Adhesive Wear Regimes on Rough Surfaces and Interaction of Micro-contacts

We develop an analytical model of adhesive wear between two unlubricated rough surfaces, forming micro-contacts under normal load. The model is based on an energy balance and a crack initiation criteria. We apply the model to the problem of self-affine rough surfaces under normal load, which we solve using the boundary element method. We discuss how self-affinity of the surface roughness, and the complex morphology of the micro-contacts that emerge for a given contact pressure, challenge the definition of contact junctions. Indeed, in the context of adhesive wear, we show that elastic interactions between nearby micro-contacts can lead to wear particles whose volumes enclose the convex hull of these micro-contacts. We thereby obtain a wear map describing the instantaneous produced wear volume as a function of material properties, roughness parameters and loading conditions. Three distinct wear regimes can be identified in the wear map. In particular, the model predicts the emergence of a severe wear regime above a critical contact pressure, when interactions between micro-contacts are favored.

Wetting and Capillarity Effects On Bubble Formation From Orifice Plates Submerged in Pools of Water

An experimental study of bubble growth from submerged orifice plates in pools of water is carried out to scale and correlate the effects of surface wettability and orifice diameter D0 on ebullience. Measurements of bubble growth on surfaces with nine different contact angles (38° ≤ θ ≤ 128°) with varying air flow rates (1 to 300 ml/min) were made using high speed videography and image processing. In the static or constant-volume regime, below a critical contact angle θc, the bubble base remains attached to the orifice and the equivalent departure diameter Db is independent of contact angle θ. On the other hand, above the critical contact angle, the bubble base spreads on the surface resulting in larger Db. For θ > θc, Db is strongly dependent on θ and increases with it. Using minimum energy method, it is shown that the wettability effects can be scaled and correlated by a modified capillary length, defined as a function of the Laplace length and contact angle. The proposed correlation provides predictions of Db that agree with experimental data of this study as well as those available in the literature to within ±15 %. Moreover, for a hydrophobic surface when D0 > twice the modified capillary length, the bubble grows inside the orifice; for a hydrophilic surface this scales with twice the capillary length and effect of θ is not seen.

Direct observation of the impact of water droplets on oil replenishment in EHD lubricated contacts

Water is one of the most significant causes of lubrication failure. There is little research on the direct observation of the impact of water on lubrication properties. In this study, the influence of water on oil replenishment under different elastohydrodynamic (EHD) lubricating conditions is evaluated using optical interferometry and infrared microscopy, and a dimensionless criterion when water influences the film thickness is proposed. Evidence shows that the scour displacing effect and emulsification of water/oil are the main reasons for the reduction in film thickness. Once a water droplet enters an oil reservoir around the critical contact zone, it hardly moves away. This aggravates starvation, reduces the center film thickness of the contact, and leads to lubrication failure of the mechanical components.

Percolation and Reynolds Flow in Elastic Contacts of Isotropic and Anisotropic, Randomly Rough Surfaces

In this work, we numerically study the elastic contact between isotropic and anisotropic, rigid, randomly rough surfaces and linearly elastic counterfaces as well as the subsequent Reynolds flow through the gap between the two contacting solids. We find the percolation threshold to depend on the fluid flow direction when the Peklenik number indicates anisotropy unless the system size clearly exceeds the roll-off wave length parallel to the easy flow direction. A critical contact area near 0.415 is confirmed. Heuristically corrected effective-medium treatments satisfactorily provide Reynolds fluid flow conductances, e.g., for isotropic roughness, we identify accurate closed-form expressions, which only depend on the mean gap and the relative contact area. Graphic Abstract

Comparative ACE2 variation and primate COVID-19 risk

The emergence of SARS-CoV-2 has caused over a million human deaths and massive global disruption. The viral infection may also represent a threat to our closest living relatives, nonhuman primates. The contact surface of the host cell receptor, ACE2, displays amino acid residues that are critical for virus recognition, and variations at these critical residues modulate infection susceptibility. Infection studies have shown that some primate species develop COVID-19-like symptoms; however, the susceptibility of most primates is unknown. Here, we show that all apes and African and Asian monkeys (catarrhines), exhibit the same set of twelve key amino acid residues as human ACE2. Monkeys in the Americas, and some tarsiers, lemurs and lorisoids, differ at critical contact residues, and protein modeling predicts that these differences should greatly reduce SARS-CoV-2 binding affinity. Other lemurs are predicted to be closer to catarrhines in their susceptibility. Our study suggests that apes and African and Asian monkeys, and some lemurs, are likely to be highly susceptible to SARS-CoV-2. Urgent actions have been undertaken to limit the exposure of great apes to humans, and similar efforts may be necessary for many other primate species.

The neurosociology of Auguste Comte

This article explores the role of phrenology in the work of Auguste Comte. I begin by reviewing the historical and contemporary significance of this doctrine to show the direct lineage that exists between phrenology and what we now call cognitive neuroscience. I then demonstrate the impact of phrenology on Comte’s sociological theory and make the claim that his paradigm exemplifies what TenHouten called ‘neurosociology.’ Following this, I show how Comte’s social epistemology rejected biological reductionism and considered neurophysiology a subfield of sociology. This results in a somewhat startling assertion: Comte saw sociology as a cognitive science. After outlining Comte’s original vision for sociology as a discipline, I place Comte’s system in critical contact with 21st century neuroscience and suggest fruitful ways to move the neurosociology paradigm forward. In this I demonstrate that Comte’s vision for sociology is worth pursuing today, even while the specifics of his doctrine are not.

Measurement of wetting angles for powders by sessile drop method

It has been established that the time to establish the critical contact angle of wetting of the surface of a pressed sample made of polycrystalline quartz powder with various liquids is mainly determined by the viscosity of the liquid. It is shown that in order to determine the critical contact angle of wetting of powder materials, it is necessary to fix the time moment at which the diameter of the base of a liquid drop on the surface of a powder sample has a maximum value.

Salt creeping as a self-amplifying crystallization process

Salt creeping is a ubiquitous phenomenon in which crystals precipitate far from an evaporating salt solution boundary, which constitutes a major problem in outdoor electronics, civil engineering, artworks, and agriculture. We report a novel experimental approach that allows to quantitatively describe the creeping mechanism and demonstrate its universality with respect to different salts. We show that there exists a critical contact angle below which salt creeping occurs, provided also the nucleation of multiple crystals is favored. The precipitation of new crystals happens ahead of the contact line by the meniscus that progressively advances over the crystals forming also nanometric precursor films. This enlarges the evaporative area, causing an exponential increase in the crystal mass in time. The self-amplifying process then results in a spectacular three-dimensional crystal network at macroscopic distances from the solution reservoir. These findings also allow us to control the creeping by using crystallization modifiers.