Contact area between a viscoelastic solid and a hard, randomly rough, substrate

Core-shell dry adhesives for rough surfaces via electrically responsive self-growing strategy

Bioinspired dry adhesives have an extraordinary impact in the field of robotic manipulation and locomotion. However, there is a considerable difference between artificial structures and biological ones regarding surface adaptability, especially for rough surfaces. This can be attributed to their distinct structural configuration and forming mechanism. Here, we propose a core-shell adhesive structure that is obtained through a growth strategy, i.e., an electrically responsive self-growing core-shell structure. This growth strategy results in a specific mushroom-shaped structure with a rigid core and a soft shell, which exhibits excellent adhesion on typical target surfaces with roughness ranging from the nanoscale to the microscale up to dozens of micrometers. The proposed adhesion strategy extends dry adhesives from smooth surfaces to rough ones, especially for rough surfaces with roughness up to dozens or hundreds of micrometers, opening an avenue for the development of dry adhesive-based devices and systems.

Film-Terminated Fibrillar Microstructures with Improved Adhesion on Skin-like Surfaces

Adhesives for interaction with human skin and tissues are needed for multiple applications. Micropatterned dry adhesives are potential candidates, allowing for a conformal contact and glue-free adhesion based on van der Waals interactions. In this study, we investigate the superior adhesion of film-terminated fibrillar microstructures (fibril diameter, 60 μm; aspect ratio, 3) in contact with surfaces of skin-like roughness (Rz 50 μm). Adhesion decays only moderately with increasing roughness, in contrast to unstructured samples. Sinusoidal model surfaces adhere when their wavelengths exceed about four fibril diameters. The film-terminated microstructure exhibits a saturation of the compressive force during application, implying a pressure safety regime protecting delicate counter surfaces. Applications of this novel adhesive concept are foreseen in the fields of wearable electronics and wound dressing.

A Numerical Model for Investigating the Effect of Viscoelasticity on the Partial Slip Solution

A single paragraph of about 200 words maximum. For research articles, abstracts should give a pertinent overview of the work. We strongly encourage authors to use the following style of structured abstracts, but without headings: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study; (2) Methods: briefly describe the main methods or treatments applied; (3) Results: summarize the article's main findings; (4) Conclusions: indicate the main conclusions or interpretations. The abstract should be an objective representation of the article and it must not contain results that are not presented and substantiated in the main text and should not exaggerate the main conclusions.

The Theoretical Basis of Face Contact Pressure Design of the Zero-Leakage Mechanical Seal

The value of face contact pressure has an important influence on the wear life and leakage rate of the mechanical seal. For a long time, people have chosen face contact pressure based on experience and lack of theoretical support, which greatly reduces the effectiveness of the mechanical seal. Based on the percolation theory, the critical porosity of zero-leakage at the wetting and non-wetting sealing interface working in liquid medium is first discussed. The influence of end-face frictional heat on end-face friction and wear is then investigated. The design criteria for the face contact pressure of mechanical seals with zero-leakage and long-life operation are established. Afterwards, the face contact pressure range of the mechanical seal working in conventional different liquid medium is calculated, and the influence of different working conditions on the face contact pressure range change is analyzed. Existing studies have shown that mechanical seals can achieve zero-leakage and long-life operation. Under the rotating and stationary rings’ physical parameters and given working conditions, the face contact pressure range of the sealing medium water and propane propylene is 0.477~1.132 MPa. The diesel sealing medium has a larger face contact pressure range than that of water and propane propylene, which can reach 0.477~2.183 MPa. The working condition speed, medium temperature, and medium pressure have an influence on the face contact pressure range, while the influence of the working condition speed is the most significant.

