Attractive forces slow contact formation between deformable bodies underwater

Thermodynamics tells us to expect underwater contact between two hydrophobic surfaces to result in stronger adhesion compared to two hydrophilic surfaces. However, the presence of water changes not only energetics but also the dynamic process of reaching a final state, which couples solid deformation and liquid evacuation. These dynamics can create challenges for achieving strong underwater adhesion/friction, which affects diverse fields including soft robotics, biolocomotion, and tire traction. Closer investigation, requiring sufficiently precise resolution of film evacuation while simultaneously controlling surface wettability, has been lacking. We perform high-resolution in situ frustrated total internal reflection imaging to track underwater contact evolution between soft-elastic hemispheres of varying stiffness and smooth–hard surfaces of varying wettability. Surprisingly, we find the exponential rate of water evacuation from hydrophobic–hydrophobic (adhesive) contact is three orders of magnitude lower than that from hydrophobic–hydrophilic (nonadhesive) contact. The trend of decreasing rate with decreasing wettability of glass sharply changes about a point where thermodynamic adhesion crosses zero, suggesting a transition in mode of evacuation, which is illuminated by three-dimensional spatiotemporal height maps. Adhesive contact is characterized by the early localization of sealed puddles, whereas nonadhesive contact remains smooth, with film-wise evacuation from one central puddle. Measurements with a human thumb and alternatively hydrophobic/hydrophilic glass surface demonstrate practical consequences of the same dynamics: adhesive interactions cause instability in valleys and lead to a state of more trapped water and less intimate solid–solid contact. These findings offer interpretation of patterned texture seen in underwater biolocomotive adaptations as well as insight toward technological implementation.

ENTROPY GENERATION IN MHD FLOW THROUGH A DEFORMABLE POROUS LAYER WITH CONSTANT INJECTION/ SUCTION AT THE BOUNDARY

The present paper examines the entropy generation on MHD flow of viscous fluid over a deformable vertical porous layer with constant injection/ suction velocity at the boundary walls of the layer. The combined phenomenon of the solid deformation and fluid movement in the porous medium are taken into consideration. The influence of relevant non-dimensional parameters on the fluid velocity, solid displacement, temperature and concentration profiles are discussed. Also, the impact of Brinkman number, volume fraction parameter and drag parameter on entropy generation and Bejan number are discussed.

The Influence of Mechanical Deformations on Surface Force Measurements

Surface Force Balance (SFB) experiments have been performed in a dry atmosphere and across an ionic liquid, combining the analysis of surface interactions and deformations, and illustrate that the mechanical deformations of the surfaces have important consequences for the force measurements. First, we find that the variation of the contact radius with the force across the ionic liquid is well described only by the Derjaguin–Muller–Toporov (DMT) model, in contrast with the usual consideration that SFB experiments are always in the Johnson–Kendall–Roberts (JKR) regime. Secondly, we observe that mica does not only bend but can also experience a compression, of order 1nm with 7μm mica. We present a modified procedure to calibrate the mica thickness in a dry atmosphere, and we show that the structural forces measured across the ionic liquid cannot be described by the usual exponentially decaying harmonic oscillation, but should be considered as a convolution of the surface forces across the liquid and the mechanical response of the confining solids. The measured structural force profile is fitted with a heuristic formulation supposing that mica compression is dominant over liquid compression, and a scaling criterion is proposed to distinguish situations where the solid deformation is negligible or dominant.

Progress on In Situ and Operando X-ray Imaging of Solidification Processes

In this review, we present an overview of significant developments in the field of in situ and operando (ISO) X-ray imaging of solidification processes. The objective of this review is to emphasize the key challenges in developing and performing in situ X-ray imaging of solidification processes, as well as to highlight important contributions that have significantly advanced the understanding of various mechanisms pertaining to microstructural evolution, defects, and semi-solid deformation of metallic alloy systems. Likewise, some of the process modifications such as electromagnetic and ultra-sound melt treatments have also been described. Finally, a discussion on the recent breakthroughs in the emerging technology of additive manufacturing, and the challenges thereof, are presented.

