Wetting of anisotropic sinusoidal surfaces - experimental and numerical study of directional spreading

Directional wettability, i.e. variation of wetting properties depending on the surface orientation, can be achieved by anisotropic surface texturing. A new high precision process can produce homogeneous sinusoidal surfaces (in particular parallel grooves) at the micro-scale, with a nano-scale residual roughness five orders of magnitude smaller than the texture features. Static wetting experiments have shown that this pattern, even with a very small aspect ratio, can induce a strong variation of contact angle depending on the direction of observation. A comparison with numerical simulations (using Surface Evolver software) shows good agreement and could be used to predict the fluid-solid interaction and droplet behaviour on textured surfaces. Two primary mechanisms of directional spreading of water droplets on textured stainless steel surface have been identified. The first one is the mechanical barrier created by the textured surface peaks, this limits spreading in perpendicular direction to the surface anisotropy. The second one is the capillary action inside the sinusoidal grooves accelerating spreading along the grooves. Spreading has been shown to depend strongly on the history of wetting and internal drop dynamics

Are the Ogle Microlenses in the Galactic Bar?

The analysis of the first two years of OGLE data revealed 9 microlensing events of the galactic bulge stars, with the characteristic time scales in the range $8.6 < t_0 < 62$ days, where $t_0 = R_E / V$. The optical depth to microlensing is larger than $( 3.3 \pm 1.2 ) \times 10^{-6}$, in excess of current theoretical estimates, indicating a much higher efficiency for microlensing by either bulge or disk lenses. We argue that the lenses are likely to be ordinary stars in the galactic bar, which has its long axis elongated towards us. A relation between $t_0$ and the lens masses remains unknown until a quantitative model of bar microlensing becomes available. At this time we have no evidence that the OGLE events are related to dark matter. The geometry of lens distribution can be determined observationally when the microlensing rate is measured over a larger range of galactic longitudes, like $-10^o < l < +10^o$, and the relative proper motions of the galactic bulge (bar) stars are measured with the HST.Comment: 10 pages, 2 figures, revised version accepted for the publication in ApJL, uses AAS LaTeX aaspp.sty macro, PostScript figures and PostScript version of the paper available through anonymous ftp from astro.princeton.edu, directory stanek/tau, or on reques

Modelling the Galactic Bar Using Red Clump Giants

The color-magnitude diagrams of $\sim 7 \times 10^5$ stars obtained for 12 fields across the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump giants. We find that the distributions of the apparent magnitudes of the red clump stars are systematically fainter when moving towards lower galactic $l$ fields. The most plausible explanation of this distinct trend is that the Galactic bulge is a bar, whose nearest end lies at positive galactic longitude. We model this Galactic bar by fitting for all fields the observed luminosity functions in the red clump region of the color-magnitude diagram. We find that almost regardless of the analytical function used to describe the 3-D stars distribution of the Galactic bar, the resulting models have the major axis inclined to the line of sight by $20-30\deg$, with axis ratios corresponding to $x_0:y_0:z_0=3.5:1.5:1$. This puts a strong constraint on the possible range of the Galactic bar models. Gravitational microlensing can provide us with additional constrains on the structure of the Galactic bar.Comment: submitted to the New Astronomy, 27 pages, 11 figures; also available at ftp://www.astro.princeton.edu/stanek/Barmodel and through WWW at http://www.astro.princeton.edu/~library/prep.htm

Color-Magnitude Diagram Distribution of the Bulge Red Clump Stars - Evidence for the Galactic Bar

The color-magnitude diagrams of $\sim 5\times 10^5$ stars obtained for 13 fields towards the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump stars. We find that the distributions of the extinction-adjusted apparent magnitudes of the red clump stars in fields lying at $l=\pm5\deg$ in galactic longitude differ by $0.37\pm0.025\;mag$. Assuming that the intrinsic luminosity distribution of the red clump stars is the same on both sides of the Galactic center, this implies that the distances to the red clump stars in the two fields differ by a factor of $1.185\pm0.015$. A plausible explanation of the observed difference in the luminosity distribution is that the Galactic bulge is a triaxial structure, or bar, which is inclined to the line of sight by no more than $45\deg$, with the part of the bar at the positive galactic longitude being closer to us. This agrees rather well with other studies indicating the presence of the bar in the center of the Galaxy. Color-magnitude diagram data are accessible over the computer network with anonymous {\tt ftp}.Comment: 10 pages, 6 figures, uses AAS LaTeX aaspp.sty macro, PostScript figures available through the anonymous ftp or on request, accepted for the publication in the ApJ

Modelling the Galactic Bar Using Red Clump Stars

The color-magnitude diagrams of $\sim 1 \times 10^6$ stars obtained for 19 fields towards the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump stars. We found that the distributions of the extinction-adjusted apparent magnitudes of red clump stars in fields lying at $l=\pm5\deg$ in galactic longitude differ by $\sim 0.4\; mag$. A plausible explanation of this observed difference in the luminosity distribution is that the Galactic bulge is a triaxial structure, or a bar, which is inclined to the line of sight by no more than $45\deg$. The part of the bar at the positive galactic longitude is closer to us. Work is now under way to model the Galactic bar by fitting the observed luminosity functions in the red clump region for various fields. Preliminary results indicate that the angle of the inclination of the bar to the line of sight can be as small as $\sim20\deg$. Gravitational microlensing can provide us with additional constrains on the structure of the Galactic bar.Comment: 4 pages, uuencoded compressed PostScript, talk presented at the IAU Colloquium 157 "Barred Galaxies"; also available through WWW at http://www.astro.princeton.edu/~library/prep.htm

The optical gravitational lensing experiment: variable stars in globular clusters; 1, fields 5139A-C in Omega Centauri

Three fields covering the central part of the globular cluster Omega Cen were surveyed in a search for variable stars. We present V-band light curves for 22 periodic variables: 9 SX~Phe stars, 7 contact binaries, 5 detached or semi-detached binaries, and one spotted variable (FK Com or RS CVn type star). Only 2 of these variables were previously known. All SX Phe stars and all contact binaries from our sample belong to blue stragglers. Observed properties of these stars are consistent with their cluster membership. Of particular interest is detection of two well detached binaries with periods P=1.50 day and P=2.47 day. Further study of these two binaries can provide direct information about properties of turnoff stars in Omega Cen. An uncomplete light curve of a Mira variable known as V2 was obtained. We present V vs. V-I color-magnitude diagrams for the monitored part of the cluster