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Giuseppe Carbone - One of the best experts on this subject based on the ideXlab platform.
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filamentary superhydrophobic teflon surfaces moderate Apparent Contact Angle but superior air retaining properties
Journal of Colloid and Interface Science, 2016Co-Authors: Rosa Di Mundo, Francesco Bottiglione, Fabio Palumbo, Michele Notarnicola, Giuseppe CarboneAbstract: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.
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an effective medium approach to predict the Apparent Contact Angle of drops on super hydrophobic randomly rough surfaces
Journal of Physics: Condensed Matter, 2015Co-Authors: Francesco Bottiglione, Giuseppe CarboneAbstract: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.
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the effect of drop volume and micropillar shape on the Apparent Contact Angle of ordered microstructured surfaces
Soft Matter, 2014Co-Authors: L Afferrante, Giuseppe CarboneAbstract: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.
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Microstructured superhydrorepellent surfaces: effect of drop pressure on fakir-state stability and Apparent Contact Angles.
Journal of Physics: Condensed Matter, 2010Co-Authors: Luciano Afferrante, Giuseppe CarboneAbstract:In this paper we present a generalized Cassie–Baxter equation to take into account the effect of drop pressure on the Apparent Contact Angle θapp. Also we determine the limiting pressure pW which causes the impalement transition to the Wenzel state and the pull-off pressure pout at which the drop detaches from the substrate. The calculations have been carried out for axial-symmetric pillars of three different shapes: conical, hemispherical-topped and flat-topped cylindrical pillars. Calculations show that, assuming the same pillar spacing, conical pillars may be more inclined to undergo an impalement transition to the Wenzel state, but, on the other hand, they are characterized by a vanishing pull-off pressure which causes the drop not to adhere to the substrate and therefore to detach very easily. We infer that this property should strongly reduce the Contact Angle hysteresis as experimentally observed in Martines et al (2005 Nano Lett. 5 2097–103). It is possible to combine large resistance to impalement transition (i.e. large value of pW) and small (or even vanishing) detaching pressure pout by employing cylindrical pillars with conical tips. We also show that, depending on the particular pillar geometry, the effect of drop pressure on the Apparent Contact Angle θapp may be more or less significant. In particular we show that in the case of conical pillars increasing the drop pressure causes a significant decrease of θapp in agreement with some experimental investigations (Lafuma and Quere 2003 Nat. Mater. 2 457), whereas θapp slightly increases for hemispherical or flat-topped cylindrical pillars.
Edward Bormashenko - One of the best experts on this subject based on the ideXlab platform.
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Apparent Contact Angles for reactive wetting of smooth, rough, and heterogeneous surfaces calculated from the variational principles.
Journal of Colloid and Interface Science, 2018Co-Authors: Edward BormashenkoAbstract: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.
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A Generalized Electrowetting Equation: Its Derivation and Consequences
arXiv: Chemical Physics, 2015Co-Authors: Edward Bormashenko, Oleg GendelmanAbstract: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.
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Wetting of Composite Surfaces: When and Why Is the Area Far from The Triple Line Important?
Journal of Physical Chemistry C, 2013Co-Authors: Edward Bormashenko, Yelena BormashenkoAbstract: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.
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Wetting of real solid surfaces: new glance on well-known problems
Colloid and Polymer Science, 2013Co-Authors: Edward BormashenkoAbstract:The wetting of solid surfaces is treated. The Young and receding Contact Angles are experimentally unattainable values for the majority of solid surfaces. Actually, we always observe the Apparent Contact Angle. This makes the characterization of wetting of real surfaces problematic. It is proposed in this paper to characterize wetting of real surfaces with the advancing Contact Angle and the minimum work of adhesion calculated according to the Dupre equation. The advancing Contact Angle, which depends slightly on the experimental technique used for its measurement, corresponds to the maximal solid/liquid surface tension and correspondingly to the minimal work of adhesion, calculated according to the Dupre equation.
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A variational approach to wetting of composite surfaces: is wetting of composite surfaces a one-dimensional or two-dimensional phenomenon?
Langmuir, 2009Co-Authors: Edward BormashenkoAbstract:Accurate treatment of the variational problem of wetting a composite surface clarifies the physical character of wetting. It is shown that only the area adjacent to a triple line exerts an influence on the Apparent Contact Angle. This conclusion is true for flat composite surfaces and also for rough surfaces governed by Cassie and Wenzel wetting regimes. A surface density of defects in the vicinity of the triple line dictates the Apparent Contact Angle.
M A Fortes - One of the best experts on this subject based on the ideXlab platform.
