Microstructured superhydrorepellent surfaces: Effect of drop pressure on fakir-state stability and apparent contact angles

In this paper we present a generalized Cassi-Baxter equation to take into account the effect of drop pressure on the apparent contact angle theta_{app}. Also we determine the limiting pressure p_{W} which causes the impalement transition to the Wenzel state and the pull-off pressure p_{out} at which the drop detaches from the substrate. The calculations have been carried out for axial-symmetric pillars of three different shapes: conical, hemispherical topped and flat topped cylindrical pillars. Calculations show that, assuming the same pillar spacing, conical pillars may be more incline to undergo an impalement transition to the Wenzel state, but, on the other hand, they are characterized by a vanishing pull-off pressure which causes the drop not to adhere to the substrate and therefore to detach very easily. We infer that this property should strongly reduce the contact angle hysteresis as experimentally osberved in Ref. \cite{Martines-Conical-Shape}. It is possible to combine large resistance to impalement transition (i.e. large value of p_{W}) and small (or even vanishing) detaching pressure p_{out} by employing cylindrical pillars with conical tips. We also show that depending on the particular pillar geometry, the effect of drop pressure on the apparent contact angle theta_{app} may be more or less significant. In particular we show that in case of conical pillars increasing the drop pressure causes a significant decrease of theta_{app} in agreement with some experimental investigations \cite{LafunaTransitio}, whereas theta_{app} slightly increases for hemispherical or flat topped cylindrical pillars.Comment: 21 pages, 13 figure

The multiple V-shaped double peeling of elastic thin films from elastic soft substrates

M. P. is supported by the European Commission H2020 under Graphene Flagship Core 1 No. 696656 (WP14 “Polymer composites”) and FET Proactive “Neurofibres” Grant No. 732344

Non-linear double-peeling: Experimental vs. theoretical predictions

The double peeling of detachment of non-linear adhesive tapes from a flat Poly(methylmethacrylate) (PMMA) surface has been investigated from both experimental and theoretical point of view. Double peeling tests show that, as the detachment process advances, the peeling angle stabilizes on a limiting value {\theta}lim corresponding to a critical pull-off force Fc above which the tape is completely detached from the substrate. This observed behavior is in good agreement with results obtained following the new theory of multiple peeling and taking into account the hardening-softening non-linear behavior of the experimentally tested adhesive tapes and clarifies some aspects of the experimental data. In particular, the theoretical model shows that the value of the limiting peeling angle depends on the geometry of the adhesive tape as well as on the stiffness properties and on the interfacial energy {\Delta}{\gamma}. Finally, theoretical predictions confirm that solutions with a peeling angle lower than {\theta}lim are unstable.Comment: 14 pages, 8 figure

Optimal adhesion control via cooperative hierarchy, grading, geometries and non-linearity of anchorages

Optimization of dry adhesion in biological organisms is achieved using various strategies at different scale levels. In the past, studies have shown how contact splitting is used effectively by animals such as geckos and insects to increase the total peeling line of contacts and therefore the adhesion force. Also, tapering of contacts or grading of their mechanical properties has been shown to be instrumental in the achievement of improved adhesion efficiency. On a more macroscopic scale, structures such as spider web anchorages exploit hierarchical structure or nonlinear constitutive material properties to improve resilience and to achieve tunability in adhesion/detachment characteristics. Here, we analyse some of these properties and propose some mechanisms for the optimization of adhesion that have thus far been neglected in modelling approaches, and could be potentially exploited for the design of bioinspired adhesives. We consider hierarchical structure, contact tapering, grading of mechanical properties, and their interaction. It emerges that these mechanisms contribute on various size scales to the achievement of optimal adhesive properties through structural complexity and hierarchical organization

Existence of a Tribo-Modified Surface Layer on SBR Elastomers: Balance Between Formation and Wear of the Modified Layer