Quantification and Imaging of Nanoscale Contact with Förster Resonance Energy Transfer

Adhesion is caused by molecular interactions that only take place if the surfaces are in nanoscale contact (NSC); i.e., the distance between the surfaces is in the range of 0.1-0.4 nm. However, there are several difficulties measuring the NSC between surfaces, mainly because regions that appear to be in full contact at low magnification may show no NSC when observed at higher magnifications. Thus, the measurement area of NSC is very small with imaging techniques, and an experimental technique to evaluate NSC for large contact areas has not been available thus far. Here, we are proposing Förster resonance energy transfer (FRET) spectroscopy/microscopy for this purpose. We demonstrate that NSC in a distance range of 1-10 nm can be evaluated. Our experiments reveal that, for thin films pressed under different loads, NSC increases with the applied pressure, resulting in a higher FRET signal and a corresponding increase in adhesion force/energy when separating the films. Furthermore, we show that local variations in molecular contact can be visualized with FRET microscopy. Thus, we are introducing a spectroscopic technique for quantification (FRET spectroscopy) and imaging (FRET microscopy) of NSC between surfaces, demonstrated here for the application of surface adhesion. This could be of interest for all fields where adhesion or nanoscale surface contact are playing a role, for example, soft matter, biological materials, and polymers, but also engineering applications, like tribology, adhesives, and sealants.

How to Measure the Area of Real Contact of Skin on Glass

The contact between the fingertip and an object is formed by a collection of micro-scale junctions, which collectively constitute the real contact area. This real area of contact is only a fraction of the apparent area of contact and is directly linked to the frictional strength of the contact (i.e., the lateral force at which the finger starts sliding). As a consequence, a measure of this area of real contact can help probe into the mechanism behind the friction of skin on glass. In this article, we present two methods to measure the variations of contact area; one that improves upon a tried-and-true fingertip imaging technique to provide ground truth, and the other that relies on the absorption and reflection of acoustic energy. To achieve precise measurements, the ultrasonic method exploits a recently developed model of the interaction that incorporates the non-linearity of squeeze film levitation. The two methods are in good agreement ($\rho =0.94$) over a large range of normal forces and vibration amplitudes. Since the real area of contact fundamentally underlies fingertip friction, the methods described in the article have importance for studying human grasping, understanding friction perception, and controlling surface-haptic devices.

Modeling Adhesive Hysteresis

When an elastomer approaches or retracts from an adhesive indenter, the elastomer’s surface can suddenly become unstable and reshape itself quasi-discontinuously, e.g., when small-scale asperities jump into or snap out of contact. Such dynamics lead to a hysteresis between approach and retraction. In this study, we quantify numerically and analytically the ensuing unavoidable energy loss for rigid indenters with flat, Hertzian and randomly rough profiles. The range of adhesion turns out to be central, in particular during the rarely modeled approach to contact. For example, negligible traction on approach but quite noticeable adhesion for nominal plane contacts hinges on the use of short-range adhesion. Central attention is paid to the design of cohesive-zone models for the efficient simulation of dynamical processes. Our study includes a Griffith’s type analysis for the energy lost during fracture and regeneration of a flat interface. It reveals that the leading-order corrections of the energy loss are due to the finite-range adhesion scale at best, with the third root of the linear mesh size, while leading-order errors in the pull-off force disappear linearly.

Maximizing Contact of Supersoft Bottlebrush Networks with Rough Surfaces To Promote Particulate Removal

Efficient removal of particulates from a rough surface with a soft material through a "press and peel" method (i.e., an adhesion and release approach) depends on good conformal contact at the interface; a material should be sufficiently soft to maximize contact with a particle while also conforming to rough surface features to clean the entire substrate surface. Here, we investigate the use of bottlebrush networks-extremely soft elastomers composed of macromolecules with polymeric side chains-as materials for cleaning model substrates of varying roughness. Formed through free-radical polymerization of mono- and dimethacrylate functionalized polysiloxanes, these solvent-free supersoft elastomers exhibit moduli comparable to those of solvated gels, allowing for a lower moduli regime of elastomers to be used in contact experiments than previously possible. By varying the macromonomer to cross-linker ratio, we study the effect of modulus on conformal contact and cleaning for materials that are as soft as gels while minimizing/negating physical and/or chemical concerns that using a traditional material may involve (e.g., changes in component concentrations, solvent evaporation, and syneresis). We study cleaning efficacy by quantifying the conformal contact between soft materials and rough substrates via a contact adhesion-based measurement. These results give insight into the correlation between shear modulus and conformal contact with surfaces of varying feature height. Not only does a decrease in shear modulus leads to improved conformal contact with rough surfaces, but also it facilitates adhesion to particulates situated on the rough surface, thus aiding removal. We highlight this property control with a case study illustrating the removal of an artificial soil mixture from a rough, acrylic surface via peeling rather than rubbing, which can cause damage to delicate surfaces.

