We consider the case of soft contacts in mixed lubrication conditions. We develop a novel, two scales contact algorithm in which the fluid- and asperity-asperity interactions are modeled within a deterministic or statistic scheme depending on the length scale at which those interactions are observed. In particular, the effects of large-scale roughness are deterministically calculated, whereas those of small-scale roughness are included by solving the corresponding homogenized problem. The contact scheme is then applied to the modeling of dynamic seals. The main advantage of the approach is the tunable compromise between the high-computing demanding characteristics of deterministic calculations and the much lower computing requirements of the homogenized solutions.

We present a general theory for the contribution from contact electrification to the work necessary to separate two solid bodies. The theory depends on the surface charge density correlation function 〈σ(x) σ(0)〉 which we deduce from Kelvin Force Microscopy (KFM) maps of the surface electrostatic potential. For silicon rubber (polydimethylsiloxane, PDMS) we discuss in detail the relative importance of the different contributions to the observed work of adhesion.

We present measurements of friction coefficient of lubricated laser surface textured (LST) microstructures with two different geometries. The former is made of a square lattice of microholes; the latter is constituted by a series of microgrooves. We analyze sliding velocities spanning more than two orders of magnitude to cover the entire range from the boundary to the hydrodynamic regime. In all cases, the interfacial pressure is limited to values (relevant to particular manufacturing processes) which allow to neglect macroscopic elastic deformations, piezo-viscosity and oil compressibility effects. The measured Stribeck curves data are compared with those obtained for the flat control surface and show that the regular array of microholes allows to reduce friction over the entire range of lubrication regimes with a decrease of about 50 % in the hydrodynamic regime. On the contrary, the parallel microgrooves lead to an increase of friction compared to the flat control surface with a maximum increase of about 80-100 % in the mixed lubrication regime. These remarkably opposite friction results are then explained with the aid of numerical simulations. Our findings confirm that LST may have cutting edge applications in engineering, not only in classical applications (e.g., to reduce piston-ring friction losses in internal combustion engines) but also, in particular, in technological processes, such as hydroforming, superplastic forming, where the mapping of the frictional properties of the mold has a crucial role in determining the final properties of the mechanical component.

We review the adhesion fundamentals and related physical aspects occurring from the nanoscale up to the length scale of macroscopic interacting solids. In particular, we provide theoretical basis for predicting surface and adhesion energies at the atomic scale, where adhesion originates, as well as the most relevant experimental techniques for the quantitative evaluation of the work of adhesion. The role of non-equilibrium processes, as well as of mesoscale interaction dynamics, will be assessed in term of the resulting adhesion regimes at the macro-scale. Mean field and numerical models are finally provided for the prediction of adhesion between real solids, in order to determine macroscopic physical quantities such as pull-off force and true contact area.

We generalize the Persson contact mechanics and rubberfrictiontheory to the case where both surfaces have surface roughness. The solids can be rigid, elastic, or viscoelastic and can be homogeneous or layered. We calculate the contact area, the viscoelastic contribution to the friction force, and the average interface separation as a function of the sliding speed and the nominal contact pressure. We illustrate the theory with numerical results for the classical case of a rubber block sliding on a road surface. We find that with increasing sliding speed, the influence of the roughness on the rubber block decreases to the extent that only the roughness of the stiff counter face needs to be considered.

We report on a mean field theory of textured surface lubrication. We study the fluid flow dynamics occurring at the interface as a function of the texture characteristics, e.g. texture area density, shape and distribution of microstructures, and local slip lengths. The present results may be very important for the investigation of tailored microtextured surfaces for low-friction hydrodynamic applications.