In most of the tribological contacts, the composition and tribological properties of the original interface will change during use. The tribo-films, with modified properties compared to the bulk, are dynamic structures that play a significant role in friction. The existence of a tribo-modified surface layer and its importance on the overall friction of elastomers has been shown both theoretically and experimentally before. The characteristics of the modified surface layer deserve specific attention since the tribological properties of elastomers in contact with a rough counter-surface are determined by these modified surfaces together with the properties of bulk of the material. Both the formation of the modified layer and the break down (wear) of it are of importance in determining the existence and thickness of the tribo-modified layer. In this study, the importance of the wear is emphasized by comparing two styrene butadiene rubber-based elastomers in contact with a granite sphere. A current status of perception of the removal and the stability of the modified surface layers on rubbers is introduced as well as experimental work related to this matter and discussion within literature. Pin-on-disk friction tests are performed on two SBR-based samples in contact with a granite sphere under controlled environmental conditions to form the modified surface layer. Although the hysteresis part of the friction force which has a minor contribution in the overall friction is not markedly different, the total measured friction coefficient differs significantly. Mechanical changes both inside and outside the wear track are determined by atomic force microscope nano-indentations at different timescales to examine the modified surface layer on the test samples. The specific wear rates of the two tribo-systems are compared, and the existence of the modified surface layer, the different measured friction coefficient and the running-in distances toward steady-state friction are explained considering different wear rates. A conceptual model is presented, correlating the energy input into the tribo-system and the existence of a modified surface layer

A note on Merwin's measurements of forward flow in rolling contact

The first quantitative analysis of the forward flow in frictionless rolling contact, firstly discovered experimentally by Crook [Proc. Inst. Mech. Eng. London 171 (1957) 187], was conducted by Merwin [Plastic deformation of surfaces in rolling, Ph.D. Dissertation, Cambridge University, UK, 1962] who attempted to model the ratchetting phenomenon in excess of shakedown (the cumulative forward flow due to continuous shear strain increase observed in experiments) as a function of load using a simple perfect plasticity model and a simplified solution to the elasto-plastic problem. However, later FEM analysis [J. Appl. Mech., Trans. ASME 52 (1985) 67, 75] and more refined calculations still based on perfect plasticity but using distributed dislocations [J. Mech. Phys. Solids 33 (1987) 61], found that the ratchet rate was much higher than what measured in experiments, showing the Merwin’s approximate solution method was not effective. However, later analysis have concentrated on sophisticated non-linear hardening laws, also because the ratchetting strain rate was found to slowly decay in rail steel materials. This note is focused on another, less known, aspect of the original Merwin’s analysis: his material data were limited to monotonic curves, but his yield limit choice corresponds for around 1% for mild steel and Dural, but to nearly 25% deformation in copper, indicating that hardening plays a significant role into the mechanics of the problem, and that Merwin had taken this into account a posteriori by looking at the load where ratchetting begins.The paper suggests that the cyclic strain growth can be divided into two sequential phenomena: the first, assuming there is no long term material ratchetting (MR), i.e. a calculation based upon elastic properties and a monotonic stress–plastic strain curve, and a second, steady state, for a hardened structure, depending only on MR. In the first phase, we assume the plastic flow is dominated by structural ratchetting (SR), i.e. assuming the ratchetting is well described by the perfectly plastic prediction, where the yield limit is increased according to the level of deformation. This process leads to a quick saturation and the following deformation is attributed to the steady-state material response which we denominate MR. Further, it is shown that experimental measurements of Merwin have more to do with MR than SR

Rate-dependent adhesion of viscoelastic contacts, Part I: Contact area and contact line velocity within model randomly rough surfaces

International audienceIn this work, we investigate dissipative effects involved during the detachment of a smooth spherical glass probe from a viscoelastic silicone substrate patterned with micro-asperities. As a baseline, the pull-off of a single asperity, millimeter-sized contact between a glass lens and a smooth poly(dimethylsiloxane) (PDMS) rubber is first investigated as a function of the imposed detachment velocity. From a measurement of the contact radius () and normal load during unloading phase, the dependence of the strain energy release rate on the velocity of the contact line = ∕ is determined under the assumption that viscoelastic dissipation is localized at the edge of the contact. These data are incorporated into Muller's model (Muller, 1999) in order to predict the time-dependence of the contact size. Similar pull-off experiments are carried out with the same PDMS substrate patterned with spherical micro-asperities with a prescribed height distribution. From in situ optical measurements of the micro-contacts, scaling laws are identified for the contact radius and the contact line velocity. On the basis of the observed similarity between macro and microscale contacts, a numerical solution is developed to predict the reduction of the contact radius during unloading