thickness of fatty acids at the air/water interface. Separating the effect of optical density of head groups, the refractive indices calculated from the chain length data were independent of ionic environment. The magnitude of δΔ for fatty acids on aqueous substrates increased in the order HCl, CdCl2, and PbCl2. This was due to an increase in the refractive index of fatty acid head groups caused by association of divalent cations. Ellipsometry was also found to be a useful technique to monitor in-situ the degree of divalent cation association.

We report on molecular dynamics simulations of incompressible Lennard-Jones fluids sheared between two solid walls. Boundary conditions and flow fields were studied for both single fluid and immiscible two-fluid systems. For single fluid systems, equal wall and fluid densities or strong wall-fluid couplings crystallized the first layers of fluid atoms. Different wall densities and weaker coupling led to normal Couette flow with a no-slip boundary condition. Our simulations of two fluid systems indicate that there is always slip within ∼ 2 atomic spacings from the contact line. This slip appears to be associated with the breakdown of hydrodynamics at atomic scales. Changes with capillary number in the interface shape and dynamic contact angle were consistent with previous analytic results.