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A W Neumann - One of the best experts on this subject based on the ideXlab platform.

  • Contact Angle hysteresis: Study by dynamic cycling Contact Angle measurements and variable Angle spectroscopic ellipsometry on polyimide
    Langmuir, 2004
    Co-Authors: Andreas Hennig, U. Staudinger, K. Sahre, M. Rogalli, Klaus Jochen Eichhorn, Manfred Stamm, A W Neumann, Karina Grundke
    Abstract:

    The phenomenon of Contact Angle hysteresis was studied on smooth films of polyimide, a polymer type used in the microelectronic industry, by dynamic cycling Contact Angle measurements based on axisymmetric drop shape analysis-profile in combination with variable Angle spectroscopic ellipsometry (VASE). It was found that both advancing and receding Contact Angles became smaller with increasing the number of cycles and are, therefore, not a property of the dry solid alone. The changes of the wetting behavior during these dynamic cycling Contact Angle measurements are attributed mainly to swelling and/or liquid retention. To reveal the water-induced changes of the polymer film, the polyimide surface was studied before and after the Contact with a water droplet by VASE. Both the experimental ellipsometric spectrum for Delta and that for Psi as well as the corresponding simulations show characteristic shifts due to the Contact with water. The so-called effective medium approximation was applied to recover information about the thickness and effective optical constants of the polymer layer from the ellipsometrically measured values of Delta and Psi. On the basis of these results, the swelling and retention behavior of the polyimide films in Contact with water droplets were discussed.

  • Study of the advancing and receding Contact Angles: liquid sorption as a cause of Contact Angle hysteresis.
    Advances in Colloid and Interface Science, 2002
    Co-Authors: C.n.c Lam, Michael L. Hair, A W Neumann
    Abstract:

    Two types of experiments were used to study the behavior of both advancing and receding Contact Angles, namely the dynamic one-cycle Contact Angle (DOCA) and the dynamic cycling Contact Angle (DCCA) experiments. For the preliminary study, DOCA measurements of different liquids on different solids were performed using an automated axisymmetric drop shape analysis-profile (ADSA-P). From these experimental results, four patterns of receding Contact Angle were observed: (1) time-dependent receding Contact Angle; (2) constant receding Contact Angle; (3) 'stick/slip'; (4) no receding Contact Angle. For the purpose of illustration, results from four different solid surfaces are shown. These solids are: FC-732-coated surface; poly(methyl methacrylate/n-butyl methacrylate) [P(MMA/nBMA)]; poly(lactic acid) (DL-PLA); and poly(lactic/glycolic acid) 50/50 (DL-PLGA 50/50). Since most of the surfaces in our studies exhibit time dependence in the receding Contact Angle, a more extended study was conducted using only FC-732-coated surfaces to better understand the possible causes of decreasing receding Contact Angle and Contact Angle hysteresis. Contact Angle measurements of 21 liquids from two homologous series (i.e. n-alkanes and 1-alcohols) and octamethylcyclotetrasiloxane (OCMTS) on FC-732-coated surfaces were performed. It is apparent that the Contact Angle hysteresis decreases with the chain length of the liquid. It was found that the receding Contact Angle equals the advancing Angle when the alkane molecules are infinitely large. These results strongly suggest that the chain length and size of the liquid molecule could contribute to Contact Angle hysteresis phenomena. Furthermore, DCCA measurements of six liquids from the two homologous series on FC-732-coated surfaces were performed. With these experimental results, one can construe that the time dependence of Contact Angle hysteresis on relatively smooth and homogeneous surfaces is mainly caused by liquid retention/sorption. The results also suggested that the Contact Angle hysteresis will eventually approach a steady state, where the rate of liquid retention-evaporation or sorption process would balance out each other. If the existence of Contact Angle hysteresis can be attributed to liquid sorption/retention, one should only use the advancing Contact Angles (measured on a dry surface) in conjunction with Young's equation for surface energetic calculations.

