Incipient sliding of adhesive contacts

A model is proposed herein to investigate the incipient sliding of contacts in the presence of both friction and adhesion, where the interfacial response is modeled based on traction-separation laws. A Maugis-like parameter is defined to characterize the response in the tangential direction. Subsequently, the model is used to investigate the contact between a smooth cylinder and a flat body, where adhesion-friction interactions are strong. A range of behaviors are observed when a tangential displacement is imposed: When the parameter is low, the contact pressure exhibits a relatively constant profile; when it is high, a pressure spike is observed at the edge of the contact. This difference is caused by a significant interface compliance in the former case, which limits the amount of slip. The results for the mid-range values of the Maugis-like parameter can qualitatively replicate various experiments performed using polydimethylsiloxane (PDMS) balls.

A Note by K. L. Johnson on the History of the JKR Theory

The history of the following note is as follows. In 2003, I invited Kenneth Johnson to Berlin to give a talk on adhesion in a seminar at the Institute of Mechanics. His lecture on the topic "Mechanics of adhesion of spherical surfaces" took place on Monday, January 26, 2004. In the run-up to the seminar, Professor Johnson sent me a historical note dated November 18, 2003. In my opinion, this note, which was written in the form of a paper, may be of interest for experts in contact mechanics and tribology. Prof. Johnson did not publish it, so it remained a private communication. For a publication he might have made a revision and would possibly have credited other important contributions. But this we can only guess at, and therefore the note is published below in the form I received it from Kenneth L. Johnson, with only a few misprints corrected. It is interesting as a historical document from Ken Johnson, who played a key role in development of theory of adhesive contacts.

A JKR-Like Solution for Viscoelastic Adhesive Contacts

A closed-form solution for the adhesive contact of soft spheres of linear elastic material is available since 1971 thanks to the work of Johnson, Kendall, and Roberts (JKR). A similar solution for viscoelastic spheres is still missing, though semi-analytical and numerical models are available today. In this note, we propose a closed-form analytical solution, based on JKR theory, for the detachment of a rigid sphere from a viscoelastic substrate. The solution returns the applied load and contact penetration as functions of the contact radius and correctly captures the velocity-dependent nature of the viscoelastic pull-off. Moreover, a simple approach is provided to estimate the stick time, i.e., the delay between the time the sphere starts raising from the substrate and the time the contact radius starts reducing. A simple formula is also suggested for the viscoelastic pull-off force. Finally, a comparison with experimental and numerical data is shown.

Molecular Biomechanics Controls Protein Mixing and Segregation in Adherent Membranes

Cells interact with their environment by forming complex structures involving a multitude of proteins within assemblies in the plasma membrane. Despite the omnipresence of these assemblies, a number of questions about the correlations between the organisation of domains and the biomechanical properties of the involved proteins, namely their length, flexibility and affinity, as well as about the coupling to the elastic, fluctuating membrane, remain open. Here we address these issues by developing an effective Kinetic Monte Carlo simulation to model membrane adhesion. We apply this model to a typical experiment in which a cell binds to a functionalized solid supported bilayer and use two ligand-receptor pairs to study these couplings. We find that differences in affinity and length of proteins forming adhesive contacts result in several characteristic features in the calculated phase diagrams. One such feature is mixed states occurring even with proteins with length differences of 10 nm. Another feature are stable nanodomains with segregated proteins appearing on time scales of cell experiments, and for biologically relevant parameters. Furthermore, we show that macroscopic ring-like patterns can spontaneously form as a consequence of emergent protein fluxes. The capacity to form domains is captured by an order parameter that is founded on the virial coefficients for the membrane mediated interactions between bonds, which allow us to collapse all the data. These findings show that taking into account the role of the membrane allows us to recover a number of experimentally observed patterns. This is an important perspective in the context of explicit biological systems, which can now be studied in significant detail.

