ADHESIVE CONTACT OF VISCOELASTIC SPHERES: A HAND-WAVING INTRODUCTION

We give an overview of the general features of the linear viscoelastic adhesive contact model. The two main features are (1) a delay between the contraction of the contact radius and the onset of the indenter retraction, and (2) the enhancement of the adherence force. We emphasize the role played by stress relaxation within the contact zone in these phenomena and give simple forms of the viscoelastic adhesive contact equations to account for it. Two characteristic timescales are identified, respectively associated with the crack tip and the contact zone. Their asymmetric roles in the growing and receding contact phases is evidenced. Energy release rates for both phases are calculated together with their irreversible components.     

WHEN LESS IS MORE: EXPERIMENTAL EVIDENCE FOR TENACITY ENHANCEMENT BY DIVISION OF CONTACT AREA

Most recent data on hairy systems demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems, some hairy systems seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and their density gets higher as the body mass increases. Previous authors explained this general trend by applying the JKR theory, according to which splitting up the contact into finer subcontacts increases adhesion. Fundamental importance of contact splitting for adhesion on smooth and rough substrata has been previously explained by a very small effective elastic modulus of the fibre array. This article provides the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface made out of polyvinylsiloxane (PVS) has significantly higher adhesion on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.     

WHEN LESS IS MORE: EXPERIMENTAL EVIDENCE FOR TENACITY ENHANCEMENT BY DIVISION OF CONTACT AREA

Most recent data on hairy systems demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems, some hairy systems seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and their density gets higher as the body mass increases. Previous authors explained this general trend by applying the JKR theory, according to which splitting up the contact into finer subcontacts increases adhesion. Fundamental importance of contact splitting for adhesion on smooth and rough substrata has been previously explained by a very small effective elastic modulus of the fibre array. This article provides the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface made out of polyvinylsiloxane (PVS) has significantly higher adhesion on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.     

Creep Effects in Nanometer-scale Contacts to Viscoelastic Materials: A Status Report

Effects of creep on the behavior of nanometer-scale contacts to viscoelastic materials are described from the viewpoint of the contact mechanics theory developed by Ting. The two most important effects are: (1) The time at which maximum contact area and maximum deformation occur can be delayed substantially from the time of maximum applied load. (2) The deformation at separation is related to the loss tangent. These long-range effects due to creep are distinct from the much shorter-range crack tip effects induced by adhesion at the periphery of the contact and associated with the names Barquins and Maugis. Consideration of relevant time scales reveals that creep effects are expected to dominate in SFM-scale contacts for a wide range of compliant viscoelastic materials. Guidelines for selection of optimal experimental parameters for nanometer-scale studies are presented. The need for a comprehensive theory is emphasized.     

Creep Effects in Nanometer-scale Contacts to Viscoelastic Materials: A Status Report

Effects of creep on the behavior of nanometer-scale contacts to viscoelastic materials are described from the viewpoint of the contact mechanics theory developed by Ting. The two most important effects are: (1) The time at which maximum contact area and maximum deformation occur can be delayed substantially from the time of maximum applied load. (2) The deformation at separation is related to the loss tangent. These long-range effects due to creep are distinct from the much shorter-range crack tip effects induced by adhesion at the periphery of the contact and associated with the names Barquins and Maugis. Consideration of relevant time scales reveals that creep effects are expected to dominate in SFM-scale contacts for a wide range of compliant viscoelastic materials. Guidelines for selection of optimal experimental parameters for nanometer-scale studies are presented. The need for a comprehensive theory is emphasized.     

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.     

MEASUREMENT OF VISCOELASTIC EFFECTS WITH A DEEP CHANNEL SURFACE VISCOMETER

The simple surface fluid represents a class of surface stress-deformation behavior. We develop for this form of material behavior a class of solutions, known as the curvilineal surface flows, that can be analyzed without assuming specific forms for the memory functions. The deep channel surface viscometer and the oscillating deep channel surface viscometer belong to this class of flows.     

THERMAL CONTACT CONDUCTANCE OF A COATED PAPER/METAL INTERFACE

A better understanding of the parameters associated with heating of the coated paper during drying process would permit a more accurate design and control of the process to achieve an improved coated paper quality and printability. Following the application of the coating, the coated paper passes through non-contact dryers (i.e. infrared dryers and impinging hot air dryers). When the coating has coalesced, further drying is achieved with steam-heated, contact dryers. One parameter affecting the heat transfer from a metallic dryer drum to the coated paper is the thermal contact conductance at the interface between the coated paper and the drum. In this paper, the thermal contact conductance of a coated paper/metal interface is determined and compared to that of the uncoated paper. Two types of base stock and one type of coating are considered. The thermal contact conductance values are given as a function of the applied interface pressure.     

