Adhesion Hysteresis Research Articles

Effect of Chemical Functionality on Adhesion Hysteresis

Studies of adhesion and adhesion hysteresis were carried out using cross-linked poly(dimethyl siloxane) (PDMS) semispherical surfaces and self-assembled model surfaces containing different chemical functionalities, using the JKR method, the contact mechanics of solids spreading their interfacial area under load. Hysteresis resulting from fast relaxation processes in the PDMS elastomer network was practically eliminated using stepwise loading and unloading protocols. The interfacial H-bonding between PDMS and both Si−OH and −COOH surfaces was shown to be an important chemical interaction causing significant adhesion hysteresis. The number of H-bonds between PDMS and Si−OH surfaces increased with contact time under compressive load, indicating pressure-induced reorganization of the PDMS network near the interface that increased the number of H-bonds. The interaction between PDMS and functionalized biphenylthiolate monolayers exhibited a smaller hysteresis, which is believed to be caused by dipolar interacti...

Adhesion Between Hydrolyzed Surfaces of Poly(dimethylsiloxane) Networks

The JKR technique was used to study the adhesion hysteresis between hydrolyzed poly(dimethylsiloxane), PDMS, networks. PDMS model networks were synthesized and then hydrolyzed for various lengths of time in a 0.1 M HCl aqueous solution. It was found that as the exposure time of the networks to the HCl solution increased, the adhesion hysteresis increased. This hysteresis could be reduced by up to 80% by exposing the hydrolyzed networks to hexamethyldisilazane, which replaces the silanol end-functionality with a trimethylsilyl group. This dramatic reduction of the hysteresis suggests that the bulk of the adhesion hysteresis arises from a surface reconstruction reaction between silanol groups. We have also analyzed the unloading rate dependence of the adhesion hysteresis and found that it is probably due to the rate of bond dissociation.

Adhesion Hysteresis Studies of Extracted Poly(dimethylsiloxane) Using Contact Mechanics

We present results of adhesion studies of poly(dimethylsiloxane) (PDMS) crosslinked hemispheres, before and after extraction, carried out using contact mechanics (the JKR apparatus.) Three types of PDMS networks were prepared with increasing crosslinker-to-PDMS ratio (r = 1.7, 2.2, 2.7). Adhesion hysteresis was observed only after the PDMS hemispheres were extracted (toluene, Soxhlet, 20−24 h.) The size of hysteresis increased systematically with the decreasing ratio (increasing percentage of extracted weight.) Brief extraction yielded smaller hysteresis, which disappeared after 12 days. It is concluded that the entanglement of tethered chains and their interdigitation with cross-linked PDMS network at the interface may be the dominating mechanisms for the observed adhesion hysteresis.

Use of Capacitance To Measure Surface Forces. 2. Application to the Study of Contact Mechanics

We performed capacitance measurements between silver layers on the back side of thin (<1 μm) mica substrates in surface force apparatus to determine the contact area with very high precision (<0.1% of the total area) and at a rate that is faster (∼1 kHz) than that in more traditional methods based on interferometry. To demonstrate the capabilities of the technique, we measured the adhesion and adhesion hysteresis between two mica surfaces. A peculiar discontinuous decrease in contact area, with decreasing load, between two dry mica surfaces is observed. We also studied the possible effect of shear forces on the value of the contact area due to adhesive forces, as in JKR theory.

Adhesion of solids

We report results of studies of adhesion of two solids on a microscopic level. We show that the Johnson-Kendall-Roberts theory [Proc. R. Soc. London A 324, 301 (1971)] remains valid even at this level, and that the effects of roughness and an intervening fluid can be accounted for by adjusting the value of the work of adhesion. We study adhesion hysteresis and demonstrate that, in our system, it arises from bulk effects. We also find that the detachment of spherical particles from solid surfaces occurs at smaller values of the shear rate than predicted by continuum theory, due to slip between the particle and the solid surface which is not taken into account by continuum theories.

Nanotribological Properties of Octadecyltrichlorosilane Self-Assembled Ultrathin Films Studied by Atomic Force Microscopy: Contact and Tapping Modes

Atomic force microscopy, working in contact and tapping modes, has been used to probe the mechanical response of octadecyltrichlorosilane, CH3−(CH2)17−SiCl3 (OTS), self-assembled ultrathin films grafted on silicon-treated surfaces. To assess the adhesion hysteresis and the elasticity of the films, we have performed loading and unloading experiments in contact mode. The effect of compression is observed while the load is being applied to the film. Once the load is fully removed, the film remains compressed showing a clear deviation from perfect elasticity, with relaxation times in the range of 0.1 s. In the tapping mode, the loading/unloading experiments show again the deviation from perfect elastic recovery of the films. Upon loading, three different regions are encountered. These can be associated with an initial disorder of the chains produced by the indenting tip, followed by a smooth compression and a final hardening effect due to repulsive forces between the compressed chains. The tapping mode is als...