Friction in rough contacts of linear viscoelastic surfaces with anisotropic statistical properties

Viscoelastic rheology can have great effects on the contact mechanics of randomly rough surfaces with anisotropic statistical properties. In this paper, we investigate such effects in the framework of Persson's theory. We calculate the forces that arise in viscoelastic contacts because of the viscous dissipation occurring in the bulk material. Interestingly, the non-symmetric distribution of the normal contact pressure entails the occurrence of a force in the direction perpendicular to the sliding one. Such force does not carry out any work, and hence does not dissipate energy, but it is important for the global equilibrium of the system. Results are also compared with numerical exact calculations finding a quite good agreement. However, with the present method, we cannot capture the stretching of the contact clusters due to the viscoelasticity and, hence, the resulting change in the anisotropy of the deformed surface.

Linear and Nonlinear Viscoelastic Modulus of Rubber

We study the linear and nonlinear viscoelastic properties of two tire tread compounds. We discuss the difference in nonlinear response between the oscillatory tensile and shear modes. We also analyze strain relaxation (creep) data for the same systems. We discuss what type of measurements are most suitable for obtaining the viscoelastic modulus used in rubber friction calculations.

Wafer-Scale van der Waals Heterostructures with Ultraclean Interfaces via the Aid of Viscoelastic Polymer

Two-dimensional (2D) van der Waals (vdW) heterostructures exhibit novel physical and chemical properties, allowing the development of unprecedented electronic, optical, and electrochemical devices. However, the construction of wafer-scale vdW heterostructures for practical applications is still limited due to the lack of well-established growth and transfer techniques. Herein, we report a method for the fabrication of wafer-scale 2D vdW heterostructures with an ultraclean interface between layers via the aid of a freestanding viscoelastic polymer support layer (VEPSL). The low glass transition temperature ( T_g) and viscoelastic nature of the VEPSL ensure absolute conformal contact between 2D layers, enabling the easy pick-up of layers and attaching to other 2D layers. This eventually leads to the construction of random sequence 2D vdW heterostructures such as molybdenum disulfide/tungsten disulfide/molybdenum diselenide/tungsten diselenide/hexagonal boron nitride. Furthermore, the VEPSL allows the conformal transfer of 2D vdW heterostructures onto arbitrary substrates, irrespective of surface roughness. To demonstrate the significance of the ultraclean interface, the fabricated molybdenum disulfide/graphene heterostructure employed as an electrocatalyst yielded excellent results of 73.1 mV·dec^-1 for the Tafel slope and 0.12 kΩ of charge transfer resistance, which are almost twice as low as that of the impurity-trapped heterostructure.

An Experimental Study of the Leakage Mechanism in Static Seals

This paper investigates the fluidic leak rate through sealing contact surfaces by comparison between model calculation and experiment measurement. The focus is on an experimental device designed to measure the leak rate of the static seals with a simpler structure, smaller errors, stronger stability, and more functions. Using the device, experiments were carried out to four test pieces with different surface characteristics, whose leak rates were measured separately. Compared with the calculation results obtained from the fractal surface leak rate prediction model, the correctness and the application range of the model were verified, and the effects of different surface topographies and material properties of the four test pieces on the leak rate were analyzed as well. The experimental device was also used to perform single-factor comparison experiments, which were then combined with the theoretical prediction model to analyze the effects of the sealing surface contact load, fluid pressure, and surface apparent size on the leak rate, so that theoretical support and experimental evidence for selecting the parameters of sealing device was provided.

