Wrapping an adhesive sphere with a sheet

We study the adhesion of an elastic sheet on a rigid spherical substrate. Gauss'Theorema Egregium shows that this operation necessarily generates metric distortions (i.e. stretching) as well as bending. As a result, a large variety of contact patterns ranging from simple disks to complex branched shapes are observed as a function of both geometrical and material properties. We describe these different morphologies as a function of two non-dimensional parameters comparing respectively bending and stretching energies to adhesion. A complete configuration diagram is finally proposed

Triagem de Manchester nas Síndromes Coronárias Agudas

INTRODUCTION: A growing number of hospitals have implemented the Manchester Triage System (MTS) in their Emergency Department (ED), so as to better prioritize the evaluation of those attending these departments. OBJECTIVES: To assess whether the MTS was used effectively in patients admitted to the hospital with a diagnosis of acute coronary syndrome (ACS). METHODS: We evaluated 114 consecutive patients admitted to the Cardiology Department with a diagnosis of ACS. We recorded the color assigned in the MTS, mean time from arrival in the ED to MTS, mean time from MTS to first medical assessment (1-MA) and mean time from 1-MA to admission. We also analyzed the correlation between the type of ACS and clinical presentation and its relation with MTS. RESULTS: Of the 114 patients, one was coded red (0.9%), 71 orange (62.3%), 12 green (11%), and two were not assigned a color code according to MTS because they were admitted via a Medical Emergency and Resuscitation Vehicle. Mean time from arrival in the ED to MTS was 5.2 +/- 0.6 min and from MTS to MA was 20 +/- 2.5 min. In patients triaged as orange the time from MTS to MA was 15.1 +/- 1.5 min, as yellow 36.2 +/- 7 min, and as green 35.2 +/- 20.6 min (p = 0.003). Mean time from 1-MA to admission was 144.4 +/- 17 min, with no differences according to triage code or ACS type. Clinical presentation influenced triage and the speed of 1-MA and admission, patients with typical presentation being evaluated and admitted more quickly. CONCLUSIONS: Most patients admitted for ACS are initially triaged as orange or yellow, an indication for prompt assessment in the ED; this has a positive effect on time to first medical assessment, but has no effect on time to hospital admission

Hydrokinetic simulations of nanoscopic precursor films in rough channels

We report on simulations of capillary filling of high-wetting fluids in nano-channels with and without obstacles. We use atomistic (molecular dynamics) and hydrokinetic (lattice-Boltzmann) approaches which point out clear evidence of the formation of thin precursor films, moving ahead of the main capillary front. The dynamics of the precursor films is found to obey a square-root law as the main capillary front, z^2(t) ~ t, although with a larger prefactor, which we find to take the same value for the different geometries (2D-3D) under inspection. The two methods show a quantitative agreement which indicates that the formation and propagation of thin precursors can be handled at a mesoscopic/hydrokinetic level. This can be considered as a validation of the Lattice-Boltzmann (LB) method and opens the possibility of using hydrokinetic methods to explore space-time scales and complex geometries of direct experimental relevance. Then, LB approach is used to study the fluid behaviour in a nano-channel when the precursor film encounters a square obstacle. A complete parametric analysis is performed which suggests that thin-film precursors may have an important influence on the efficiency of nanochannel-coating strategies.Comment: 16 pages, 8 figures; To be published on JSTAT: Journal of statistical mechanics: Theory and experiment

Capillary filling with pseudo-potential binary Lattice-Boltzmann model

We present a systematic study of capillary filling for a binary fluid by using a mesoscopic lattice Boltzmann model for immiscible fluids describing a diffusive interface moving at a given contact angle with respect to the walls. The phenomenological way to impose a given contact angle is analysed. Particular attention is given to the case of complete wetting, that is contact angle equal to zero. Numerical results yield quantitative agreement with the theoretical Washburn law, provided that the correct ratio of the dynamic viscosities between the two fluids is used. Finally, the presence of precursor films is experienced and it is shown that these films advance in time with a square-root law but with a different prefactor with respect to the bulk interface.Comment: 13 pages, 8 figures, accepted for publication on The European journal of physics

Self-similarity of contact line depinning from textured surfaces

The mobility of drops on surfaces is important in many biological and industrial processes, but the phenomena governing their adhesion, which is dictated by the morphology of the three-phase contact line, remain unclear. Here we describe a technique for measuring the dynamic behaviour of the three-phase contact line at micron length scales using environmental scanning electron microscopy. We examine a superhydrophobic surface on which a drop’s adhesion is governed by capillary bridges at the receding contact line. We measure the microscale receding contact angle of each bridge and show that the Gibbs criterion is satisfied at the microscale. We reveal a hitherto unknown self-similar depinning mechanism that shows how some hierarchical textures such as lotus leaves lead to reduced pinning, and counter-intuitively, how some lead to increased pinning. We develop a model to predict adhesion force and experimentally verify the model’s broad applicability on both synthetic and natural textured surfaces.National Science Foundation (U.S.) (CAREER Award 0952564)DuPont MIT AllianceNational Science Foundation (U.S.). Graduate Research Fellowship ProgramNational Science Foundation (U.S.) (Award ECS-0335765

Wettability Switching Techniques on Superhydrophobic Surfaces

The wetting properties of superhydrophobic surfaces have generated worldwide research interest. A water drop on these surfaces forms a nearly perfect spherical pearl. Superhydrophobic materials hold considerable promise for potential applications ranging from self cleaning surfaces, completely water impermeable textiles to low cost energy displacement of liquids in lab-on-chip devices. However, the dynamic modification of the liquid droplets behavior and in particular of their wetting properties on these surfaces is still a challenging issue. In this review, after a brief overview on superhydrophobic states definition, the techniques leading to the modification of wettability behavior on superhydrophobic surfaces under specific conditions: optical, magnetic, mechanical, chemical, thermal are discussed. Finally, a focus on electrowetting is made from historical phenomenon pointed out some decades ago on classical planar hydrophobic surfaces to recent breakthrough obtained on superhydrophobic surfaces