Adhesion and friction of self-assembled organic monolayers

Recent advances in the study of friction and adhesion using self-assembled monolayers (SAMs) include the use of microscopic measurements to investigate adhesion hysteresis and the effects of hydrogen bonding and acid-base interactions on this process. Also, recent molecular dynamics simulations have enabled several important observations to be made concerning friction between surfaces and the role that surface molecules play in systems.

Time-dependent effects in surface forces

Recent studies employ the surface forces apparatus to measure shear and normal forces in molecularly thin films. Coming from these studies are several key results. Friction is related to adhesion hysteresis rather than adhesional force. The confinement-induced freezing of liquid films produces solids much softer than bulk glass. In systems of strongly adsorbed layers, friction depends on the degree of interpenetration. End-adsorbed polymer layers in good solvent reduce the effective viscosity of the solvent by orders of magnitude.

Adhesion and composites

Adhesion is vital to composites because cracks are stopped at adhesive interfaces, deflected along them, or converted into interfacial dislocations. Failure is therefore dominated by the adhesion between the components of the composite. This paper describes new ideas about adhesion and considers the relevance of these ideas to composite materials. In particular it emphasises the difference between adhesion at the molecular level and adhesion in engineering terms. There is no doubt that molecules of solid materials are attracted to each other by van der Waals and other influences. Also, they can be separated by applying mechanical forces. The problem is explaining the connection, i.e. the mechanism, between molecular attractions and mechanical measurements. Ideas such as 'keying', 'glueing' and 'friction' require critical assessment because they operate differently at the molecular scale. Interesting mechanisms such as 'adhesive hysteresis', 'adhesive stringing', and 'adhesive aggregation' deserve evaluati...

Measurement of interfacial adhesion between glassy polymers using the JKR method

AbstractThe surface and interfacial energies of polymers are measured using the JKR‐type experiments. A novel method has been developed to prepare samples of glassy polymers for adhesion measurements. A thin layer of a polymer is coated on the surface of an O2‐plasma modified cross‐linked poly(dimethylsiloxane) [PDMS] spherical cap resulting in the formation of a composite. Using the JKR theory, the surface energies of polystyrene [PS] and poly(methyl methacrylate) [PMMA] are determined from the measurements of the contact radius as a function of applied load. The results of the JKR‐type experiments are compared to adhesion measurements done using the surface forces apparatus (SFA). Adhesion hysteresis was observed for PS‐PS contact as well as PMMA‐PMMA contact. However, no hysteresis was observed for PDMS‐PDMS, PDMS‐PS, and PDMS‐PMMA contacts. The exact origin of the hysteresis is not clear at present. The current evidence suggests that hysteresis is due to rearrangement of the interface during contact.

JKR Studies of Acrylic Elastomer Adhesion to Glassy Polymer Substrates

Adhesion between a model acrylic elastomer and a glassy polymeric substrate has been investigated by the JKR technique. Hemispherical lenses of lightly cross-linked poly(n-butyl acrylate) (PNBA) were subjected to loading/unloading cycles on flat poly(methyl methacrylate) (PMMA) substrates. Significant adhesion hysteresis is observed at all accessible rates of unloading. The amount of hysteresis is seen to increase systematically with unloading rate and is characterized by the measured energy release rate (G). The presence of free PNBA chains, whether in the elastomer or applied directly to the interface, significantly reduces adhesion of the elastomer to the glassy substrate. Unloading profiles of the crack growth rate dependence of G yield a threshold value (G(0) = 70 ± 30 mJ/m2) which is close to the expected thermodynamic work of adhesion (W) between PNBA and PMMA. However, fits of the JKR model to loading data result in values of G which are significantly lower than W. We also examine the underlying a...