Adhesion between rubber and glass in dry and lubricated condition

We study the adhesion between differently processed glass and filled bromobutyl rubber in dry conditions, in water, and in silicone oil. The boundary line between contact and non-contact in adhesion experiments can be considered as a mode I crack, and we show that viscoelastic energy dissipation, close to the opening (or closing) crack tip and surface roughness, strongly affects the work of adhesion. We observe strong adhesion hysteresis and, in contrast to the Johnson-Kendall-Roberts theory prediction for elastic solids, this results in a pull-off force (and work of adhesion) which depends on the loading force and contact time. In particular, for the system immersed in water and silicone oil, we register very weak adhesive bonding. For glass ball with baked-on silicone oil, the pull-off force is nearly independent of the contact time, but this is not observed for the unprocessed glass surface.

Importance of Substrate Functionality on the Adhesion and Debonding of a Pressure-Sensitive Adhesive under Water

We investigate the effect of an aqueous environment on the adhesion of a model acrylic pressure-sensitive adhesive (PSA) composed of 2-ethylhexyl acrylate-co-acrylic acid. We use probe-tack adhesion measurements accompanied by in situ imaging of the contact region during bonding and debonding. Within the probe-tack tests, we use both hydrophilic (piranha and plasma treatment) and hydrophobic (C₁₈-silanization) surface treatments to investigate the contribution of the probe's surface energy on the underwater adhesion. In examining contact formation in air and underwater, we find that the presence of water when contact is made leads to different modes of PSA relaxation and contact formation. For all probes investigated, the adhesive strength between the PSA and the probe decreases when measured underwater. Additionally, we observe that the presence of water during debonding has a more pronounced effect on the adhesive strength of the PSA when probed by a hydrophilic surface as opposed to a hydrophobic surface. Using fingering wavelength analysis, we estimate the surface energy of the PSA in situ and find that when submerged in water, the PSA has a significantly higher surface energy compared to in air. Therefore, combining the observation of different modes of contact formation, the increase in surface energy, and the importance of the surface energy of the probe, we suggest that the decrease in adhesive strength in water can be explained by the hydration of the PSA and by trapped water defects between the PSA and the probe.

Evolution of the Contact Area with Normal Load for Rough Surfaces: from Atomic to Macroscopic Scales

The evolution of the contact area with normal load for rough surfaces has great fundamental and practical importance, ranging from earthquake dynamics to machine wear. This work bridges the gap between the atomic scale and the macroscopic scale for normal contact behavior. The real contact area, which is formed by a large ensemble of discrete contacts (clusters), is proven to be much smaller than the apparent surface area. The distribution of the discrete contact clusters and the interaction between them are key to revealing the mechanism of the contacting solids. To this end, Green's function molecular dynamics (GFMD) is used to study both how the contact cluster evolves from the atomic scale to the macroscopic scale and the interaction between clusters. It is found that the interaction between clusters has a strong effect on their formation. The formation and distribution of the contact clusters is far more complicated than that predicted by the asperity model. Ignorance of the interaction between them leads to overestimating the contacting force. In real contact, contacting clusters are smaller and more discrete due to the interaction between the asperities. Understanding the exact nature of the contact area with the normal load is essential to the following research on friction.

Composite Pillars with a Tunable Interface for Adhesion to Rough Substrates

The benefits of synthetic fibrillar dry adhesives for temporary and reversible attachment to hard objects with smooth surfaces have been successfully demonstrated in previous studies. However, surface roughness induces a dramatic reduction in pull-off stresses and necessarily requires revised design concepts. Toward this aim, we introduce cylindrical two-phase single pillars, which are composed of a mechanically stiff stalk and a soft tip layer. Adhesion to smooth and rough substrates is shown to exceed that of conventional pillar structures. The adhesion characteristics can be tuned by varying the thickness of the soft tip layer, the ratio of the Young's moduli and the curvature of the interface between the two phases. For rough substrates, adhesion values similar to those obtained on smooth substrates were achieved. Our concept of composite pillars overcomes current practical limitations caused by surface roughness and opens up fields of application where roughness is omnipresent.