  • Contact Angle measurement and Contact Angle interpretation
    Advances in Colloid and Interface Science, 1999
    Co-Authors: Daniel Y Kwok, A W Neumann
    Abstract:

    Recent progress in the correlation of Contact Angles with solid surface tensions are summarized. The measurements of meaningful Contact Angles in terms of surface energetics are also discussed. It is shown that the controversy with respect to measurement and interpretation of Contact Angles are due to the fact that some (or all) of the assumptions made in all energetic approaches are violated when Contact Angles are measured and processed. For a large number of polar and non-polar liquids on different solid surfaces, the liquid–vapor surface tension times cosine of the Contact Angle, γlvcosθ, is shown to depend only on the liquid–vapor surface tension γlv, and the solid–vapor surface tension γsv when the appropriate experimental techniques and procedures are used. Equations which follow these experimental patterns and which allow the determination of solid surface tensions from Contact Angles are discussed. Universality of these experimental Contact Angle patterns is illustrated; other reasons which may cause data to deviate from the patterns slightly are discussed. It is found that surface tension component approaches do not reflect physical reality. Assuming the fact that solid surface tension is constant for one and the same solid surface, experimental Contact Angle patterns are employed to deduce a functional relationship to be used in conjunction with Young's equation for determining solid surface tensions. The explicit form of such a relation is obtained by modifying Berthelot's rule together with experimental data; essentially constant solid surface tension values are obtained, independent of liquid surface tension and molecular structure. A new combining rule is also derived based on an expression similar to one used in molecular theory; such a combining rule should allow a better understanding of the molecular interactions between unlike solid–liquid pairs from like pairs. Existing static Contact Angles for 34 different types of solid surfaces from Zisman et al. are evaluated in terms of their solid surface tensions using experimental Contact Angle patterns. A FORTRAN computer program has been implemented to automate these procedures. It is found that literature Contact Angles do not have to be discarded completely; they can be used to determine solid surface tensions, with caution. The surface tensions for the 34 solid surfaces from Zisman et al. are also reported.

  • Contact Angle measurements and Contact Angle interpretation 1 Contact Angle measurements by axisymmetric drop shape analysis and a goniometer sessile drop technique
    Langmuir, 1997
    Co-Authors: Daniel Y Kwok, Karina Grundke, T Gietzelt, H J Jacobasch, A W Neumann
    Abstract:

    Low-rate dynamic Contact Angles and static advancing Contact Angles of 13 liquids were measured, respectively, by axisymmetric drop shape analysis (ADSA) and a conventional goniometer technique, on two copolymers, poly(propene-alt-N-(n-propyl)maleimide) and poly(propene-alt-N-(n-hexyl)maleimide); both have polar groups. In the case of the former technique for measuring Contact Angles (at very low velocity of the three-phase Contact line), very complex Contact Angle responses were observed for some solid−liquid systems. In a specific case, slip and stick Contact Angle behavior occurs where the Contact Angle increases steadily by as much as 35° at constant three-phase Contact radius and subsequently decreases sharply due to a sudden jump in the three-phase Contact line. Thus, circumspection is necessary in the decision whether or not the experimental Contact Angles can be used to interpret surface energetics in conjunction with Young's equation and whether the solid−liquid systems violate the basic assumpti...

Anthony B Brennan - One of the best experts on this subject based on the ideXlab platform.