The Effect of Chemical Functionality on Adhesion Hysteresis: A Study using the JKR Method

ABSTRACTThe adhesion of crosslinked PDMS surfaces to self-assembled monolayers with different chemical functionality was investigated using the JKR method, the contact mechanics of solids spreading their interfacial area under load. Interfacial H-bonding was shown to be an important chemical interaction causing significant adhesion hysteresis. The number of H-bonds between PDMS and silanol groups on SiO2/Si surfaces increased with time of the contact under a constant load, indicating pressure-induced reorganization of the PDMS network near the interface. The interaction between PDMS and carboxylic acid groups showed somewhat smaller hysteresis which suggests weaker H-bonding strength. The interaction between PDMS and functionalized biphenyl groups exhibited small hysteresis which is believed to be caused by dipolar interaction. whereas that between PDMS and nonpolar perfluorocarbon groups showed negligible hysteresis. The distinction in the behavior of the unloading data between H-bonding related interaction and dipolar interaction seems to indicate the difference in the nature between non-specific (van der Waals, dipolar) and specific (donor-acceptor, H-bond, acid-base) interactions.

Sensitivity of Frictional Forces to pH on a Nanometer Scale: A Lateral Force Microscopy Study

An atomic force microscope (AFM) operating in the lateral (frictional) force mode (LFM) was used to investigate the pH dependence of kinetic friction between a silicon nitride tip and an oxidized silicon surface under aqueous electrolyte solutions. It was found that, besides the known pH dependence of the normal force curves, the frictional force is highly sensitive to the pH value of the solution. A linear relationship between frictional force and adhesion hysteresis (i.e., the dissipated energy), as recently shown to be valid for some systems by Israelachvili et al., is supported. The implication is that many chemical species on surfaces can be readily differentiated using AFM-LFM by varying the pH value of the surrounding medium and that a possible nanochemical imaging method could be developed, based upon this principle.

Adhesion Energy Hysteresis and Friction between Ultrathin Polyglutamate Films Measured with the Surface Forces Apparatus

Adhesion and friction forces between deposited monolayers of poly({gamma}-methyl L-glutamate){sub x} co-({gamma}-n-octadecyl L-glutamate){sub y}--a hairy rod polymer having a rigid main chain rod and alkyl side chains containing either 1 or 18 C atoms--were measured using the surface forces apparatus technique. By performing dynamic (time-dependent) adhesion energy and friction experiments as a function of temperature T, the authors obtained the energy loss spectra for these quasi two-dimensional films. The measured adhesion energy losses or hysteresis as a function of temperature are shown to be related to the interdigitation of the octadecyl side chains, caused by different molecular motions that have been previously shown to be active at different temperatures. Friction experiments probed the energy dissipation processes arising from lateral motion. Assuming that the molecular dissipation processes are the same in both types of measurements (adhesion and friction), they used a previously derived simple formula to relate quantitatively the adhesion energy hysteresis and friction forces.

Molecular dynamic modeling of interfacial energy

Molecular dynamic modeling is used to calculate the changes in the energy between two surfaces when the surfaces are first allowed to approach one another and, subsequently, separated. The equations of motion of the atoms, which were assumed to interact via a Lennard-Jones potential, were integrated using Verlet's algorithm. They were implemented in an environment of periodic boundary conditions, feedback loop temperature and pressure controllers, with direct computation of the stress and strain tensors. This approach allows one to calculate the temperature dependence of the 'leap-to-contact' phenomenon, the thermodynamic work of adhesion, the work needed to separate the surfaces, and the forces of attraction and separation. Effects that occur during approach and separation, such as surface roughening and vacancy formation, were included in the energetics calculations. Sound waves and the resulting thermal transients were also modeled. The adhesion hysteresis and irreversible behaviors during approach and...

Irreversibility, Energy Dissipation, and Time Effects in Intermolecular and Surface Interactions

AbstractThe use of thermodynamic functions such as surface energy (y) or the reversible work of adhesion (W) implicitly assumes that two surfaces are interacting under conditions of thermodynamic equilibrium. Likewise, the concept of the adhesion force or pressure between two surfaces, molecules, or colloidal particles also suggests the existence of well‐defined time‐independent values for these quantities. Yet most interactions involving molecules, surfaces, and complex fluid systems are irreversible, involving the transfer of energy from one system to another; that is, they involve “energy dissipation”. We review the molecular origins nonequilibrium (irreversible) interactions of surfaces that give rise to adhesion hysteresis, contact angle hysteresis, friction, and other thin‐film properties, and discuss the important role of “time” in such processes which can have a significant effect on what we measure. We also consider the possible central role of nonequilibrium interactions in biological systems, where nature often makes use of the finite time of molecular processes for regulating the interactions of proteins, membranes, and cells.