Onset of frictional sliding of rubber-glass contact under dry and lubricated conditions

Rubber friction is critical in many applications ranging from automotive tyres to cylinder seals. The process where a static rubber sample transitions to frictional sliding is particularly poorly understood. The experimental and simulation results in this paper show a completely different detachment process from the static situation to sliding motion under dry and lubricated conditions. The results underline the contribution of the rubber bulk properties to the static friction force. In fact, simple Amontons' law is sufficient as a local friction law to produce the correct detachment pattern when the rubber material and loading conditions are modelled properly. Simulations show that micro-sliding due to vertical loading can release initial shear stresses and lead to a high static/dynamic friction coefficient ratio, as observed in the measurements.

Fracture and adhesion of soft materials: a review

Soft materials are materials with a low shear modulus relative to their bulk modulus and where elastic restoring forces are mainly of entropic origin. A sparse population of strong bonds connects molecules together and prevents macroscopic flow. In this review we discuss the current state of the art on how these soft materials break and detach from solid surfaces. We focus on how stresses and strains are localized near the fracture plane and how elastic energy can flow from the bulk of the material to the crack tip. Adhesion of pressure-sensitive-adhesives, fracture of gels and rubbers are specifically addressed and the key concepts are pointed out. We define the important length scales in the problem and in particular the elasto-adhesive length Γ/E where Γ is the fracture energy and E is the elastic modulus, and how the ratio between sample size and Γ/E controls the fracture mechanisms. Theoretical concepts bridging solid mechanics and polymer physics are rationalized and illustrated by micromechanical experiments and mechanisms of fracture are described in detail. Open questions and emerging concepts are discussed at the end of the review.

Debonding mechanisms of soft materials at short contact times

A carefully controlled, custom-built adhesion testing device was developed which allows a precise, short dwell time on the order of milliseconds to be applied during a contact adhesion experiment. The dwell time dependence of the adhesive strength of crosslinked poly(dimethylsiloxane) (PDMS) in contact with glass and uncrosslinked styrene butadiene rubber (SBR) in contact with glass and with itself was tested with a spherical probe in a confined Johnson-Kendall-Roberts (JKR) geometry. Analysis of the contact images revealed several unique separation mechanisms which are dependent on dwell time and interfacial properties. PDMS-glass interfaces show essentially no dependence of adhesion on the dwell time while the adhesive strength and separation mechanisms of SBR interfaces are shown to vary drastically for dwell times ranging from 40 to 10,000 ms. This influence of dwell time is particularly pronounced for polymer-polymer (SBR-SBR) interfaces. Observations of cavitation due to trapped air pockets in the center of the contact at very short contact times illustrate a transition between a defect-controlled debonding and an interface-controlled debonding which has not been previously reported.

Role of hydrophobicity on interfacial fluid flow: theory and some applications

The fluid flow through a seal interface depends on the percolating non-contact channels morphology, size and length, and on the interfacial surface energies. In particular, hydrophobic interfaces may expel fluids and decrease the fluid flow of seals, while increasing the sliding friction. We present results of interfacial fluid flow experiments on a hydrostatic column device which demonstrate how interfacial hydrophobicity can block fluid flow at interfaces and reduce the leak rate of seals. The presented results may help to understand the role of interfacial hydrophobicity in many practical applications, some of which we discuss briefly in this paper, e.g., rubber wiper blades on hydrophobic (usually wax-coated) glass, the locomotion of insects on surfaces in water, and syringes.