Nature has successfully combined soft matter and hydration lubrication to achieve ultralow friction even at relatively high contact pressure (e.g., articular cartilage). Inspired by this, hydrogels are used to mimic natural aqueous lubricating systems. However, hydrogels usually cannot bear high load because of solvation in water environments and are, therefore, not adopted in real applications. Here, a novel composite surface of ordered hydrogel nanofiber arrays confined in anodic aluminum oxide (AAO) nanoporous template based on a soft/hard combination strategy is developed. The synergy between the soft hydrogel fibers, which provide excellent aqueous lubrication, and the hard phase AAO, which gives high load bearing capacity, is shown to be capable of attaining very low coeffcient of friction (<0.01) under heavy load (contact pressures ≈2 MPa). Interestingly, the composite synthetic material is very stable, cannot be peeled off during sliding, and exhibits desirable regenerative (self-healing) properties, which can assure long-term resistance to wear. Moreover, the crosslinked polymethylacrylic acid hydrogels are shown to be able to promptly switch between high friction (>0.3) and superlubrication (≈10−3) when their state is changed from contracted to swollen by means of acidic and basic actuation. The mechanisms governing ultralow and tunable friction are theoretically explained via an in-depth study of the chemomechanical interactions responsible for the behavior of these substrate-infiltrated hydrogels. These findings open a promising route for the design of ultra-slippery and smart surface/interface materials.

We study the molecular reorientation induced by a textured external field in a nematic liquid crystal (nLC). In particular, we consider an infinitely wide cell with strong planar anchoring boundary conditions, subjected to a spatially periodic piecewise magnetic field. In the framework of the Frank’s continuum theory, we use the perturbation analysis to study in detail the field-induced splay-bend Freedericksz transition. A numerical approach, based on the finite differences method, is instead employed to solve the fully nonlinear equations. At high field strengths, an analytic approach allows us to draw the bulk profile of the director in terms of elliptic integrals. Finally, through the application of the Bruggeman texture hydrodynamics theory, we qualitatively discuss on the LCs piecewise director configuration under sliding interfaces, which can be adopted to actively regulate friction. Our study opens the pathway for the application of highly controlled nLC texturing for tribotronics.

We present the calculation results of optimal texture geometries which maximize the supported load for a three-dimensional thrust bearing pad. By making use of the recently developed mean field theory of texture hydrodynamics, we develop an efficient multigrid optimization procedure based on the sequential genetic and conjugate gradient optimization. We show that our model allows to determine optimal solutions based on a two-scale hierarchy of structures, and the existence of particularly effective optimal geometries is presented and discussed.

The texture hydrodynamics occurring in optimized steady-state lubricated contacts is theoretically investigated recurring to the recent Bruggeman texture hydrodynamics model. In particular, an attempt is made to unravel the intimate relationship between the texture-induced local flow properties and the macroscopic characteristics of the contact such as friction and supported load. The existence of different mechanisms of fluid flow harvesting and channeling is discussed, highlighting their strict relationship to the particular texture physical and topological properties. Whilst their single-species optimization does not produce particularly interesting performances in comparison to classical geometries, their optimized synergistic interaction, instead, provides a remarkable load generation and friction reduction almost independently from the macroscopic geometrical characteristics of the contact. The provided discussion can be easily extended to other contact geometries, such as for journal bearings, wet clutches and dynamic sealings, as well as to bio-tribology and soft contact applications.

We study the contact mechanics of a smooth hard cylinder rolling on a flat surface of a linear viscoelastic solid. Using the measured viscoelastic modulus of unfilled and filled (with carbon black) nitrile rubber, we compare numerically exact results for the rolling friction with the prediction of a simple analytical theory. For the unfilled rubber, the two theories agree perfectly while some small difference exists for the filled rubber. The rolling friction coefficient depends nonlinearly on the normal load and the rolling velocity.

We discuss on a recently presented theory of textured surface hydrodynamic lubrication (Scaraggi, Phys Rev E, 2012). The model, based on the Bruggeman effective medium approach, allows to analytically evaluate the effects of a generic texture shape, distribution, and area density on the macroscopic hydrodynamic characteristics of the contact, such as friction and supported load. In this study, we apply the cited theory to practical cases, and in particular we derive the flow and shear stress tensors for two limiting conditions, i.e., for isotropic (circular inclusion in isotropic medium) and perfectly anisotropic (infinite slit inclusion) flow conductivities. These results are then used to perform near-optimum design calculations for the simplest case of one- and two-dimensional thrustbearing geometries. Finally, a comparison with published results is presented and discussed. The developed theory may be a very useful tool in the process of evaluating the lubrication performances of sliding microtextured surfaces and for the near-optimum design of a textured pair, where texturing could be achieved by both physical (e.g., microstructuring) and chemical surface manipulation.

