Wetting phase connectivity and irreducible saturation in simple granular media

We present drainage simulations that allow trapping of wetting phase in a simple but nontrivial granular medium, a dense random packing of equal spheres. The basis for the simulations is a network model derived directly from the known locations and dimensions of pore space features. This provides a means of evaluating the morphology of trapped wetting phase. The possible morphologies depend on the assumed connectivity of the wetting phase. At one extreme, we assume that the entire wetting phase except for pendular rings is connected. At the opposite extreme, we illustrate a low level of connectivity by assuming that pendular rings are trapped as soon as the pores surrounding them are drained; any wetting phase not yet drained from pore throats connecting these pores is also assumed to be trapped. Finally we consider a set of criteria involving larger neighborhoods within the network, which allows trapping in individual pores. Irreducible wetting phase saturations obtained in the latter case agree with experimental data. The numbers of pendular rings and liquid bridges are also consistent with observations. Because the agreement does not involve adjustable parameters, we conclude that a relatively simple, local evaluation of trapping criteria can yield physically representative wetting phase configurations.     

Highly selective directed assembly of functional actomyosin on Au surfaces

The capability of assembling biomotors onto specific locations of solid substrates is a key for development of biomotor-based nanomechanical systems. We developed a method to direct the assembly of the heavy meromyosin fragment from rabbit skeletal muscle myosin onto specific locations of Au substrates utilizing surface molecular patterns. In this strategy, chemically directed patterns of streptavidin were achieved to direct highly specific assembly of biotinylated heavy meromyosin on the substrates--a strategy applicable for patterning a variety of biotinylated molecules--while BSA was utilized to avoid nonspecific adsorption. In vitro motility assays of filament sliding were used to confirm functionality of assembled actomyosin.     

An electron-microscopic study of Na-attapulgite particles

A scheme for the separation, purification and preparation of sub-micron, homoionic, Na-attapulgite from a natural mineral deposit is presented together with represen-tative analysis of the particle size distribution. Transmission electron microscopic examination indicated that particles were predominantly <1μm long and “lath-like” with aspect ratios (length:width:thickness) 100:3:1 which provide for a variety of modes of particle/particle interaction. The scheme involves physical and mild chemical treatment of the natural material and appears to preserve the physico-chemical integrity of the attapulgite particles and provides material within a narrow size range. Received: 30 March 1998 Accepted: 27 April 1998     

An initial assessment of the use of gradient elution in microemulsion and micellar liquid chromatography

Novel microemulsion and micellar HPLC separations have been achieved using gradient elution and columns packed with reverse phase material. Initial attempts at gradient microemulsion liquid chromatography proved impossible on use of a microemulsion successfully used in capillary electrophoresis. Optimisation of the microemulsion composition allowed the generation of stable microemulsions to achieve separations in HPLC. The novel use of organic-solvent micellar chromatography in gradient elution mode was shown to give efficient separations. A range of efficient separations of pharmaceuticals and related impurities were obtained. Acidic, basic, and neutral solutes were resolved covering a wide range of water solubilities and polarities. Elution times were in the order of 4-15 minutes. Separations were briefly compared to those accomplished with a micellar HPLC system. It is proposed that gradient elution in both microemulsion and micellar HPLC can be regarded as a highly successful means of achieving resolution of complex mixtures and should be considered for routine analysis and further investigation.     

Time delays in lead-salt semiconductor diode lasers

Time delays of typically 15–17 have been measured directly for PbS_1–xSe_x, Pb_1–xSn_xSe and Pb_1–xSn_xTe diode lasers at injection levels just above threshold in each case. The corresponding minority carrier lifetimes, as determined using the one-carrier injection model, were typically 2–4.     

THE AVAILABILITY OF SOLAR RADIATION BELOW 290 nm AND ITS IMPORTANCE IN PHOTOMODIFICATION OF POLYMERS

Abstract— On a percentage basis, ozone is a very minor component of the atmosphere; at STP it would make a layer only about 2 mm thick. On almost every other basis (biological, meteorological, paleontological, photochemical, etc.) it is a major component, due mainly to the tremendous reduction in solar ultraviolet flux which it causes in the 220–290 nm region. Since no data are available for Λ < 285 nm, a rational basis for estimating the flux reaching the earth's surface in this region is discussed. Variations in ozone concentration are of great importance, and it is possible to have more radiation with Λ < 270 nm fall on a surface in one extreme day than in several years of typical days. Often, persons involved in studies of polymer degradation by sunlight mention that a negligible fraction (1 ppm) of the radiant flux reaching the earth's surface is associated with wavelengths below 290 nm and infer that studies at shorter wavelengths will not be of much practical value. Such inferences are questionable for at least two reasons. (1) The quantum flux density below 290 nm is about 10¹⁶ photons cm^-2 month^-1, so that considerable long-term damage is possible since most of the flux will be absorbed in a layer only a few microns thick. (2) Even if solar radiation below 290 nm were completely absent, the existence of correlations between absorption peaks in the near ultraviolet and visible, and in the infrared with ionization potentials typically 6–12 eV or 200-100 nm) is evidence that we may expect studies in the ultraviolet and extreme ultraviolet (EUV) to provide important clues to the problem of improving the resistance of polymers to sunlight.