Grain contact adhesion hysteresis: A mechanism for attenuation of seismic waves

We propose a new mechanism for attenuation of seismic waves in rocks. It is shown that grain contact adhesion hysteresis leads to energy dissipation without having to invoke frictional sliding at grain contacts. Adhesion hysteresis gives rise to frequency independent attenuation that increases with strain amplitude for large strains and decreases with overburden stress. We can obtain qualitatively correct trends that show the effect of fluid saturation, pore structure and grain size through simple models for granular sedimentary rocks.

Friction and energy dissipation at the atomic scale: A review

Discussions of energy dissipation during friction processes have captured the attention of engineers and scientists for over 300 years. Why then do we know so little about either dissipation or friction processes? A simple answer is that we cannot see what is taking place at the interface during sliding. Recently, however, devices such as the atomic force microscope have been used to perform friction measurements, characterize contact conditions, and even describe the ‘‘worn surface.’’ Following these and other experimental developments, friction modeling at the atomic level—particularly molecular dynamics (MD) simulations—has brought scientists a step closer to ‘‘seeing’’ what takes place during sliding contact. With these investigations have come some answers and new questions about the modes and mechanisms of energy dissipation at the sliding interface. This article will review recent theoretical and experimental studies of friction processes at the atomic scale. Theoretical treatments range from simple, analytical models of two-dimensional, coupled ball-spring systems at 0 K, to more complex MD simulations of three-dimensional arrays of hydrogen- and hydrocarbon-terminated surfaces at finite temperatures. Results are presented for the simplest yet most practical cases of sliding contact: sliding without wear. Sliding without friction is seen in weakly interacting systems. Simple models can easily explain the energetics of such friction processes, but MD studies are needed to explore the dynamics (excitation modes, energy pathways,...) of thermally excited atoms interacting in three-dimensional fields. These studies provide the first atomic-scale models for anisotropic friction and boundary lubrication. Friction forces at atomic interfaces must ultimately be measured at the macroscopic level; these measurements, which depend on the mechanical properties of the measuring system, are discussed. Two rather unique experimental studies of friction are also reviewed. The first employs a ‘‘surface force apparatus’’ to measure adhesion and friction between surfactant monolayers. The correlation of adhesion hysteresis and friction provides a new mechanism of friction; moreover, the interpretation for the effect—hysteresis from entanglement of the molecular chains during a phase transformation—implies that the dynamics are taking place at an accessible time scale (seconds to minutes). The second study extends the time domain at which friction can be measured to the nanosecond scale. A quartz-crystal oscillator is used to monitor the viscosity of monolayer liquids and solids against solid surfaces. Interfaces slip angstroms in nanoseconds. Modelers have suggested a variety of mechanisms for this atomic-scale friction process, from defect-mediated sliding to electron drag effects. The article ends by identifying the vast, barely charted time-space domain (micro-to-pico time and length scales) in which experiments are needed to further understand the dynamic aspects of friction processes.

Adhesion: Molecules and Mechanics

There is a difference between adhesion at the molecular level and adhesion in engineering. There is no doubt that molecules of solid materials stick together and can be separated mechanically. The problem is explaining the connection between molecular attractions and mechanical measurements. False ideas such as keying and gluing require critical assessment because they confuse molecules and mechanics. Mechanisms such as adhesive hysteresis, stringing, and clustering deserve evaluation. A rational theory of these phenomena should be based on the theoretical concept of reversible work of adhesion and on the measured quantity of adhesive energy, which includes the extra energy required to restructure the interface as surfaces move.

Recent advances in molecular level understanding of adhesion, friction and lubrication

In order to investigate the mechanism of boundary friction, adhesion and friction experiments were carried out with a variety of surfactant-monolayer-coated surfaces using the surface forces apparatus. The friction forces were measured as a function of monolayer coverage and phase state, load, temperature, shear rate and relative humidity. We report on two important findings. (i) Stick-slip motion can occur not only between a “static” and a “kinetic” state but also between a static or kinetic state and some “superkinetic” state that exhibits ultralow friction. (ii) The friction coefficient is not directly correlated with adhesion but rather with the adhesion hysteresis, which is a measure of the energy dissipated during an irreversible adhesion ( e.g. loading-unloading) cycle. We also conclude that chain interdigitation is the most important molecular mechanism that gives rise to “boundary” friction and adhesion hysteresis of monolayer-coated surfaces.