David H. Van Winkle

Layering in colloidal fluids near a smooth repulsive wall

We have directly observed layering in three‐dimensional colloidal fluids caused by the presence of a smooth repulsive glass wall. The system we study is a colloidal suspension of highly charged monodisperse latex spheres in water, with diameter 0.3 μm, charge ∼104 electronic charges, and mean separation ∼0.8–2 μm. This allows direct atomic resolution of the colloid using ordinary optical microscopy and digital imaging techniques. The layering of the colloid is manifested as a density modulation perpendicular to the wall. For fluids with low bulk densities, we find the density profile perpendicular to the wall to be essentially identical to the pair distribution function in a thin slab of spheres parallel to the wall, with the exception of the spacing of the first peak. For a fluid with bulk density approaching that of the crystal, we find a smaller peak spacing than that of the in‐plane pair distribution function, indicating incipient crystallization.

Ordered phases in concentrated DNA solutions

Aqueous solutions of concentrated DNA, a strong polyelectrolyte, in 1:1 electrolyte form liquid-crystalline phases analogous to those observed for other semi-rigid neutral polymers and weak polyelectrolytes. Phase transitions were examined in detail for DNA fragments with a contour length (500 A) approximating the persistence length. These fragments form at least three lyotropic phases. The lowest density or “precholesteric” phase appears to be a nematic with a slight and easily variable twist. The intermediate density phase is a true cholesteric with a pitch of ≈2.1 μm. With increasing concentration the cholesteric phase unwinds prior to formation of the third, high density, columnar phase. Phase transition boundaries were determined as functions of DNA concentration from 10 to 300 mg/ml solvent, supporting electrolyte concentration from 0.01 to 1.0 M, and temperature from 20°C to 60°C. Critical concentrations for formation of anisotropic phase (Ci) and disappearance of isotropic phase (Ca) were only moderately dependent on temperature. Ci, Ca, and the pitch of the cholesteric phase were surprisingly insensitive to the supporting electrolyte concentration. The insensitivity can be most simply related to the high concentrations required for anisotropic phase formation by these rather short, highly charged rod-like DNA fragments. At high concentrations the DNA counterions contribute significantly to the effective ionic strength, hence overall charge screening, and the counterion atmosphere as monitored by 23Na NMR, appears to be perturbed by inter-rod interactions.

DNA electrophoresis in agarose gels: A simple relation describing the length dependence of mobility

Electrophoretic mobilities of DNA molecules ranging in length from 100 to 10 000 base pairs (bp) were measured in gels of eleven concentrations of agarose from 0.5 to 1.5%. Excellent fits of the dependence of mobility on DNA length were obtained with the relationship

DNA electrophoresis in agarose gels: Effects of field and gel concentration on the exponential dependence of reciprocal mobility on DNA length

{1 \over {\mu \left( {L} \right)}} = {1 \over {\mu _l }} - \left( {{1 \over {\mu _l }} - {1 \over {\mu _s }}} \right)e^{ - L/\gamma }

Terraces in the cholesteric phase of DNA liquid crystals

showing an e–L/γ crossover, where L is the length of a DNA fragment and γ is a crossover length ranging from 8000 to 12 000 bp. The other parameters in the fit are νs the mobility of short DNA with unit charge in the limit as length is extrapolated to zero, and νl, the mobility of long DNA as length is extrapolated to infinity. This exponential relationship should be a useful interpolation function for determining DNA lengths over a wide range. The simplicity of this relationship may be of more fundamental significance and suggests that some common feature dominates the electrophoresis of double stranded DNA fragments in agarose gels, regardless of length.

Single molecule observation of DNA electrophoresis in pluronic f127.

Electrophoretic mobilities of DNA molecules ranging in length from 200 to 48 502 base pairs (bp) were measured in agarose gels with concentrations T = 0.5% to 1.3% at electric fields from E = 0.71 to 5.0 V/cm. This broad data set determines a range of conditions over which the new interpolation equation ν(L) = (β+α(1+exp(–L/γ))–1 can be used to relate mobility to length with high accuracy. Mobility data were fit with χ2 > 0.999 for all gel concentrations and fields ranging from 2.5 to 5 V/cm, and for lower fields at low gel concentrations. Analyses using so‐called reptation plots (Rousseau, J., Drouin, G., Slater, G. W., Phys. Rev. Lett. 1997, 79, 1945–1948) indicate that this simple exponential relation is obeyed well when there is a smooth transition from the Ogston sieving regime to the reptation regime with increasing DNA length. Deviations from this equation occur when DNA migration is hindered, apparently by entropic‐trapping, which is favored at low fields and high gel concentrations in the ranges examined.

Lipid Multilayer Grating Arrays Integrated by Nanointaglio for Vapor Sensing by an Optical Nose

Near the transition to the columnar phase, the cholesteric liquid crystal phase in an aqueous solution of DNA fragments with contour lengths approximating the persistence length undergoes an unwinding of the cholesteric pitch. Unwinding of the cholesteric with planar alignment of the fragments was studied by polarized light microscopy. Terraces or ‘‘Grandjean planes’’ of cholesteric are seen as uniformly birefringent fields of distinct hues (typically blue), bounded by lines which moved as the local concentration of DNA increased. These lines are interpreted as disclination lines, bounding regions of different total twist, which move as the intrinsic pitch of the cholesteric varies with concentration.

Raman scattering from freely suspended liquid crystal films

Single molecule fluorescence microscopy is used to follow the motion of long DNA molecules undergoing electrophoresis in Pluronic gels. We find that for low fields most DNA molecules follow tortuous paths through the gels, at an angle up to 90 degrees from the field direction, while some molecules find paths along the field lines. In high fields, virtually all of the DNA molecules follow the field lines. In many cases, the molecules travel as compact coils with optically discernible radii smaller than in free solution. In other cases, the molecules extend and contract or travel in an extended configuration.

Electrohydrodynamic Flow in Thick Liquid Crystal Cells

Lipid multilayer gratings are recently invented nanomechanical sensor elements that are capable of transducing molecular binding to fluid lipid multilayers into optical signals in a label free manner due to shape changes in the lipid nanostructures. Here, we show that nanointaglio is suitable for the integration of chemically different lipid multilayer gratings into a sensor array capable of distinguishing vapors by means of an optical nose. Sensor arrays composed of six different lipid formulations are integrated onto a surface and their optical response to three different vapors (water, ethanol and acetone) in air as well as pH under water is monitored as a function of time. Principal component analysis of the array response results in distinct clustering indicating the suitability of the arrays for distinguishing these analytes. Importantly, the nanointaglio process used here is capable of producing lipid gratings out of different materials with sufficiently uniform heights for the fabrication of an optical nose.

Quantification of Protein-Induced Membrane Remodeling Kinetics In Vitro with Lipid Multilayer Gratings

Using Raman scattering we have studied the molecular vibrations in freely suspended liquid crystal films as thin as two molecular layers. The effect of the free surfaces on several aspects of molecular order were examined by comparing the Raman spectra obtained on both thin and thick films. The liquid crystal materials 8CB, 8SI, and 65OBC were studied in the crystal, hexatic, and fluid smectic phases. Changes were observed in both intramolecular and intermolecular vibrations as a function of thickness.