THERMAL CONTACT CONDUCTANCE OF A COATED PAPER/METAL INTERFACE

A better understanding of the parameters associated with heating of the coated paper during drying process would permit a more accurate design and control of the process to achieve an improved coated paper quality and printability. Following the application of the coating, the coated paper passes through non-contact dryers (i.e. infrared dryers and impinging hot air dryers). When the coating has coalesced, further drying is achieved with steam-heated, contact dryers. One parameter affecting the heat transfer from a metallic dryer drum to the coated paper is the thermal contact conductance at the interface between the coated paper and the drum. In this paper, the thermal contact conductance of a coated paper/metal interface is determined and compared to that of the uncoated paper. Two types of base stock and one type of coating are considered. The thermal contact conductance values are given as a function of the applied interface pressure.     

Adherence and Rolling Kinetics of a Rigid Cylinder in Contact with a Natural Rubber Surface

Equilibrium conditions of a rigid cylinder in contact with the flat and smooth surface of a natural rubber sample are studied using concepts of fracture mechanics, such as the strain energy release rate or the stress intensity factor. It is shown that an equilibrium contact area exists if the applied force per unit axial length is greater than a negative critical value, closely related to the cylinder radius and mechanical and superficial properties of the elastic solid. Due to the intervention of molecular attraction forces, of van der Waals type, a light cylinder rolls under an inclined rubber surface and it is displayed that the rolling speed is the same when the cylinder rolls upon the same inclined surface. It has been verified that if a flat rubber substrate, with an adequate length, is rotated at constant angular velocity, a steel cylinder rolls alternately upon and under the surface, unceasingly without falling down.     

Adhesion of ultrafine particles—A micromechanical approach

In particle processing and product handling of fine , ultrafine and nanosized particles , the well-known flow problems of dry cohesive powders in process apparatuses or storage and transportation containers include bridging, channelling, widely spread residence time distribution associated with time consolidation or caking effects, chemical conversions and deterioration of bioparticles. Avalanching effects and oscillating mass flow rates in conveyors lead to feeding and dosing problems. Finally, insufficient apparatus and system reliability of powder processing plants are also related to these flow problems. Thus, it is very essential to understand the fundamentals of particle adhesion with respect to product quality assessment and process performance in particle technology.The state-of-the-art in constitutive modelling of elastic, elastic-adhesion, elastic-dissipative, plastic-adhesion and plastic-dissipative contact deformation response of a single isotropic contact of two smooth spheres is briefly discussed. Then the new models are shown that describe the elastic-plastic force-displacement and moment-angle behaviour of adhesive and frictional contacts.Using the model “stiff particles with soft contacts”, a sphere-sphere interaction of van der Waals forces without any contact deformation describes the “stiff” attractive term. A plate-plate model is used to calculate the “soft” micro-contact flattening and adhesion. Various contact deformation paths for loading, unloading and contact detachment are discussed. Thus, the varying adhesion forces between particles depend directly on this “frozen” irreversible deformation. Thus, the adhesion force is found to be load dependent. Their essential contribution on the tangential force in an elastic-plastic frictional contact with partially sticking within the contact plane and microslip, the rolling resistance and the torque of mobilized frictional contact rotation is shown.     

SYNTHESIS AND CHARACTERIZATION OF A NEW ACRYLIC ADHESIVE MIXTURE FOR USE IN OCULAR STRABISMUS SURGERY

A new series of acrylic adhesive mixtures was specifically designed for use in strabismus surgery, more precisely to join the rectus muscles to the sclera. These two-part adhesives consisted of a mixture of ethyl cyanoacrylate (CN) and ethyl carboxyacrylate (ECA). ECA acted as a plasticizer imparting flexibility to the CN, and also as a nonreactive diluent, serving to reduce the exotherm in the reaction between CN and eye tissues. In this article, the synthesis of the ethyl carboxyacrylate is described, and the properties of different ethyl cyanoacrylate + ethyl carboxyacrylate mixtures were studied. The curing reaction of the adhesive mixtures was monitored using Fourier transform infrared (FTIR) spectroscopy and scanning differential calorimetry (DSC). The rheological properties of the cured CN-ECA adhesive films were studied using plate-plate rheometer experiments. To quantify the adhesion, single lap-shear tests produced between a rubber and the adhesive mixtures were performed and, to evaluate the adhesion to eye tissues, tensile strength measurements of superior rectus muscle/adhesive mixture/sclera joints were carried out. The 70CN-30ECA (v/v) adhesive mixture provided the most adequate balance between adhesion and mechanical properties in the joining of the superior rectus muscle to the sclera. The glass transition temperature of the CN-ECA adhesive mixtures linearly decreased with increase in the ECA content, and a lower degree of conversion during polymerization was obtained by increasing the ECA content. As a consequence, the CN-ECA mixtures were less stiff than CN, giving better performance in the joining of the rectus muscles to the sclera. Finally, the adhesion of CN was sufficiently decreased in CN-ECA mixtures, and the locus of failure was directed to the adhesive film in the joint between the rectus muscles and the sclera.     