  • potential for tunable static and dynamic Contact Angle anisotropy on gradient microscale patterned topographies
    Langmuir, 2009
    Co-Authors: Christopher J Long, James F Schumacher, Anthony B Brennan
    Abstract:

    Translationally symmetric topographies can be designed to induce anisotropy of static and dynamic Contact Angles. The validity of ignoring directionality of topography in Contact Angle characterization was evaluated using microscale patterned topographies. Seven patterned topographies comprising elongated discontinuous microfeatures oriented along parallel paths and one topography comprising ridges were fabricated in a poly(dimethyl siloxane) elastomer (PDMSe). The static Contact Angle, advancing Contact Angle, receding Contact Angle, Contact Angle hysteresis, and slip Angle were measured using water on each surface at three in-plane perspectives, with respect to the feature orientation. Static and dynamic Contact Angle anisotropies were investigated on the topographies to evaluate the effect of discontinuities along the feature lengths on the anisotropy that has been shown on channels or ridges in previous reports. Discontinuous feature topographies exhibited a statistically significant anisotropy of 2°−...

  • potential for tunable static and dynamic Contact Angle anisotropy on gradient microscale patterned topographies
    Langmuir, 2009
    Co-Authors: Christopher J Long, James F Schumacher, Anthony B Brennan
    Abstract:

    Translationally symmetric topographies can be designed to induce anisotropy of static and dynamic Contact Angles. The validity of ignoring directionality of topography in Contact Angle characterization was evaluated using microscale patterned topographies. Seven patterned topographies comprising elongated discontinuous microfeatures oriented along parallel paths and one topography comprising ridges were fabricated in a poly(dimethyl siloxane) elastomer (PDMSe). The static Contact Angle, advancing Contact Angle, receding Contact Angle, Contact Angle hysteresis, and slip Angle were measured using water on each surface at three in-plane perspectives, with respect to the feature orientation. Static and dynamic Contact Angle anisotropies were investigated on the topographies to evaluate the effect of discontinuities along the feature lengths on the anisotropy that has been shown on channels or ridges in previous reports. Discontinuous feature topographies exhibited a statistically significant anisotropy of 2 degrees-6 degrees between the perpendicular and parallel directions, with respect to the static and dynamic Contact Angles. The ridges topography exhibited much larger 5 degrees-42 degrees anisotropy in the Contact Angles. The discontinuities along the feature lengths greatly reduced, but did not eliminate, the anisotropies compared to the ridges. This evidence of Contact Angle anisotropy indicates a need to identify the orientation of topography, in relation to Contact Angle measurements. It also implies a need to consider directionality in the design of microfluidic devices and self-cleaning surfaces.

Abraham Marmur - One of the best experts on this subject based on the ideXlab platform.

  • Contact Angle measurement on rough surfaces
    Journal of Colloid and Interface Science, 2004
    Co-Authors: Tammar S Meiron, Abraham Marmur, Sam I Saguy
    Abstract:

    A new method for the measurement of apparent Contact Angles at the global energy minimum on real surfaces has been developed. The method consists of vibrating the surface, taking top-view pictures of the drop, monitoring the drop roundness, and calculating the Contact Angle from the drop diameter and weight. The use of the new method has been demonstrated for various rough surfaces, all having the same surface chemistry. In order to establish the optimal vibration conditions, the proper ranges for the system parameters (i.e., drop volume, vibration time, frequency of vibration, and amplitude of vibration) were determined. The reliability of the method has been demonstrated by the fact that the ideal Contact Angles of all surfaces, as calculated from the Wenzel equation using the measured apparent Contact Angles, came out to be practically identical. This ideal Contact Angle has been compared with three methods of calculation from values of advancing and receding Contact Angles.

  • line tension and the intrinsic Contact Angle in solid liquid fluid systems
    Joint International Conference on Information Sciences, 1997
    Co-Authors: Abraham Marmur
    Abstract:

    Abstract An approximate theory for calculating the line tension in a solid–liquid–fluid system is presented and demonstrated for a solid–liquid–vapor system. The line tension is shown to depend on the Contact Angle. Consequently, the classical equation for the intrinsic Contact Angle, in terms of the line tension, is modified. The magnitude of the line tension is shown to be less than 5 × 10−9N, and its sign is shown to be positive for acute Contact Angles and negative for obtuse Contact Angles. The deviation from the Young Contact Angle is shown to be negligible for drops of macroscopic size on an ideal solid surface.