Distribution of Gaps and Adhesive Interaction between Contacting Rough Surfaces

Understanding the distribution of interfacial separations between contacting rough surfaces is integral for providing quantitative estimates for adhesive forces between them. Assuming nonadhesive, frictionless contact of self-affine surfaces, we derive the distribution of separations p(g) between surfaces near the contact edge. The distribution diverges as g-1/3 for small gaps, and we use numerical simulations with fine resolution to confirm the scaling. The characteristic scale over which the prediction persists is h0' drep, the product of the rms surface slope and the mean diameter of contacting regions. We show that these results remain valid for weakly adhesive contacts and connect these observations to recent theories for adhesion between rough surfaces.

Emerging functions of cytoskeletal proteins in immune diseases

ABSTRACTImmune cells are especially dependent on the proper functioning of the actin cytoskeleton, and both innate and adaptive responses rely on it. Leukocytes need to adhere not only to substrates but also to cells in order to form synapses that pass on instructions or kill infected cells. Neutrophils literally squeeze their cell body during blood extravasation and efficiently migrate to the inflammatory focus. Moreover, the development of immune cells requires the remodeling of their cytoskeleton as it depends on, among other processes, adhesive contacts and migration. In recent years, the number of reports describing cytoskeletal defects that compromise the immune system has increased immensely. Furthermore, a new emerging paradigm points toward a role for the cellular actin content as an essential component of the so-called homeostasis-altering molecular processes that induce the activation of innate immune signaling pathways. Here, we review the role of critical actin-cytoskeleton-remodeling proteins, including the Arp2/3 complex, cofilin, coronin and WD40-repeat containing protein 1 (WDR1), in immune pathophysiology, with a special focus on autoimmune and autoinflammatory traits.

Adhesion and friction in hard and soft contacts: theory and experiment

This paper is devoted to an analytical, numerical, and experimental analysis of adhesive contacts subjected to tangential motion. In particular, it addresses the phenomenon of instable, jerky movement of the boundary of the adhesive contact zone and its dependence on the surface roughness. We argue that the “adhesion instabilities” with instable movements of the contact boundary cause energy dissipation similarly to the elastic instabilities mechanism. This leads to different effective works of adhesion when the contact area expands and contracts. This effect is interpreted in terms of “friction” to the movement of the contact boundary. We consider two main contributions to friction: (a) boundary line contribution and (b) area contribution. In normal and rolling contacts, the only contribution is due to the boundary friction, while in sliding both contributions may be present. The boundary contribution prevails in very small, smooth, and hard contacts (as e.g., diamond-like-carbon (DLC) coatings), while the area contribution is prevailing in large soft contacts. Simulations suggest that the friction due to adhesion instabilities is governed by “Johnson parameter”. Experiments suggest that for soft bodies like rubber, the stresses in the contact area can be characterized by a constant critical value. Experiments were carried out using a setup allowing for observing the contact area with a camera placed under a soft transparent rubber layer. Soft contacts show a great variety of instabilities when sliding with low velocity — depending on the indentation depth and the shape of the contacting bodies. These instabilities can be classified as “microscopic” caused by the roughness or chemical inhomogeneity of the surfaces and “macroscopic” which appear also in smooth contacts. The latter may be related to interface waves which are observed in large contacts or at small indentation depths. Numerical simulations were performed using the Boundary Element Method (BEM).

THE EFFECT OF THE SIZE AND SHAPE OF WOOD PARTICLES ON THE TENSILE STRENGTH PERPENDICULAR TO THE PLANE OF THE PARTICLEBOARD: EXPERIMENTS AND MODELING

One of the problems of sustainable development is the technologies improvement for the rational use of wood and other raw materials of plant origin. The literature reflects a large amount of applied research that was conducted to justify new technologies for the production of particle boards (PB). The main attention in the known works is paid to the influence of the particle size distribution on the strength of PB. The influence of particle shape on the PB strength has been studied to a lesser extent. In this regard, this article considers the influence of the shape and size of particles on the tensile strength perpendicular to the plane of the PB. A geometric analysis of the particle shape is performed. It was taken into account that the PB strength depends on the shape and size of the particles, as well as on the number of adhesive contacts between particles. To obtain quantitative estimates, formulas were substantiated confirming that an increase in the length of the particles and a decrease in their transverse dimensions lead to an increase in the PB strength. Experimental research methods were used, and mathematical modeling of the sample failure area was performed.