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Apparent Contact Angle

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Giuseppe Carbone – 1st expert on this subject based on the ideXlab platform

  • filamentary superhydrophobic teflon surfaces moderate Apparent Contact Angle but superior air retaining properties
    Journal of Colloid and Interface Science, 2016
    Co-Authors: Rosa Di Mundo, Francesco Bottiglione, Fabio Palumbo, Michele Notarnicola, Giuseppe Carbone

    Abstract:

    Abstract Hypothesis Micro-scale textured Teflon surfaces, resulting from plasma etching modification, show extremely high water Contact Angle values and fairly good resistance to water penetration when hit by water drops at medium-high speed. This behavior is more pronounced when these surfaces present denser and smaller micrometric reliefs. Tailoring the top of these reliefs with a structure which further stabilizes the air may further increase resistance to wetting (water penetration) under static and dynamic conditions. Experiments Conditions of the oxygen fed plasma were tuned in order to explore the possibility of obtaining differently topped structures on the surface of the polymer. Scanning Electron Microscopy (SEM) was used to explore topography and X-ray Photoelectron Spectroscopy (XPS) to assess chemical similarity of the modified surfaces. Beside the usual advancing and receding water Contact Angle (WCA) measurements, surfaces were subjected to high speed impacting drops and immersion in water. Findings At milder, i.e. shorter time and lower input power, plasma conditions formation of peculiar filaments is observed on the top of the sculpted reliefs. Filamentary topped surfaces result in a lower WCA than the spherical ones, appearing in this sense less superhydrophobic. However, these surfaces give rise to the formation of a more pronounced air layer when placed underwater. Further, when hit by water drops falling at medium/high speed, they show a higher resistance to water penetration and a sensitively lower surface-liquid Contact time. The Contact time is as low as previously observed only on heated solids. This behavior may be ascribed to the cavities formed beneath the filaments which, similarly with the salvinia leaf structures, require a surplus of pressure to be filled by water. Also, it suggests a different concept of superhydrophobicity, which cannot be expected on the basis of the conventional water Contact Angle characterization.

  • an effective medium approach to predict the Apparent Contact Angle of drops on super hydrophobic randomly rough surfaces
    Journal of Physics: Condensed Matter, 2015
    Co-Authors: Francesco Bottiglione, Giuseppe Carbone

    Abstract:

    The Apparent Contact Angle of large 2D drops with randomly rough self-affine profiles is numerically investigated. The numerical approach is based upon the assumption of large separation of length scales, i.e. it is assumed that the roughness length scales are much smaller than the drop size, thus making it possible to treat the problem through a mean-field like approach relying on the large-separation of scales. The Apparent Contact Angle at equilibrium is calculated in all wetting regimes from full wetting (Wenzel state) to partial wetting (Cassie state). It was found that for very large values of the roughness Wenzel parameter (rW > −1/cos θY, where θY is the Young’s Contact Angle), the interface approaches the perfect non-wetting condition and the Apparent Contact Angle is almost equal to 180 ◦ . The results are compared with the case of roughness on one single scale (sinusoidal surface) and it is found that, given the same value of the Wenzel roughness parameter rW, the Apparent Contact Angle is much larger for the case of a randomly rough surface, proving that the multi-scale character of randomly rough surfaces is a key factor to enhance superhydrophobicity. Moreover, it is shown that for millimetre-sized drops, the actual drop pressure at static equilibrium weakly affects the wetting regime, which instead seems to be dominated by the roughness parameter. For this reason a methodology to estimate the Apparent Contact Angle is proposed, which relies only upon the micro-scale properties of the rough surface.

