Dynamics of sliding drops on superhydrophobic surfaces

We use a free energy lattice Boltzmann approach to investigate numerically the dynamics of drops moving across superhydrophobic surfaces. The surfaces comprise a regular array of posts small compared to the drop size. For drops suspended on the posts the velocity increases as the number of posts decreases. We show that this is because the velocity is primarily determined by the contact angle which, in turn, depends on the area covered by posts. Collapsed drops, which fill the interstices between the posts, behave in a very different way. The posts now impede the drop behaviour and the velocity falls as their density increases.Comment: 7 pages, 4 figures, accepted for publication in Europhys. Let

Jetting Micron-Scale Droplets onto Chemically Heterogeneous Surfaces

We report experiments investigating the behaviour of micron-scale fluid droplets jetted onto surfaces patterned with lyophobic and lyophilic stripes. The final droplet shape depends on the droplet size relative to that of the stripes. In particular when the droplet radius is of the same order as the stripe width, the final shape is determined by the dynamic evolution of the drop and shows a sensitive dependence on the initial droplet position and velocity. Numerical solutions of the dynamical equations of motion of the drop provide a close quantitative match to the experimental results. This proves helpful in interpreting the data and allows for accurate prediction of fluid droplet behaviour for a wide range of surfaces.Comment: 14 pages, accepted for publication in Langmui

Rheology of cholesteric blue phases

Blue phases of cholesteric liquid crystals offer a spectacular example of naturally occurring disclination line networks. Here we numerically solve the hydrodynamic equations of motion to investigate the response of three types of blue phases to an imposed Poiseuille flow. We show that shear forces bend and twist and can unzip the disclination lines. Under gentle forcing the network opposes the flow and the apparent viscosity is significantly higher than that of an isotropic liquid. With increased forcing we find strong shear thinning corresponding to the disruption of the defect network. As the viscosity starts to drop, the imposed flow sets the network into motion. Disclinations break-up and re-form with their neighbours in the flow direction. This gives rise to oscillations in the time-dependent measurement of the average stress.Comment: 4 pages, 4 figure

Control of drop positioning using chemical patterning

We explore how chemical patterning on surfaces can be used to control drop wetting. Both numerical and experimental results are presented to show how the dynamic pathway and equilibrium shape of the drops are altered by a hydrophobic grid. The grid proves a successful way of confining drops and we show that it can be used to alleviate {\it mottle}, a degradation in image quality which results from uneven drop coalescence due to randomness in the positions of the drops within the jetted array.Comment: 3 pages, 4 figure

Parameter-free Stark Broadening of Hydrogen Lines in DA White Dwarfs

We present new calculations for the Stark broadening of the hydrogen line profiles in the dense atmospheres of white dwarf stars. Our improved model is based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but it also includes non-ideal gas effects from the Hummer & Mihalas occupation probability formalism directly inside the line profile calculations. This approach improves upon previous calculations that relied on the use of an ad-hoc free parameter to describe the dissolution of the line wing opacity in the presence of high electric microfields in the plasma. We present here the first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has no free parameters. The atmospheric parameters obtained from optical and UV spectroscopic observations using these improved models are shown to differ substantially from those published in previous studies.Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference Proceedings for the 16th European White Dwarf Worksho

The Ultramassive White Dwarf EUVE J1746-706

We have obtained new optical and extreme ultraviolet (EUV) spectroscopy of the ultramassive white dwarf EUVE J1746-706. We revise Vennes et al.'s (1996a, ApJ, 467, 784) original estimates of the atmospheric parameters and we measure an effective temperature of 46,500 +/- 700 K and a surface gravity log g = 9.05 +/- 0.15 (~1.2 M_o), in agreement with Balmer line profiles and the EUV continuum. We derive an upper limit on the atmospheric abundance of helium of He/H = 1.3 x 10^{-4} and a neutral hydrogen column density in the local interstellar medium N_HI = 1.8 +/- 0.4 x 10^{19} cm^{-2} from the EUV spectrum. Our upper limit corresponds to half the helium abundance observed in the atmosphere of the ultramassive white dwarf GD 50. We discuss the possibility that EUVE J1746-706 represents an earlier phase of evolution relative to GD 50 and may, therefore, help us understand the origin and evolution of massive white dwarfs.Comment: 6 pages, 4 postscript figures, uses aastex, to be published in ApJ Letter

A Renormalization group approach for highly anisotropic 2D Fermion systems: application to coupled Hubbard chains

I apply a two-step density-matrix renormalization group method to the anisotropic two-dimensional Hubbard model. As a prelude to this study, I compare the numerical results to the exact one for the tight-binding model. I find a ground-state energy which agrees with the exact value up to four digits for systems as large as $24 \times 25$. I then apply the method to the interacting case. I find that for strong Hubbard interaction, the ground-state is dominated by magnetic correlations. These correlations are robust even in the presence of strong frustration. Interchain pair tunneling is negligible in the singlet and triplet channels and it is not enhanced by frustration. For weak Hubbard couplings, interchain non-local singlet pair tunneling is enhanced and magnetic correlations are strongly reduced. This suggests a possible superconductive ground state.Comment: 8 pages, 11 figures, expanded version of cond-mat/060856

Dimensional crossover and metal-insulator transition in quasi-two-dimensional disordered conductors

We study the metal-insulator transition (MIT) in weakly coupled disordered planes on the basis of a Non-Linear Sigma Model (NL$\sigma$M). Using two different methods, a renormalization group (RG) approach and an auxiliary field method, we calculate the crossover length between a 2D regime at small length scales and a 3D regime at larger length scales. The 3D regime is described by an anisotropic 3D NL$\sigma$M with renormalized coupling constants. We obtain the critical value of the single particle interplane hopping which separates the metallic and insulating phases. We also show that a strong parallel magnetic field favors the localized phase and derive the phase diagram.Comment: 16 pages (RevTex), 4 poscript figure