  • Contact Angle and thin film equilibrium
    Journal of Colloid and Interface Science, 1992
    Co-Authors: Abraham Marmur
    Abstract:

    Abstract The “mechanical” equilibrium condition for a drop on a solid surface is developed, taking into account the three-phase molecular interactions near the Contact line. From this condition, equations for the shape of the liquid-fluid interface and the Contact Angle are derived. It is shown that the equilibrium condition does not explicitly contain the colloidal interaction energy. Rather, the interfacial tensions are considered to be functions of the position and inclination of the liquid-fluid interface. The mathematical approach is demonstrated first for simplified situations with constant interfacial tensions. Then, general equations are developed for the shape of the liquid-fluid interface and for the Contact Angle. It is also shown that the shape and thickness of the thin film in the case of complete wetting can naturally result from the equilibrium condition.

  • Contact Angle equilibrium the intrinsic Contact Angle
    Journal of Adhesion Science and Technology, 1992
    Co-Authors: Abraham Marmur
    Abstract:

    The conceptual and mathematical difficulties associated with Contact Angle equilibrium are reviewed and analyzed. The main discussion is centered around the theory of the intrinsic Contact Angle and its dependence on the three-phase mutual interactions in the vicinity of the Contact line. Rigorous thermodynamic derivation of the equilibrium condition is presented, emphasizing the question of the correct presentation of the colloidal interactions near the Contact line. This is followed by the presentation of various case studies, in order of increasing complexity. The line tension concept is also discussed, and its implications are compared with those of the general thermodynamic treatment.

Sashikumaar Ganesan - One of the best experts on this subject based on the ideXlab platform.

  • simulations of impinging droplets with surfactant dependent dynamic Contact Angle
    Journal of Computational Physics, 2015
    Co-Authors: Sashikumaar Ganesan
    Abstract:

    An arbitrary Lagrangian-Eulerian (ALE) finite element scheme for computations of soluble surfactant droplet impingement on a horizontal surface is presented. The numerical scheme solves the time-dependent Navier-Stokes equations for the fluid flow, scalar convection-diffusion equation for the surfactant transport in the bulk phase, and simultaneously, surface evolution equations for the surfactants on the free surface and on the liquid-solid interface. The effects of surfactants on the flow dynamics are included into the model through the surface tension and surfactant-dependent dynamic Contact Angle. In particular, the dynamic Contact Angle ( ? d ) of the droplet is defined as a function of the surfactant concentration at the Contact line and the equilibrium Contact Angle ( ? e 0 ) of the clean surface using the nonlinear equation of state for surface tension. Further, the surface forces are included into the model as surface divergence of the surface stress tensor that allows to incorporate the Marangoni effects without calculating the surface gradient of the surfactant concentration on the free surface. In addition to a mesh convergence study and validation of the numerical results with experiments, the effects of adsorption and desorption surfactant coefficients on the flow dynamics in wetting, partially wetting and non-wetting droplets are studied in detail. It is observed that the effects of surfactants are more in wetting droplets than in the non-wetting droplets. Further, the presence of surfactants at the Contact line reduces the equilibrium Contact Angle further when ? e 0 is less than 90?, and increases it further when ? e 0 is greater than 90?. Nevertheless, the presence of surfactants has no effect on the Contact Angle when ? e 0 = 90 ? . The numerical study clearly demonstrates that the surfactant-dependent Contact Angle has to be considered, in addition to the Marangoni effect, in order to study the flow dynamics and the equilibrium states of surfactant droplet impingement accurately. The proposed numerical scheme guarantees the conservation of fluid mass and of the surfactant mass accurately.