Modelling coupled fluid flow and solid deformation with the viscoplastic rheology

<p>The coupling between fluid flow and solid deformation plays important roles in earth dynamics at different timescales and length-scales. Related processes include, magma migration and focusing in the Mid-Ocean Ridges, fluid migration after slab dehydration in the subduction zone, channelized fluid flow observed as seismic chimney in the continental margin, as so on. Here we study how localized fluid channels can develop through asymmetric compaction and decompaction processes of the solid matrix by solving coupled two-phase equations with viscoplastic rheology. Previous studies produced fluid channels with decompaction weakening, while negative effective pressure (P<sub>t</sub>-P<sub>f</sub>) is inevitable due to the simplified rheology formulation. We develop a viscoplastic rheology formulation that considers the effects of shear stress and plastic failure on the volumetric deformation, consistent with experimental data.</p><p>Our model results show that this new rheology can produce channelized fluid flow without negative effective pressure in the model. Our numerical results also clarified that it is the flow instability of the coupled two-phase system that cause the formation of fluid channels. The ratio between shear viscosity and bulk viscosity determines how fast the flow instability develops and manifests. The geometry of the Reservoir, on the other hand, can affect where the channels form. We further study the effects of different background and reservoir porosity, different rock layer, permeability exponents, decompaction weakening factor, and so on. These results provide a better understanding of the two-phase system and its potential applications in geological environments.   </p><p> </p>

On the consolidation behaviour of double-porosity geomaterials

<p>Many natural and engineered geomaterials have double-porosity structure where two dominant pore systems coexist. Examples include structured soils where the two pore systems are inter-aggregate pores and intra-aggregate pores, and fissured rocks where the two pore systems are fissures and matrix pores. Although such double-porosity materials are frequently observed in geosciences and geoengineering applications, it remains mostly unclear how fluid flow and solid deformation interact differently in single- and double-porosity materials. The presentation explores this question through numerical simulation of consolidation – a paradigmatic problem in poromechanics – based on a recently developed modelling framework for fluid-infiltrated, inelastic materials with double porosity. Built on a combination of continuum principles of thermodynamics and standard plasticity theory, the framework can capture deformation, flow, and their coupling that occur individually in each pore system. Simulation results using this framework suggest that double-porosity structure gives rise to a two-staged consolidation behaviour, where the second stage appears similar to secondary compression in clays. It is also found that the simulated two-staged behaviour bears a striking semblance to experimentally observed consolidation processes in shales. These findings suggest that double-porosity structure may exert dominant control over the long-term hydro-mechanical behaviour of geomaterials.</p>

Modelling the effect of mechanical deformation on the peatland carbon stock resilience

<p>Mathematical models of peatland growth have been developed for many purposes, including understanding the effect of past or future climate change on peatland carbon accumulation. This is important because peatland contains a vast amount of carbon and has a significant role in the global carbon cycle through carbon dioxide and methane exchange with the atmosphere. In general, the models produced so far suffer from the fact that the mechanical process has an essential role in the peatland carbon stock resilience because they only focus on ecohydrological feedback. We propose a one-dimensional mathematical model that includes ecological, hydrological, and mechanical feedback on the peatland through the poroelasticity concept, which coupling between fluid flow and solid deformation. The formulation is divided into two categories, fully saturated and unsaturated, to accommodate peatland characteristics. We compare the numerical solution of the fully saturated case with analytical solutions of Terzaghi’s problem for validation. We assume that peat is an elastic material with flat, impermeable, and stiff substrate properties.  Based on the initial simulation results,  we find that compression reduces the thickness of acrotelm, leading to the shorter residence time of plant litter, and consequently, higher cumulative carbon is obtained. Furthermore, mechanical deformation of the pore structure effectively maintains carbon stock in the peatland against climate change because it reduces water table depth fluctuations.</p>