Contact between a smooth microsphere and an anisotropic rough surface

This article discusses the effects of asperities on elastic and adhesive contact between a smooth sphere and a rough surface. Two numerical methods are introduced: an asperity-superposition method and a direct-simulation method. In the first method, geometric parameters such as asperity heights, orientations, and radii of curvature are identified by a least-squares regression of neighboring surface heights measured using an atomic force microscope. The rough surface is reconstructed by the superposition of these asperities. The modeling of adhesive and elastic contacts begins with the modeling of a single parabolic-shaped asperity contact. A generalized JKR model for an arbitrary parabola is developed to suit this purpose. The contact between the rough surface (represented by the supposition of parabolic-shaped asperities) and the sphere consequently is modeled bythe mapping and integration of individual asperity contacts. In the second method, pure-elastic contact is modeled by half-space elastic theory. A contact-search algorithm is used to find solutions on the displacement and the contact-pressure distribution that satisfy both the load-displacement equation and the contactboundary conditions. Results from both methods are compared to reveal the effects of asperities on adhesion and elastic-contact pressure.     

EXPERIMENTS ON CONTACT OF A LOOP WITH A SUBSTRATE TO MEASURE WORK OF ADHESION

A technique for characterizing surface energies of solid materials is investigated experimentally and numerically. A narrow strip is bent into a loop, pushed into contact with a flat substrate, and then pulled off the substrate. Provided the loop is sufficiently flexible, the size of the contact zone during this process was expected to depend on the interfacial interactions. Larger adhesion forces should tend to increase the contact size, in a manner analogous to the JKR technique. The experiments involve a poly (dimethyl siloxane) (PDMS) loop and glass substrates with various coatings. Anticlastic bending of the loop affects the contact zone. Hysteresis is observed between the loading and unloading data. A three-dimensional finite element analysis is conducted in which adhesion forces are not included, and results from a two-dimensional elastica model of the loop are utilized for comparison purposes. The contact zone appears to be insensitive to the adhesive interactions between the loop and the substrate for the systems studied.     

WETTABILITY AND SURFACE ENERGETICS OF ROUGH FLUOROPOLYMER SURFACES

Hydrophobic solid surfaces with controlled roughness were prepared by coating glass slides with an amorphous fluoropolymer (Teflon® AF1600, DuPont) containing varying amounts of silica spheres (diameter 48?μm). Quasi-static advancing, θ_A, and receding, θ_R, contact angles were measured with the Wilhelmy technique. The contact angle hysteresis was significant but could be eliminated by subjecting the system to acoustic vibrations. Surface roughness affects all contact angles, but only the vibrated ones, θ_V, agree with the Wenzel equation. The contact angle obtained by averaging the cosines of θ_A and θ_R is a good approximation for θ_V, provided that roughness is not too large or the angles too small. Zisman's approach was employed to obtain the critical surface tension of wetting (CST) of the solid surfaces. The CST increases with roughness in accordance with Wenzel equation. Advancing, receding, and vibrated angles yield different results. The θ_A is known to be characteristic of the main hydrophobic component (the fluoropolymer). The θ_V is a better representation of the average wettability of the surface (including the presence of defects).     

WETTABILITY AND SURFACE ENERGETICS OF ROUGH FLUOROPOLYMER SURFACES

Hydrophobic solid surfaces with controlled roughness were prepared by coating glass slides with an amorphous fluoropolymer (Teflon® AF1600, DuPont) containing varying amounts of silica spheres (diameter 48 μm). Quasi-static advancing, θ_A, and receding, θ_R, contact angles were measured with the Wilhelmy technique. The contact angle hysteresis was significant but could be eliminated by subjecting the system to acoustic vibrations. Surface roughness affects all contact angles, but only the vibrated ones, θ_V, agree with the Wenzel equation. The contact angle obtained by averaging the cosines of θ_A and θ_R is a good approximation for θ_V, provided that roughness is not too large or the angles too small. Zisman's approach was employed to obtain the critical surface tension of wetting (CST) of the solid surfaces. The CST increases with roughness in accordance with Wenzel equation. Advancing, receding, and vibrated angles yield different results. The θ_A is known to be characteristic of the main hydrophobic component (the fluoropolymer). The θ_V is a better representation of the average wettability of the surface (including the presence of defects).     

