Effect of surface pattern on the adhesive friction of elastomers

In vitro models of soft tissue damage by implant-associated frictional shear stresses

Silicone elastomer medical implants are ubiquitous in medicine, particularly for breast augmentation. However, when these devices are placed within the body, disruption of the natural biological interfaces occurs, which significantly changes the native energy-dissipation mechanisms of living systems. These new interfaces can introduce non-physiological contact pressures and tribological conditions that provoke inflammation and soft tissue damage. Despite their significance, the biotribological properties of implant-tissue and implant-extracellular matrix (ECM) interfaces remain poorly understood. Here, we developed an in vitro model of soft tissue damage using a custom-built in situ biotribometer mounted onto a confocal microscope. Sections of commercially-available silicone breast implants with distinct and clinically relevant surface roughness (Ra = 0.2 ± 0.03 μm, 2.7 ± 0.6 μm, and 32 ± 7.0 μm) were mounted to spherically-capped hydrogel probes and slid against collagen-coated hydrogel surfaces as well as healthy breast epithelial (MCF10A) cell monolayers to model implant-ECM and implant-tissue interfaces. In contrast to the "smooth" silicone implants (Ra < 10 μm), we demonstrate that the "microtextured" silicone implant (10 < Ra < 50 μm) induced higher frictional shear stress (τ > 100 Pa), which led to greater collagen removal and cell rupture/delamination. Our studies may provide insights into post-implantation tribological interactions between silicone breast implants and soft tissues.

Survival of polymeric microstructures subjected to interrogatory touch

Polymeric arrays of microrelief structures have a range of potential applications. For example, to influence wettability, to act as biologically inspired adhesives, to resist biofouling, and to play a role in the “feel” of an object during tactile interaction. Here, we investigate the damage to micropillar arrays comprising pillars of different modulus, spacing, diameter, and aspect ratio due to the sliding of a silicone cast of a human finger. The goal is to determine the effect of these parameters on the types of damage observed, including adhesive failure and ploughing of material from the finger onto the array. Our experiments point to four principal conclusions [1]. Aspect ratio is the dominant parameter in determining survivability through its effect on the bending stiffness of micropillars [2]. All else equal, micropillars with larger diameter are less susceptible to breakage and collapse [3]. The spacing of pillars in the array largely determines which type of adhesive failure occurs in non-surviving arrays [4]. Elastic modulus plays an important role in survivability. Clear evidence of elastic recovery was seen in the more flexible polymer and this recovery led to more instances of pristine survivability where the stiffer polymer tended to ablate PDMS. We developed a simple model to describe the observed bending of micropillars, based on the quasi-static mechanics of beam-columns, that indicated they experience forces ranging from 10⁻⁴–10⁻⁷ N to deflect into adhesive contact. Taken together, results obtained using our framework should inform design considerations for microstructures intended to be handled by human users.

Shear-Induced Anisotropy in Rough Elastomer Contact

True contact between randomly rough solids consists of myriad individual microjunctions. While their total area controls the adhesive friction force of the interface, other macroscopic features, including viscoelastic friction, wear, stiffness, and electric resistance, also strongly depend on the size and shape of individual microjunctions. We show that, in rough elastomer contacts, the shape of microjunctions significantly varies as a function of the shear force applied to the interface. This process leads to a growth of anisotropy of the overall contact interface, which saturates in the macroscopic sliding regime. We show that smooth sphere-plane contacts have the same shear-induced anisotropic behavior as individual microjunctions, with a common scaling law over 4 orders of magnitude in the initial area. We discuss the physical origin of the observations in light of a fracture-based adhesive contact mechanics model, described in the companion article, which captures the smooth sphere-plane measurements. Our results shed light on a generic, overlooked source of anisotropy in rough elastic contacts, not taken into account in current rough contact mechanics models.

Evolution of real contact area under shear and the value of static friction of soft materials

The frictional properties of a rough contact interface are controlled by its area of real contact, the dynamical variations of which underlie our modern understanding of the ubiquitous rate-and-state friction law. In particular, the real contact area is proportional to the normal load, slowly increases at rest through aging, and drops at slip inception. Here, through direct measurements on various contacts involving elastomers or human fingertips, we show that the real contact area also decreases under shear, with reductions as large as 30[Formula: see text], starting well before macroscopic sliding. All data are captured by a single reduction law enabling excellent predictions of the static friction force. In elastomers, the area-reduction rate of individual contacts obeys a scaling law valid from micrometer-sized junctions in rough contacts to millimeter-sized smooth sphere/plane contacts. For the class of soft materials used here, our results should motivate first-order improvements of current contact mechanics models and prompt reinterpretation of the rate-and-state parameters.

Role of adhesion between asperities in the formation of elastic solid/solid contacts

We investigated the formation of a contact between a smooth sphere of elastomer and a micro-patterned elastomer substrate. We focussed our attention on the transition between a contact only established at the top of the pillars, and a mixed contact with a central zone of full contact surrounded by a top contact corona, which was observed when the normal load was increased. The full contact zone always nucleated with a finite radius, and the transition appears to be a first-order transition, with a hysteresis due to the creation of an adhesive zone between the pillars. We propose to include the effect of the new inter-pillar adhesion to produce a realistic treatment of the mechanics of these complex contacts. This new approach quantitatively accounts for the evolution of the observed jump in the radius of the full contact with the geometrical parameters of the pattern.