We present the numerical results for the viscoelastic and adhesive contribution to rubber friction for a tread rubber sliding on a hard solid with a randomly rough surface. In particular, the effect of the high- and low-frequency roughness power spectrum cut-off is investigated. The numerical results are then compared to the predictions of an analytical theory of rubber friction. We show that the friction coefficient for large load is given exactly by the theory while some difference between theory and simulations occur for small loads, due to a finite sample-size effects, whereas the contact area is almost unaffected by the low frequency cut-off. Finally, the role of a finite rubber thickness on viscoelastic friction and contact area is introduced and critically discussed. Interestingly, we show that classical rough contact mechanics scaling rules do not apply for this case.

The mean field fluid dynamics and the friction occurring in the wet sliding contact between inhomogeneous surfaces, characterized by a deterministically repeated pattern of microdefects, are modelled within the Bruggeman effective medium theory. By comparing with the results of an accurate numerical homogenization of the flow equations, and with asymptotic solutions, we discuss the validity of the mean field model and its limitations in relation to the occurrence of clustering and interference effects. Finally, an analytical upgrade to the Bruggeman approach, allowing for inclusion of the clustering effect, is presented and discussed.

We discuss the influence of geometrical and rheological non-linearities on the prediction of rubber friction and true contact area for rough sliding interactions. In particular, we compare the results of a linearly-viscoelastic linear-contact model, formulated in the Fourier space, with those obtained from non-linear finite element calculations. A sinusoidal rigid profile indenting a rubber block is here considered for simplicity, whereas the effects of non-linearity are evaluated by varying the aspect ratio, loading conditions and sliding speed of the contact interface. It is found that accurate friction predictions can be obtained through the linear viscoelastic model, provided that the roughness under investigation features moderate values of root mean square slopes, whereas non-linear finite element computations should be adopted for large root mean square slopes.

We discuss how surface roughness influences the adhesion between elastic solids. We introduce a Tabor number which depends on the length scale or magnification, and which gives information about the nature of the adhesion at different length scales. We consider two limiting cases relevant for (a) elastically hard solids with weak (or long ranged) adhesive interaction (DMT-limit) and (b) elastically soft solids with strong (or short ranged) adhesive interaction (JKR-limit). For the former cases we study the nature of the adhesion using different adhesive force laws (F ∼ un, n = 1.5-4, where u is the wall-wall separation). In general, adhesion may switch from DMT-like at short length scales to JKR-like at large (macroscopic) length scale. We compare the theory predictions to results of exact numerical simulations and find good agreement between theory and simulation results.

We study the lubricated (wet) contact mechanics of a smooth hard cylinder sliding on a randomly rough nominally flat surface of a linear viscoelastic solid. We calculate the rolling and sliding friction, and study the transition from the boundary lubrication to the elasto-hydrodynamic lubrication regime. For the viscoelastic contact the minimum (average) separation does not monotonically increase with the sliding velocity, and the Stribeck curve exhibits new structures not shown for elastic solids.

We study the time dependency of the interfacial separation and of the area of real contact between a soft elastic cylinder and a rigid solid with a randomly rough surface, squeezed with normal approach in a fluid. This problem is relevant for biological, as well as for bio-medical, seals and tires applications. An ad-hoc numerical scheme is developed to solve the transient mixed-EHD homogenized lubrication problem. We show that, for the soft contact case, the transition from the EHD to the boundary regime can be much more efficiently studied within a simplified (Grubin-like) problem formulation then with the full numerical scheme. This is not only of great conceptional importance, but also of practical importance as the latter calculation is much simpler and faster than the full scheme calculation.

We have investigated the friction properties of lubricated laser microtextured surfaces. The microtexture consists of a square lattice of micro-holes whose diameter, depth and spacing are controlled during the laser texturing process. All surfaces have the same texture area density, but different diameter and depth of the micro-holes. We measure the coefficient of friction on a range of sliding velocities covering the range from the mixed lubrication to the hydrodynamic regime. We find that the depth and diameter of the micro-holes have a huge influence in determining the amount of friction reduction at the interface. Interestingly experiments also show that optimal micro-hole depth values, minimizing the friction in the hydrodynamic regime, are remarkably effective also in the mixed lubrication regime.