HEAT TRANSFER BETWEEN ROUGH SURFACES IN CONTACT OVER A HIGHLY ELLIPTICAL CONTOUR AREA: COMPARISON OF EXPERIMENTAL AND NUMERICAL RESULTS

This paper describes experimental and numerical studies of heat transfer between surfaces in contact over a highly elliptical area in a gas or in vacuum. Measurements of the steady-state thermal resistance of a high aspect ratio contact formed between cylindrical steel surfaces with widely different radii of curvature (76 cm vs 0.95 cm) are reported for two levels of surface roughness (0.05 μm and 1.0 μm rms) and compared to 3-D numerical results obtained with the multigrid method. Theoretical results obtained by evaluating the contact conductance acting over each surface element within the contour area with a method developed previously for rough but nominally flat surfaces are shown to be in excellent agreement with the rough surface experimental data.     

FRICTION AND ADHESION MEASUREMENTS BETWEEN A FLUOROCARBON SURFACE AND A HYDROCARBON SURFACE IN AIR

The friction and adhesion between a fluorocarbon monolayer-coated surface against a hydrocarbon monolayer-coated surface has been directly measured. The friction was found to be lower than the friction between a hydrocarbon monolayer against a hydrocarbon monolayer and a fluorocarbon monolayer against a fluorocarbon monolayer. No stick-slip sliding was observed for speeds from 0.8?µm/s to 2.6?µm/s. The fluorocarbon–hydrocarbon interface was adhesive, with the energy of interaction measured to be 14.9?mJ/m²?±?1.0?mJ/m². As predicted from theory, the magnitude of the adhesion of a fluorocarbon monolayer against a hydrocarbon monolayer is between that measured for a fluorocarbon monolayer against a fluorocarbon monolayer and a hydrocarbon monolayer against a hydrocarbon monolayer. One may note that the interfacial energy, γ, follows the general trend γ_FC/FC?<?γ_HC/FC?<?γ_HC/HC, whereas the shear stress, τ, varies according to τ_FC/HC?<?τ_HC/HC?<?τ_FC/FC.     

Physico-Chemical Criteria for Maximum Adhesion. Part II: A New Comprehensive Thermodynamic Analysis

In this paper, two parameters defined as the relative work of adhesion [W_A/γ_L] and the relative interfacial energy [γ_SL/γ_L] have been examined for their assumed usefulness in correlating the thermodynamic properties of the components of the system substrate/ adhesive with its practical performance (strength). It is shown that the minimum value of [γ_SL/γ_L] relevant to conditions for the maximum adhesion becomes zero only for those systems (relatively rare) for which interaction factor Φ₀ is equal to 1.0.

Several transition points were identified for boundary conditions acquired at θ = 0° and θ = 90° which can be used to predict the properties and performance of an adhesive joint. These transition points are: a_MIN—energy modulus of the system (E. M. S.), relevant to the minimum interfacial energy; a_S—E. M. S. where self-spreading of adhesive occurs; a_CRIT—E. M. S. relevant to conditions under which the thermodynamic work of adhesion becomes negative and the system exhibits a tendency for self-delaminating or has “zero-strength”; a_CF—E. M. S. beyond which the geometry of the interface at any interfacial void or boundary of the joint may be regarded as a crack tip.

It is shown that only in those systems for which Φ₀ = 1.0 can a minimum contact angle of 0° indicate a condition for the maximum strength. If Φ₀ is known, the optimum contact angle can be estimated and hence the optimum surface energy of the substrate (adjusted by surface treatment, etc.) for the maximum adhesion.     