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Apparent Contact Angle and triple line tension of a soap bubble on a substrate
Journal of Colloid and Interface Science, 2001Co-Authors: Joao F D Rodrigues, Benilde Saramago, M A FortesAbstract: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 .
Vadim Nikolayev - One of the best experts on this subject based on the ideXlab platform.
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TRIGGERING THE BOILING CRISIS: A STUDY OF THE DRY SPOT SPREADING MECHANISM
Interfacial Phenomena and Heat Transfer, 2020Co-Authors: Vladislav Janecek, Vadim NikolayevAbstract:In this article we describe the boiling crisis (departure from nucleate boiling) as a transition between two regimes of bubble growth: " bubble departure " and " bubble spreading, " which occur for small and large heat fluxes, respectively. The threshold heat flux is the critical heat flux (CHF). Our assumptions are based on existing experimental observations. The boiling crisis is assumed to be triggered by spreading of dry spots under individual bubbles that occurs due to the Apparent Contact Angle growth, when the Apparent Contact Angle attains 90°. Such a mechanism is studied via numerical simulation of the single bubble growth. The Apparent Contact Angle is obtained from the hydrodynamic modeling of the vicinity of the triple Contact line (microregion). The microregion model is based on the lubrication approximation extended to treat high interface slopes. The microregion model is compared to previous experimental and numerical data obtained for moderate heater superheating values. The macroregion is modeled within a simplified 2D quasistationary approach that neglects the fluid motion. The CHF obtained from such a formulation is calculated as a function of gravity level and the heater wetting properties. The results agree qualitatively with existing experimental data.
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Apparent Contact Angle model at partial wetting and evaporation impact of surface forces
Physical Review E, 2013Co-Authors: Vladislav Janecek, Vadim NikolayevAbstract:: This theoretical and numerical study deals with evaporation of a fluid wedge in Contact with its pure vapor. The model describes a regime where the continuous wetting film is absent and the actual line of the triple gas-liquid-solid Contact appears. A constant temperature higher than the saturation temperature is imposed at the solid substrate. The fluid flow is solved in the lubrication approximation. The introduction of the surface forces in the case of the partial wetting is discussed. The Apparent Contact Angle (the gas-liquid interface slope far from the Contact line) is studied numerically as a function of the substrate superheating, Contact line velocity, and parameters related to the solid-fluid interaction (Young and microscopic Contact Angles, Hamaker constant, etc.). The dependence of the Apparent Contact Angle on the substrate temperature is in agreement with existing approaches. For water, the Apparent Contact Angle may be 20° larger than the Young Contact Angle for 1 K superheating. The effect of the surface forces on the Apparent Contact Angle is found to be weak.
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impact of the Apparent Contact Angle on the bubble departure at boiling
International Journal of Heat and Mass Transfer, 2012Co-Authors: Vadim Nikolayev, Vladislav JanecekAbstract:Abstract The contribution of the microregion (vicinity of the triple gas–liquid–solid Contact line) to the force balance of a vapor bubble is discussed. The force balance of a bubble attached to the heater includes the capillary adhesion force that involves the Contact Angle. From the theoretical point of view, the microscopic value of the Contact Angle should be used in this expression. However in the experimental literature on the bubble departure, the Apparent Contact Angle is commonly used instead. This usually leads to a substantial increase of the bubble adhesion and provides a good agreement with experimental data. A theoretical justification for such a substitution is proposed. The difference between the effective capillary adhesion force (that involving the Apparent Contact Angle) and its true value is provided by the hydrodynamic stress caused by the liquid flow towards the Contact line which is also at the origin of the difference between the microscopic and Apparent Contact Angles.
Kazuyuki Ueno - One of the best experts on this subject based on the ideXlab platform.
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Apparent Contact Angle calculated from a water repellent model with pinning effect
Langmuir, 2017Co-Authors: Shojiro Suzuki, Kazuyuki UenoAbstract:A set of new theoretical equations for Apparent Contact Angles is proposed. The equations are derived from an equilibrium of interfacial tensions of a three-phase Contact line pinned at the edges of a fine structure. These equations are validated by comparison with Contact-Angle measurement results for 2 μL water droplets on poly(methyl methacrylate) microstructured samples with square pillars or holes. The equilibrium Contact Angles predicted by the new equations reasonably agree with the experimental results. In contrast, the values predicted by the Cassie–Baxter equation or the Wenzel equation do not qualitatively agree with the experimental results in pillar pattern cases because the Cassie–Baxter equation and the Wenzel equation do not account for the pinning effect.