  • the effect of drop volume and micropillar shape on the Apparent Contact Angle of ordered microstructured surfaces
    Soft Matter, 2014
    Co-Authors: L Afferrante, Giuseppe Carbone

    Abstract:

    In the present paper, we propose a new theoretical approach to evaluate the shape and Apparent Contact Angle (ACA) of a drop gently deposited on microstructured superhydrophobic surfaces. We exploit the very large separation of scales between the drop size and the features of the micromorphology of the interface to propose a numerical methodology to calculate the Apparent Contact area and Apparent Contact Angle. In agreement with very recent experiments, calculations show that, in the case of surfaces made of conical micropillars, the ACA may take values very close to 180° not depending on the size of the liquid drop. At large drop volumes, the shape of the drop deviates from the spherical one as a result of the gravity effects, but it is noteworthy that the Apparent Contact Angle does not change at all. Our calculations shows that this holds true also for different pillar shapes, showing that, for any given Young Contact Angle of the solid constituting the pillars, the ACA is an intrinsic property of the surface microgeometry.

Edward Bormashenko – 2nd expert on this subject based on the ideXlab platform

  • Apparent Contact Angles for reactive wetting of smooth, rough, and heterogeneous surfaces calculated from the variational principles.
    Journal of Colloid and Interface Science, 2018
    Co-Authors: Edward Bormashenko

    Abstract:

    Abstract Apparent steady Contact Angles inherent for reactive wetting of flat, rough and chemically heterogeneous solid surfaces are calculated from the variational analysis of the reactive wetting problem. The surface (2D) and linear (1D) contributions to the free energy of the reactive wetting system constitute the Apparent Contact Angle, whereas the bulk contributions such as external fields and heat release do not influence the Apparent Contact Angles of reactive wetting. When the interfacial tensions constitute the Apparent Contact Angle, the equilibrium Contact Angles are independent on the dimensions of droplets. When the line tension is taken into account, the Apparent steady Contact Angle depends on the radius of the Contact area. The Wenzel and Cassie-like regimes of reactive wetting are considered. Reactive wetting of porous surfaces is addressed. The Cassie impregnating reactive wetting regime is treated.

  • A Generalized Electrowetting Equation: Its Derivation and Consequences
    arXiv: Chemical Physics, 2015
    Co-Authors: Edward Bormashenko, Oleg Gendelman

    Abstract:

    The thermodynamics of electrowetting is treated. A general equation of electrowetting is derived from the first principles. It is demonstrated that the well-known Lippmann Equation describes a particular case of electrowetting when the radial derivative of the capacitance of the double layer is constant. The Apparent Contact Angle of electrowetting depends on the gradient of capacity of a double layer in the vicinity of the triple line. The role of the area adjacent to the triple line in constituting the equilibrium Apparent Contact Angle of electrowetting is emphasized.

  • Wetting of Composite Surfaces: When and Why Is the Area Far from The Triple Line Important?
    Journal of Physical Chemistry C, 2013
    Co-Authors: Edward Bormashenko, Yelena Bormashenko

    Abstract:

    Apparent Contact Angles are totally governed by the area of the solid surface adjacent to the triple (three-phase) line. However, Apparent Contact Angles do not describe the wetting situation exhaustively. The wetting regime is characterized by both Apparent Contact Angle and the energy of adhesion. The energy of adhesion in turn depends on the physical and chemical properties of the entire area underneath the droplet. We demonstrate this experimentally by preparing rough surfaces exhibiting high Apparent Contact Angles accompanied with the high energy of adhesion leading to the high Contact Angle hysteresis. A droplet deposited axisymmetrically on the superhydrophobic surface comprising a nonsuperhydrophobic spot holds “sticky” wetting attended with high Apparent Contact Angles.

M A Fortes – 3rd expert on this subject based on the ideXlab platform

  • Apparent Contact Angle and triple line tension of a soap bubble on a substrate
    Journal of Colloid and Interface Science, 2001
    Co-Authors: Joao F D Rodrigues, Benilde Saramago, M A Fortes

    Abstract:

    Abstract The Contact Angle, θ, of a small bubble on a flat solid substrate was measured as a function of bubble radius, R . The observed deviation of the Contact Angle from 90° can be accounted for in terms of a negative line tension, τ. The measured values of |τ|/γ f , where γ f is the film tension, ranged between 0.15 and 0.6 mm and are proportional to the height, h , of the Plateau border, with |τ|≅1.7γ f h .