UNSTEADY HYDROMAGNETIC NATURAL CONVECTION OF A SHORT-MEMORY VISCOELASTIC FLUID IN A NON-DARCIAN REGIME: NETWORK SIMULATION

A mathematical model for the unsteady magnetohydrodynamic (MHD) laminar natural convection flow of a viscoelastic fluid from an infinite vertical porous plate to an isotropic, homogeneous, non-Darcian porous regime, with time-dependent suction, in the presence of a uniform transverse magnetic field, is studied. The generalized Beard-Walters rheological model is employed, which introduces a mixed third-order derivative into the momentum conservation equation. The transformed conservation equations are solved using the robust, well-tested computational procedure known as network simulation method (NSM). The NSM computations have shown that with an increase in viscoelasticity parameter (S) the flow accelerates considerably with time. Increasing magnetic field (M), however, retards the flow strongly with time. An increase in the Darcy number (Da) serves to augment the velocity (w) profiles, i.e., accelerate the flow in both the conducting (M ≠ 0) and nonconducting (M = 0) cases. Velocities also increase in value over time (τ). A velocity overshoot is identified close to the plate. A rise in the Forchheimer number (Fs), corresponding to an accentuation in the quadratic porous drag effect, induces a strong deceleration in the flow, in particular near the plate surface, for both conducting and nonconducting cases. Increasing buoyancy effects, as simulated via a rise in the thermal Grashof number (Gr), leads to a substantial retardation in the flow; this effect is enhanced with Lorentzian magnetic drag force. An increase in the suction parameter (A) causes a stronger adherence of the hydrodynamic boundary layer to the plate and leads to a reduction in velocities along the entire plate regime. A similar decrease in temperature (θ) is caused with increasing suction parameter (A). The results are of relevance in, for example, magneto-rheological materials processing operations and advanced hybrid magnetohydrodynamic energy systems exploiting non-Newtonian fluids.     

CONTACT ANGLE DETERMINATION OF NANOPARTICLES: REAL EXPERIMENTS AND COMPUTER SIMULATIONS

Monolayers of Stöber-silica nanoparticles (ca. 40?nm diameters) at the water–air interface have been studied in a Wilhelmy film balance. Scanning angle reflectometry and molecular dynamics computer simulation have been used to assess the contact angles of the particles. Our results indicate that the traditional film balance method of contact angle determination overestimates the real contact angles even in the lower range of particle hydrophobicities.     

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.     

THE DEVELOPMENT OF A MANDREL PEEL TEST FOR THE MEASUREMENT OF ADHESIVE FRACTURE TOUGHNESS OF EPOXY-METAL LAMINATES

A mandrel peel test is established for measuring the adhesive fracture toughness of a metal/rubber-toughened epoxy laminate system. By adopting an energy balance analysis it is possible to determine directly both adhesive fracture toughness and plastic work in bending the peel arm around the mandrel. The suitability of the procedure is examined for various types of metal peel arms, which are classified in terms of their ability to deform plastically during the test. The plastic work is also predicted theoretically, and comparisons are made between the measured and calculated values. The fracture energies determined from the mandrel tests are compared with those obtained from 90° fixed-arm peel tests. For the calculations of plastic work in bending in the fixed arm test, various options are used when modelling the tensile stress-strain behaviour of the peel arm material. In addition, the adhesive layer thickness is considered in terms of its influence on the calculation of adhesive fracture toughness.     

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.     

ON THERMODYNAMICS OF THIN FILMS: THE MECHANICAL EQUILIBRIUM CONDITION AND CONTACT ANGLES

A thermodynamic model for plane parallel thin liquid films applicable to solid-liquid-vapor systems was presented using the detailed method. The film was modeled as a bulk phase bound by two dividing surfaces. The thermodynamic thickness of the film was established as well as excess properties such as film tension. The analysis using this model yielded disjoining pressure definition identical to the literature reports. The effect of definition for contact angle on the resulting mechanical equilibrium condition was also demonstrated. It was concluded that from a theoretical perspective it is important to clearly define contact angles as the angle a sessile drop makes with either the solid phase or the thin film. However, on a practical level for most cases, the difference between using either of the two mechanical equilibrium conditions to determine film tension or contact angle will be minimal (ascertained by an order of magnitude analysis). The attempt was also made to bring about clarity concerning some of the questions in the literature regarding the thermodynamic model for thin films presented by Li and Neumann.