Plastron properties of a superhydrophobic surface

Most insects and spiders drown when submerged during flooding or tidal inundation, but some are able to survive and others can remain submerged indefinitely without harm. Many achieve this by natural adaptations to their surface morphology to trap films of air, creating plastrons which fix the water-vapor interface and provide an incompressible oxygen-carbon dioxide exchange surface. Here the authors demonstrate how the surface of an extremely water-repellent foam mimics this mechanism of underwater respiration and allows direct extraction of oxygen from aerated water. The biomimetic principle demonstrated can be applied to a wide variety of man-made superhydrophobic materials

LOCAL DISTORTIONS AND VOLUME CHANGES IN SEMICONDUCTORS - DONORS IN SILICON

Experiments giving impurity-induced lattice distortion can measure quite distinct quantities. In particular EXAFS (extended X-ray fine structure) measures nearest-neighbour distances, whereas both volume changes and recent spectroscopic data measure long-range displacements. The relationship between the two depends strongly on the inter-atomic potential. The authors analyse this for impurities in silicon by adopting a variety of current potentials. There is a significant contradiction between the EXAFS results and the other experiments for all of the inter-atomic potentials. This problem may be associated with the high oxygen concentrations of Czochralski crystals used in the EXAFS study

Porous materials show superhydrophobic to superhydrophilic switching

Switching between superhydrophobicity and superhydrophilicity in porous materials was predicted theoretically and demonstrated experimentally with the example of thermally induced contact angle change; tunability of this system was also demonstrated

Bromination of Graphene and Graphite

We present a density functional theory study of low density bromination of graphene and graphite, finding significantly different behaviour in these two materials. On graphene we find a new Br2 form where the molecule sits perpendicular to the graphene sheet with an extremely strong molecular dipole. The resultant Br+-Br- has an empty pz-orbital located in the graphene electronic pi-cloud. Bromination opens a small (86meV) band gap and strongly dopes the graphene. In contrast, in graphite we find Br2 is most stable parallel to the carbon layers with a slightly weaker associated charge transfer and no molecular dipole. We identify a minimum stable Br2 concentration in graphite, finding low density bromination to be endothermic. Graphene may be a useful substrate for stabilising normally unstable transient molecular states

Spontaneous Breakdown of Superhydrophobicity

In some cases water droplets can completely wet micro-structured superhydrophobic surfaces. The {\it dynamics} of this rapid process is analyzed by ultra-high-speed imaging. Depending on the scales of the micro-structure, the wetting fronts propagate smoothly and circularly or -- more interestingly -- in a {\it stepwise} manner, leading to a growing {\it square-shaped} wetted area: entering a new row perpendicular to the direction of front propagation takes milliseconds, whereas once this has happened, the row itself fills in microseconds ({\it ``zipping''})Comment: Accepted for publication in Physical Review Letter

Dual-scale roughness produces unusually water-repellent surfaces

Super-hydrophobicity can be achieved on relatively smooth surfaces. Short, wide pillars on slightly rough surfaces are shown to produce super-hydrophobic surfaces (see Figure) where neither the pillars nor the slight roughness suffice alone. This use of two length scales to create super-hydrophobic surfaces directly mimics the mechanism used by some plants including the lotus

Topography driven spreading

Roughening a hydrophobic surface enhances its nonwetting properties into superhydrophobicity. For liquids other than water, roughness can induce a complete rollup of a droplet. However, topographic effects can also enhance partial wetting by a given liquid into complete wetting to create superwetting. In this work, a model system of spreading droplets of a nonvolatile liquid on surfaces having lithographically produced pillars is used to show that superwetting also modifies the dynamics of spreading. The edge speed-dynamic contact angle relation is shown to obey a simple power law, and such power laws are shown to apply to naturally occurring surfaces

Menkul kıymetler borsalarındaki mevsimselliklerin İstanbul Menkul Kıymetler Borsası’nda (İMKB) incelenmesi

Ankara : Bilkent Univ., 1993.Thesis (Master's) -- Bilkent University, 1993.Includes bibliographical references leaves 64-65.This study empirically examines stock market seasonality in the Istanbul Stock Exchange Market (IMKB) , Turkey. Current evidence from the studies for other capital markets around the world provides that there are strong seasonalities in the stock returns in most of these capital markets. The seasonality , when it exists, is associated with the turn of the year, the week, as well as with holidays. The turn of the week effect appears to be negative on Monday or Tuesday returns; turn of the year effect appears to be high for January or April returns;and holiday effect appears to have higher returns on the trading days prior to holidays in most of the capital markets in developed countries. This study, however, presents the evidence that so called weekend effect and the day-of-the-week effect do not exist in IMKB. The mean returns on Thursdays are negative and it cannot be accepted statistically. I find a turn of the year effect with high January returns and holiday effect with high mean returns, averaging four times the mean return for the remaining days of the year as in the other capital markets. The returns for the IMKB daily index for 1988-1991 period are examined in this study, as well as the weekend and turn of the year effect for the individual stocks of 29 large and 34 small firms.Erbil, A. FuatM.S