Adhesion: role of bulk viscoelasticity and surface roughness

We study the adhesion between smooth polydimethylsiloxane (PDMS) rubber balls and smooth and rough poly(methyl methacrylate) (PMMA) surfaces, and between smooth silicon nitride balls and smooth PDMS surfaces. From the measured viscoelastic modulus of the PDMS rubber we calculate the viscoelastic contribution to the crack-opening propagation energy γeff(v,T) for a wide range of crack tip velocities v and for several temperatures T. The Johnson-Kendall-Roberts (JKR) contact mechanics theory is used to analyze the ball pull-off force data, and γeff(v,T) is obtained for smooth and rough surfaces. We conclude that γeff(v,T) has contributions of similar magnitude from both the bulk viscoelastic energy dissipation close to the crack tip, and from the bond-breaking process at the crack tip. The pull-off force on the rough surfaces is strongly reduced compared to that of the flat surface, which we attribute mainly to the decrease in the area of contact on the rough surfaces.

Rubber friction for tire tread compound on road surfaces

We have measured the surface topography and calculated the surface roughness power spectrum for an asphalt road surface. For the same surface we have measured the friction for a tire tread compound for velocities 10(-6) m s(-1) < v < 10(-3) m s(-1) at three different temperatures (at -8 °C, 20 °C and 48 °C). The friction data was shifted using the bulk viscoelasticity shift factor a(T) to form a master curve. We have measured the effective rubber viscoelastic modulus at large strain and calculated the rubber friction coefficient (and contact area) during stationary sliding and compared it to the measured friction coefficient. We find that for the low velocities and for the relatively smooth road surface we consider, the contribution to friction from the area of real contact is very important, and we interpret this contribution as being due to shearing of a very thin confined rubber smear film.

Spectrally enhanced imaging of occlusal surfaces and artificial shallow enamel erosions with a scanning fiber endoscope

An ultrathin scanning fiber endoscope, originally developed for cancer diagnosis, was used to image dental occlusal surfaces as well as shallow artificially induced enamel erosions from human extracted teeth (n=40). Enhanced image resolution of occlusal surfaces was obtained using a short-wavelength 405-nm illumination laser. In addition, artificial erosions of varying depths were also imaged with 405-, 404-, 532-, and 635-nm illumination lasers. Laser-induced autofluorescence images of the teeth using 405-nm illumination were also obtained. Contrast between sound and eroded enamel was quantitatively computed for each imaging modality. For shallow erosions, the image contrast with respect to sound enamel was greatest for the 405-nm reflected image. It was also determined that the increased contrast was in large part due to volume scattering with a smaller component from surface scattering. Furthermore, images obtained with a shallow penetration depth illumination laser (405 nm) provided the greatest detail of surface enamel topography since the reflected light does not contain contributions from light reflected from greater depths within the enamel tissue. Multilayered Monte Carlo simulations were also performed to confirm the experimental results.

Self-affine elastic contacts: percolation and leakage

We study fluid flow at the interfaces between elastic solids with randomly rough, self-affine surfaces. We show by numerical simulation that elastic deformation lowers the relative contact area at which contact patches percolate in comparison to traditional approaches to seals. Elastic deformation also suppresses leakage through contacts even far away from the percolation threshold. Reliable estimates for leakage can be obtained by combining Persson's contact mechanics theory with a slightly modified version of Bruggeman's effective-medium solution of the Reynolds equation.

