Liquid bridge splitting enhances normal capillary adhesion and resistance to shear on rough surfaces

Pore-scale physics of ice melting within unconsolidated porous media revealed by non-destructive magnetic resonance characterization

Melting of ice in porous media widely exists in energy and environment applications as well as extraterrestrial water resource utilization. In order to characterize the ice-water phase transition within complicated opaque porous media, we employ the nuclear magnetic resonance (NMR) and imaging (MRI) approaches. Transient distributions of transverse relaxation time T2 from NMR enable us to reveal the substantial role of inherent throat and pore confinements in ice melting among porous media. More importantly, the increase in minimum T2 provides new findings on how the confinement between ice crystal and particle surface evolves inside the pore. For porous media with negligible gravity effect, both the changes in NMR-determined melting rate and our theoretical analysis of melting front confirm that conduction is the dominant heat transfer mode. The evolution of mushy melting front and 3D spatial distribution of water content are directly visualized by a stack of temporal cross-section images from MRI, in consistency with the corresponding NMR results. For heterogeneous porous media like lunar regolith simulant, the T2 distribution shows two distinct pore size distributions with different pore-scale melting dynamics, and its maximum T2 keeps increasing till the end of melting process instead of reaching steady in homogeneous porous media.

Modulation of the Capillary Force Profile at the Solid-Solid Interface through Topographical Modifications

Modulations of interfacial adhesion at solid-solid contacts are desired in many multidisciplinary applications. This study aims at the creation of reproducible solid-solid interfaces with significantly mitigated capillary adhesion through physical modifications. First, a continuum boundary element-based mathematical model to predict capillary forces at solid-solid contacts was developed and validated. Next, the model was utilized to simulate the capillary adhesion between a glass substrate with hypothetical surface topographies, in the form of nanopillars and nanowells, and silica particles of various sizes at varied humidity conditions. This study revealed that the nanopillar surface topography was much more effective than the nanowell in suppressing the capillary condensation and could lower the capillary forces by more than one order of magnitude for the micrometer-scale and nanoscale particulates. This study suggested that topographical tuning at the solid-solid interface can significantly reduce interfacial adhesion and promote the dust-resistance characteristic of a substrate. Finally, this simulation study can guide the fabrication of solid surfaces with reproducible topography and optimized geometrical parameters to yield an extremely reduced interfacial capillary adhesion.

Reversible Perspiring Artificial "Fingertips"

Fingertip perspiration is a vital process within human predation, to which the species owes its survival and its biological success. In this paper, the unique human ability of extensive perspiration and controlled friction in self-assembled cholesteric liquid crystals is recreated, mimicking the natural processes that occur in the dermis and epidermis of human skin. This is achieved by inducing porosity in responsive, liquid-bearing material through the controlled-polymerization phase-separation process. The unique topography of human fingerprints is further emulated in the materials by balancing the parallel chirality-induced force and the perpendicular substrate-anchoring force during synthesis. As a result, artificial fingertips are capable of secreting and re-absorbing liquid upon light illumination. By demonstrating the function of the soft material in a tribological aspect, it exhibits a controllable anti-sliding property comparable to human fingertips and subsequently attains a higher degree of biomimicry. This biomimetic fingertip is envisioned being applied in a multitude of fields, ranging from biomedical instruments to interactive, human-like soft robotic devices.

A Scientific Perspective on Reducing Ski-Snow Friction to Improve Performance in Olympic Cross-Country Skiing, the Biathlon and Nordic Combined

Of the medals awarded at the 2022 Winter Olympics in Beijing, 24% were for events involving cross-country skiing, the biathlon and Nordic combined. Although much research has focused on physiological and biomechanical characteristics that determine success in these sports, considerably less is yet known about the resistive forces. Here, we specifically describe what is presently known about ski-snow friction, one of the major resistive forces. Today, elite ski races take place on natural and/or machine-made snow. Prior to each race, several pairs of skis with different grinding and waxing of the base are tested against one another with respect to key parameters, such as how rapidly and for how long the ski glides, which is dependent on ski-snow friction. This friction arises from a combination of factors, including compaction, plowing, adhesion, viscous drag, and water bridging, as well as contaminants and dirt on the surface of and within the snow. In this context the stiffness of the ski, shape of its camber, and material composition and topography of the base exert a major influence. An understanding of the interactions between these factors, in combination with information concerning the temperature and humidity of both the air and snow, as well as the nature of the snow, provides a basis for designing specific strategies to minimize ski-snow friction. In conclusion, although performance on “narrow skis” has improved considerably in recent decades, future insights into how best to reduce ski-snow friction offer great promise for even further advances.