  • On the dynamic Contact Angle in simulation of impinging droplets with sharp interface methods
    Microfluidics and Nanofluidics, 2013
    Co-Authors: Sashikumaar Ganesan
    Abstract:

    Effects of dynamic Contact Angle models on the flow dynamics of an impinging droplet in sharp interface simulations are presented in this article. In the considered finite element scheme, the free surface is tracked using the arbitrary Lagrangian–Eulerian approach. The Contact Angle is incorporated into the model by replacing the curvature with the Laplace–Beltrami operator and integration by parts. Further, the Navier-slip with friction boundary condition is used to avoid stress singularities at the Contact line. Our study demonstrates that the Contact Angle models have almost no influence on the flow dynamics of the non-wetting droplets. In computations of the wetting and partially wetting droplets, different Contact Angle models induce different flow dynamics, especially during recoiling. It is shown that a large value for the slip number has to be used in computations of the wetting and partially wetting droplets in order to reduce the effects of the Contact Angle models. Among all models, the equilibrium model is simple and easy to implement. Further, the equilibrium model also incorporates the Contact Angle hysteresis. Thus, the equilibrium Contact Angle model is preferred in sharp interface numerical schemes.

Yixiang Gan - One of the best experts on this subject based on the ideXlab platform.

  • Dynamic Contact Angle hysteresis in liquid bridges
    Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018
    Co-Authors: Zhang Shi, Yi Zhang, Mingchao Liu, Dorian A. H. Hanaor, Yixiang Gan
    Abstract:

    Abstract This work presents an experimental study of dynamic Contact Angle hysteresis using liquid bridges under cyclic compression and stretching between two identical plates. Under various loading rates, Contact Angle hystereses for three different liquids were measured by examination of advancing and receding liquid bridges, and the capillary forces were recorded. It is found that for a given liquid, the hysteretic behaviour of the Contact Angle is more pronounced at higher loading rates. By unifying the behaviour of the three liquids, power-law correlations were proposed to describe the relationship between the dynamic Contact Angle and the capillary number for advancing and receding cases. It is found that the exponents of obtained power-law correlations differ from those derived through earlier methods (e.g., capillary rise), due to the different kinematics of the Contact line. The various hysteretic loops of capillary force in liquid bridges under varied cyclic loading rates were also observed, which can be captured quantitatively by the prediction of our developed model incorporating the dynamic Contact Angle hysteresis. These results illustrate the importance of varying Contact line geometries during dynamic wetting and dewetting processes, and warrant an improved modelling approach for higher level phenomena involving these processes, e.g., multiphase flow in porous media and liquid transfer between surfaces with moving Contact lines.

  • Dynamic Contact Angle hysteresis in liquid bridges
    arXiv: Soft Condensed Matter, 2017
    Co-Authors: Zhang Shi, Yi Zhang, Mingchao Liu, Dorian A. H. Hanaor, Yixiang Gan
    Abstract:

    This work presents a combined experimental and theoretical study of dynamic Contact Angle hysteresis using liquid bridges under cyclic compression and stretching between two identical plates. Under various loading rates, Contact Angle hysteresis for three different liquids was measured by examination of advancing and receding Angles in liquid bridges, and the capillary forces were recorded. It is found that, for a given liquid, the hysteretic phenomenon of the Contact Angle is more pronounced at higher loading rates. By unifying the behaviour of the three liquids, power-law correlations were proposed to describe the relationship between the dynamic Contact Angle and the capillary number for advancing and receding cases. It is found that the exponents of obtained power-law correlations differ from those derived through earlier methods (e.g., capillary rise), due to the different kinematics of the triple-line. The various hysteretic loops of capillary force in liquid bridges under varied cyclic loading rates were also observed, which can be captured quantitatively by the prediction of our developed model incorporating the dynamic Contact Angle hysteresis. These results illustrate the importance of varying triple-line geometries during dynamic wetting and dewetting processes, and warrant an improved modelling approach for higher level phenomena involving these processes, e.g., multiphase flow in porous media and liquid transfer between surfaces with moving Contact lines.