Rubber friction: comparison of theory with experiment

We have measured the friction force acting on a rubber block slid on a concrete surface. We used both unfilled and filled (with carbon black) styrene butadiene (SB) rubber and have varied the temperature from -10 °C to 100 °C and the sliding velocity from 1 μm/s to 1000 μm/s. We find that the experimental data at different temperatures can be shifted into a smooth master-curve, using the temperature-frequency shifting factors obtained from measurements of the bulk viscoelastic modulus. The experimental data has been analyzed using a theory which takes into account the contributions to the friction from both the substrate asperity-induced viscoelastic deformations of the rubber, and from shearing the area of real contact. For filled SB rubber the frictional shear stress σ(f) in the area of real contact results mainly from the energy dissipation at the opening crack on the exit side of the rubber-asperity contact regions. For unfilled rubber we instead attribute σ(f) to shearing of a thin rubber smear film, which is deposited on the concrete surface during run in. We observe very different rubber wear processes for filled and unfilled SB rubber, which is consistent with the different frictional processes. Thus, the wear of filled SB rubber results in micrometer-sized rubber particles which accumulate as dry dust, which is easily removed by blowing air on the concrete surface. This wear process seams to occur at a steady rate. For unfilled rubber a smear film forms on the concrete surface, which cannot be removed even using a high-pressure air stream. In this case the wear rate appears to slow down after some run in time period.

Rubber friction and tire dynamics

We propose a simple rubber friction law, which can be used, for example, in models of tire (and vehicle) dynamics. The friction law is tested by comparing numerical results to the full rubber friction theory (Persson 2006 J. Phys.: Condens. Matter 18 7789). Good agreement is found between the two theories. We describe a two-dimensional (2D) tire model which combines the rubber friction model with a simple mass-spring description of the tire body. The tire model is very flexible and can be used to accurately calculate μ-slip curves (and the self-aligning torque) for braking and cornering or combined motion (e.g. braking during cornering). We present numerical results which illustrate the theory. Simulations of anti-blocking system (ABS) braking are performed using two simple control algorithms.

Rolling friction for hard cylinder and sphere on viscoelastic solid

We calculate the friction force acting on a hard cylinder or spherical ball rolling on a flat surface of a viscoelastic solid. The rolling-friction coefficient depends non-linearly on the normal load and the rolling velocity. For a cylinder rolling on a viscoelastic solid characterized by a single relaxation time Hunter has obtained an exact result for the rolling friction, and our result is in very good agreement with his result for this limiting case. The theoretical results are also in good agreement with experiments of Greenwood and Tabor. We suggest that measurements of rolling friction over a wide range of rolling velocities and temperatures may constitute a useful way to determine the viscoelastic modulus of rubber-like materials.

Fluid dynamics at the interface between contacting elastic solids with randomly rough surfaces

We study fluid dynamics at the interface between elastic solids with randomly rough surfaces. The contact mechanics model of Persson is used to take into account the elastic interaction between the solid walls, and the Bruggeman effective medium theory to account for the influence of the disorder on the fluid flow. We calculated the flow tensor which determines the pressure flow factor and, for example, the leak rate of static seals. It is shown how the perturbation treatment of Tripp can be extended to arbitrary order in the ratio between the root-mean-square roughness amplitude and the average interfacial surface separation. We introduce a matrix D(ζ), determined by the surface roughness power spectrum, which can be used to describe the anisotropy of the surface at any magnification ζ. Results are presented for the asymmetry factor γ(ζ) (generalized Peklenik number) for grinded steel and sandblasted PMMA surfaces.

Leak rate of seals: Effective-medium theory and comparison with experiment

Seals are extremely useful devices to prevent fluid leakage. We present an effective-medium theory of the leak rate of rubber seals, which is based on a recently developed contact mechanics theory. We compare the theory with experimental results for seals consisting of silicon rubber in contact with sandpaper and sand-blasted PMMA surfaces.

A comparison of tackified, miniemulsion core-shell acrylic latex films with corresponding particle-blend films: structure-property relationships

Tackifying resins (TRs) are often added to pressure-sensitive adhesive films to increase their peel strength and adhesion energy. In waterborne adhesives, the TR is dispersed in water using surfactants and then blended with colloidal polymers in water (i.e., latex). In such waterborne systems, there are problems with the colloidal stability and difficulty in applying coatings of the particle blends; the films are often hydrophilic and subject to water uptake. Here, an alternative method of making waterborne, tackified adhesives is demonstrated. The TR is incorporated within the core of colloidal polymer particles via miniemulsion polymerization. Atomic force microscopy (AFM) combined with force spectroscopy analysis reveals there is heterogeneity in the distribution of the TR in films made from particle blends and also in films made from miniemulsion polymers. Two populations, corresponding to TR-rich and acrylic-rich components, were identified through analysis of the AFM force-displacement curves. The nanoscale maximum adhesion force and adhesion energy were found to be higher in a miniemulsion film containing 12 wt % tackifying resin in comparison to an equivalent blended film. The macroscale tack and viscoelasticity are interpreted by consideration of the nanoscale structure and properties. The incorporation of tackifying resin through a miniemulsion polymerization process not only offers clear benefits in the processing of the adhesive, but it also leads to enhanced adhesion properties.

