We present a comprehensive study of water drops sliding down chemically heterogeneous surfaces formed by a periodic pattern of alternating hydrophobic and hydrophilic stripes. Drops are found to undergo a stick-slip motion whose average speed is an order of magnitude smaller than that measured on a homogeneous surface having the same static contact angle. This motion is the result of the periodic deformations of the drop interface when crossing the stripes. Numerical simulations confirm this view and are used to elucidate the principles underlying the experimental observations.

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Isothermal scans of third-sound velocity as a function of He-4 coverage on graphite exhibit pronounced dips with integral layer periodicity. The minimum He-4 coverage on graphite needed to support third sound is nearly three layers. The heat-capacity isotherm exhibits minima at layer completion but there is no noticeable feature related to the onset of superfluidity. High-resolution adsorption isotherms below 1 K show that He-4 film grows layer by layer through at least seven layers

It is well known from quantum mechanics that weak measurements offer a means of amplifying and detecting very small phenomena. We present here the first experimental observation of the Goos-Hänchen shift via a weak measurement approach. c 2013 Optical Society of America OCIS codes: 000.0000, 999.9999.A bounded beam of light reflected from or transmitted through a planar interface suffers diffractive corrections to the law of reflection or to Snell's law. The most prominent of these are the Goos-Hänchen (GH) [1] and the Imbert-Fedorov (IF) [2,3] shifts that induce spatial translations of the beam in the directions parallel and perpendicular to the plane of incidence, respectively. Angular analogous to the GH [4] and the IF effect have recently been observed, as well as the spin Hall effect of light (SHEL) [5][6][7]. This last one is connected to the IF shift being a separation orthogonal to the plane of incidence of the two spin components of the reflected or transmitted beam. These effects do not occur only for simple planar interfaces but were also observed or predicted for photonics crystals, waveguides or resonators [8][9][10]. They are not restricted to fully spatially coherent beams but they are observed also for beams with a partial degree of spatial coherence [11,12]. They are wavelike phenomena that occur also for matter waves or acoustic waves [13,14]. The measurement of these optical shifts is in general a challenging task because they are tiny phenomena. A weak measurement approach has proven to be successful for the observations of these effects. This technique is an optical analogue [15] of the quantum weak measurement concept introduced in ref. [16]. In a remarkable experiment, Hosten and Kwiat [6] where the firsts that applied this experimental scheme to the measurement of optical beam shifts. They reported the first experimental observation of the SHEL. This observation was also confirmed by other experiments [17]. Theoretical analysis of the weak measurement approach for the observation of optical beam shifts were reported in ref. [18].In this letter we present the first experimental observation of the GH effect via a weak measurement scheme. The "weak measuring device" [15] is a prism which introduces a small lateral displacement D p (D s ) for the p (s) polarization of a Gaussian beam in total internal reflection (TIR). We consider a Cartesian reference frame with y in the vertical direction, z in the propagation direction and x fixed as consequence. A polarizer and an analyzer select the initial and the final linear polarizations to be at angles α and β with respect to the horizontal in the xy plane. The weak measurement scheme works as follow [15,19]: if the polarizer and the analyzer are set to α = π 4 and β = α + π 2 + ( 1) respectively, the emerging beam is laterally shifted of a quantityWe measure the separation (∆ GH · cot( )) in between the two beams corresponding to the two polarizations settings α = π 4 and β = α + π 2 ± . The small beam displacement ∆ GH introduced by the GH effect...

We perform a joint numerical and experimental study to systematically characterize the motion of 30 μl drops of pure water and of ethanol in water solutions, sliding over a periodic array of alternating hydrophobic and hydrophilic stripes with a large wettability contrast and a typical width of hundreds of microns. The fraction of the hydrophobic areas has been varied from about 20% to 80%. The effects of the heterogeneous patterning can be described by a renormalized value of the critical Bond number, i.e., the critical dimensionless force needed to depin the drop before it starts to move. Close to the critical Bond number we observe a jerky motion characterized by an evident stick-slip dynamics. As a result, dissipation is strongly localized in time, and the mean velocity of the drops can easily decrease by an order of magnitude compared to the sliding on the homogeneous surface. Lattice Boltzmann numerical simulations are crucial for disclosing to what extent the sliding dynamics can be deduced from the computed balance of capillary, viscous, and body forces by varying the Bond number, the surface composition, and the liquid viscosity. Beyond the critical Bond number, we characterize both experimentally and numerically the dissipation inside the droplet by studying the relation between the average velocity and the applied volume forces.