Insect adhesion on rough surfaces: analysis of adhesive contact of smooth and hairy pads on transparent microstructured substrates

Insect climbing footpads are able to adhere to rough surfaces, but the details of this capability are still unclear. To overcome experimental limitations of randomly rough, opaque surfaces, we fabricated transparent test substrates containing square arrays of 1.4 µm diameter pillars, with variable height (0.5 and 1.4 µm) and spacing (from 3 to 22 µm). Smooth pads of cockroaches (Nauphoeta cinerea) made partial contact (limited to the tops of the structures) for the two densest arrays of tall pillars, but full contact (touching the substrate in between pillars) for larger spacings. The transition from partial to full contact was accompanied by a sharp increase in shear forces. Tests on hairy pads of dock beetles (Gastrophysa viridula) showed that setae adhered between pillars for larger spacings, but pads were equally unable to make full contact on the densest arrays. The beetles' shear forces similarly decreased for denser arrays, but also for short pillars and with a more gradual transition. These observations can be explained by simple contact models derived for soft uniform materials (smooth pads) or thin flat plates (hairy-pad spatulae). Our results show that microstructured substrates are powerful tools to reveal adaptations of natural adhesives for rough surfaces.

Oscillating friction on shape-tunable wrinkles

Friction on soft materials is strongly correlated with the associated deformation, which may be controlled by the surface topography. We investigate the wearless sliding friction between a rigid hemispherical indenter and a deformable textured surface, which is shape-tunable wrinkles. The size of the indenter is comparable to the wavelength of the wrinkles. We evaluate the effects on the friction of the aspect ratio of the wrinkles, the applied normal load, and the alignment direction of the wrinkles relative to the sliding direction. The frictional oscillations are observed during sliding in the direction perpendicular to the alignment using optical images and friction profiles. The correlation of friction force oscillation with deformation of the wrinkles is elucidated using Hertz contact theory. Within a cycle of frictional oscillation, the friction force increases as the front part of the indenter elastically plows the crests. When the normal load is high and/or the aspect ratio of the wrinkles is low, the indenter continues to squash the wrinkles and remains in contact with them during sliding. Consequently, the amplitude of friction force oscillation relative to the averaged friction force decreases.

History-dependent friction and slow slip from time-dependent microscopic junction laws studied in a statistical framework

To study how macroscopic friction phenomena originate from microscopic junction laws, we introduce a general statistical framework describing the collective behavior of a large number of individual microjunctions forming a macroscopic frictional interface. Each microjunction can switch in time between two states: a pinned state characterized by a displacement-dependent force and a slipping state characterized by a time-dependent force. Instead of tracking each microjunction individually, the state of the interface is described by two coupled distributions for (i) the stretching of pinned junctions and (ii) the time spent in the slipping state. This framework allows for a whole family of microjunction behavior laws, and we show how it represents an overarching structure for many existing models found in the friction literature. We then use this framework to pinpoint the effects of the time scale that controls the duration of the slipping state. First, we show that the model reproduces a series of friction phenomena already observed experimentally. The macroscopic steady-state friction force is velocity dependent, either monotonic (strengthening or weakening) or nonmonotonic (weakening-strengthening), depending on the microscopic behavior of individual junctions. In addition, slow slip, which has been reported in a wide variety of systems, spontaneously occurs in the model if the friction contribution from junctions in the slipping state is time weakening. Next, we show that the model predicts a nontrivial history dependence of the macroscopic static friction force. In particular, the static friction coefficient at the onset of sliding is shown to increase with increasing deceleration during the final phases of the preceding sliding event. We suggest that this form of history dependence of static friction should be investigated in experiments, and we provide the acceleration range in which this effect is expected to be experimentally observable.

Texture-induced modulations of friction force: the fingerprint effect

Modulations of the friction force in dry solid friction are usually attributed to macroscopic stick-slip instabilities. Here we show that a distinct, quasistatic mechanism can also lead to nearly periodic force oscillations during sliding contact between an elastomer patterned with parallel grooves, and abraded glass slides. The dominant oscillation frequency is set by the ratio between the sliding velocity and the grooves period. A model is derived which quantitatively captures the dependence of the force modulations amplitude with the normal load, the grooves period, and the slides roughness characteristics. The model's main ingredient is the nonlinearity of the friction law. Since such nonlinearity is ubiquitous for soft solids, this "fingerprint effect" should be relevant to a large class of frictional configurations and have important consequences in human digital touch.

Conformal adhesion enhancement on biomimetic microstructured surfaces

Inspired by the superior adhesive ability of the gecko foot pad, we report an experimental study of conformal adhesion of a soft elastomer thin film on biomimetic micropatterned surfaces (micropillars), showing a remarkable adhesion enhancement due to the surface patterning. The adhesion of a low-surface-energy poly(dimethylsiloxane) tape to a SU-8 micropatterned surface was found be able to increase by 550-fold as the aspect ratio increases from 0 to 6. The dependency of the adhesion enhancement on the aspect ratio is highly nonlinear. A series of peeling experiment coupled with optical interference imaging were performed to investigate the adhesion enhancement as a function of the height of the micropillars and the associated delamination mechanisms. Local elastic energy dissipation, side-wall friction, and plastic deformations were analyzed and discussed in terms of their contributions to the adhesion enhancement. We conclude that the local adhesion and friction events of pulling micropillars out of the embedded polymer film play a primary role in the observed adhesion enhancement. The technical implications of this local friction-based adhesion enhancement mechanism were discussed for the effective assembly of similar or dissimilar material components at small scales. The combined use of the micro/nanostructured surfaces with the van der Waals interactions seem to be a potentially more universal solution than the conventional adhesive bonding technology, which depends on the chemical and viscoelastic properties of the materials.