Controlled sparse and percolating cross-linking in waterborne soft adhesives

The effect of low levels of cross-linking on the adhesive and mechanical properties of waterborne pressure-sensitive adhesives was investigated. We have taken advantage of a core-shell latex particle morphology obtained by emulsion polymerization to create a heterogeneous structure of cross-links without major modification of the monomer composition. The latex particles comprise a shell containing cross-linkable diacetone acrylamide (DAAM) repeat units localized on the periphery of a slightly softer core copolymer of very similar composition. Adipic acid dihydrazide was added to the latex prior to film formation to react with DAAM repeat units and affect interfacial cross-linking between particles in the adhesive films. The honeycomb-like structure obtained after drying of the latex results in a good balance between the dissipative properties required for adhesion and the resistance to creep. The characterization of the mechanical properties of the films shows that the chosen cross-linking method creates a percolating lightly cross-linked network, swollen with a nearly un-cross-linked component. With this cross-linking method, the linear viscoelastic properties of the soft films are nearly unaffected by the cross-linking while the nonlinear tensile properties are greatly modified. As a result, the long-term shear resistance of the adhesive film improves very significantly while the peel force remains nearly the same. A simple rheological model is used to interpret qualitatively the changes in the material parameters induced by cross-linking.

Contact mechanics and rubber friction for randomly rough surfaces with anisotropic statistical properties

In this paper we extend the theory of contact mechanics and rubber friction developed by one of us (B.N.J. Persson, J. Chem. Phys. 115, 3840 (2001)) to the case of surfaces with anisotropic surface roughness. As an application we calculate the viscoelastic contribution to the rubber friction. We show that the friction coefficient may depend significantly on the sliding direction, while the area of contact depends weakly on the sliding direction. We have carried out experiments for rubber blocks sliding on unidirectionally polished steel surfaces. The experimental data are in a good qualitative agreement with the theory.

Influence of surface roughness on adhesion between elastic bodies

We study the influence of surface roughness on the adhesion between elastic solids. We present experimental data for the force necessary to pull off rubber balls from hard rough substrates. We show that the effective adhesion (or the pull-off force) can be calculated accurately from the surface roughness power spectra obtained from the measured surface height profile.

On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion

Surface roughness has a huge impact on many important phenomena. The most important property of rough surfaces is the surface roughness power spectrum C(q). We present surface roughness power spectra of many surfaces of practical importance, obtained from the surface height profile measured using optical methods and the atomic force microscope. We show how the power spectrum determines the contact area between two solids. We also present applications to sealing, rubber friction and adhesion for rough surfaces, where the power spectrum enters as an important input.

Sealing is at the origin of rubber slipping on wet roads

Loss of braking power and rubber skidding on a wet road is still an open physics problem, as neither the hydrodynamic effects nor the loss of surface adhesion that are sometimes blamed really manage to explain the 20-30% observed loss of low-speed tyre-road friction. Here we report a novel mechanism based on sealing of water-filled substrate pools by the rubber. The sealed-in water effectively smoothens the substrate, thus reducing the viscoelastic dissipation in bulk rubber induced by surface asperities-well established as a major friction contribution. Starting with the measured spectrum of asperities one can calculate the water-smoothened spectrum and from that the predicted friction reduction, which is of the correct magnitude. The theory is directly supported by fresh tyre-asphalt friction data.