DETERMINATION OF SOLID SURFACE TENSION FROM CONTACT ANGLES: THE ROLE OF SHAPE AND SIZE OF LIQUID MOLECULES

Accurate surface tension of Teflon® AF 1600 was determined using contact angles of liquids with bulky molecules. For one group of liquids, the contact angle data fall quite perfectly on a smooth curve corresponding to γ_sv = 13.61 mJ/m², with a mean deviation of only ±0.24 degrees from this curve. Results suggest that these liquids do not interact with the solid in a specific fashion. However, contact angles of a second group of liquids with fairly bulky molecules containing oxygen atoms, nitrogen atoms, or both deviate somewhat from this curve, up to approximately 3 degrees. Specific interactions between solid and liquid molecules and reorientation of liquid molecules in the close vicinity of the solid surface are the most likely causes of the deviations. It is speculated that such processes induce a change in the solid–liquid interfacial tension, causing the contact angle deviations mentioned above. Criteria are established for determination of accurate solid surface tensions.     

DETERMINATION OF SOLID SURFACE TENSION FROM CONTACT ANGLES: THE ROLE OF SHAPE AND SIZE OF LIQUID MOLECULES

Accurate surface tension of Teflon® AF 1600 was determined using contact angles of liquids with bulky molecules. For one group of liquids, the contact angle data fall quite perfectly on a smooth curve corresponding to γ_sv = 13.61 mJ/m², with a mean deviation of only ±0.24 degrees from this curve. Results suggest that these liquids do not interact with the solid in a specific fashion. However, contact angles of a second group of liquids with fairly bulky molecules containing oxygen atoms, nitrogen atoms, or both deviate somewhat from this curve, up to approximately 3 degrees. Specific interactions between solid and liquid molecules and reorientation of liquid molecules in the close vicinity of the solid surface are the most likely causes of the deviations. It is speculated that such processes induce a change in the solid-liquid interfacial tension, causing the contact angle deviations mentioned above. Criteria are established for determination of accurate solid surface tensions.     

MEASURING INTERFACIAL ADHESION BETWEEN A SOFT VISCOELASTIC LAYER AND A RIGID SURFACE USING A PROBE METHOD

A reliable measure of the adhesion between a very deformable material and a solid surface is rather difficult, since the interface boundary conditions and the bulk deformation of the layer are closely and very nonlinearly coupled. In this article, a new methodology to assess the adhesion of a soft viscoelastic layer on a solid surface is proposed, where we have used a specific experimental geometry minimizing the bulk deformation of the layer. A flat-ended probe is first put in contact with a thin layer of soft material and removed at a constant velocity. The probe is then stopped at a preset level of tensile force and the time for complete debonding of the layer from the probe is measured. For our model system, comprised of a soft acrylic removable adhesive and a silicone-coated surface, the higher the applied force the faster the interfacial fracture occurs, leading to an experimental curve of the adhesion energy as a function of average crack velocity. We find that the methodology is relatively simple to implement and should be widely applicable for weakly adhering soft layers of arbitrary viscoelastic properties. The assumptions involved in such an analysis and their inherent limitations are also illustrated experimentally and critically discussed.     

MEASURING INTERFACIAL ADHESION BETWEEN A SOFT VISCOELASTIC LAYER AND A RIGID SURFACE USING A PROBE METHOD

A reliable measure of the adhesion between a very deformable material and a solid surface is rather difficult, since the interface boundary conditions and the bulk deformation of the layer are closely and very nonlinearly coupled. In this article, a new methodology to assess the adhesion of a soft viscoelastic layer on a solid surface is proposed, where we have used a specific experimental geometry minimizing the bulk deformation of the layer. A flat-ended probe is first put in contact with a thin layer of soft material and removed at a constant velocity. The probe is then stopped at a preset level of tensile force and the time for complete debonding of the layer from the probe is measured. For our model system, comprised of a soft acrylic removable adhesive and a silicone-coated surface, the higher the applied force the faster the interfacial fracture occurs, leading to an experimental curve of the adhesion energy as a function of average crack velocity. We find that the methodology is relatively simple to implement and should be widely applicable for weakly adhering soft layers of arbitrary viscoelastic properties. The assumptions involved in such an analysis and their inherent limitations are also illustrated experimentally and critically discussed.