Inspired by the dynamic wet adhesive systems in nature, various artificial adhesive surfaces have been developed but still face different challenges. Crucially, the theoretical mechanics of wet adhesives has never been sufficiently revealed. Here, we develop a novel adhesive mechanism for governing wet adhesion and investigate the biological models of honeybee arolium for reproducing the natural wet adhesive systems. Micro-nano structures of honeybee arolium and arolium-prints were observed by Cryogenic scanning electron microscopy (Cryo-SEM), and the air pockets were found in the contact interface notably. Subsequently, the adhesive models with a three-phase composite interface (including air pockets, liquid secretion, and hexagonal frames of arolium), were formed to analyze the wet adhesion of honeybee arolium. The results of theoretical calculations and experiments indicated an enhanced adhesive mechanism of the honeybee by liquid self-sucking effects and air-embolism effects. Under these effects, normal and shear adhesion can be adjusted by controlling the proportion of liquid secretion and air pockets in the contact zone. Notably, the air-embolism effects contribute to the optimal coupling of smaller normal adhesion with greater shear adhesion, which is beneficial for the high stride frequency of honeybees. These works can provide a fresh perspective on the development of bio-inspired wet adhesive surfaces.

Three-dimensional (3D) frictional contact model of functionally graded magneto-electro-elastic (FGMEE) material with a conducting spherical punch under electromagnetic fields is presented. Two types of imperfect bonding interface of layers, dislocation-like interface and force-like interface, are considered. Frequency response functions (FRFs) for multilayered MEE material with imperfect interface subjected to unit mechanical, electric, and magnetic loads are derived. The FRFs are used with the semi-analytical method (SAM) to solve present multiphysical contact problem. The present model is verified by comparing with literatures and the finite element method (FEM) and used to study the contact problem of FGMEE film imperfectly bonded on homogenous MEE half-space under electromagnetic fields. Parametric studies are conducted to reveal the effects of imperfect interfaces and also film properties including gradient index and thickness.

Lubricant oil is crucial to the rolling bearings as the main medium of lubricating, cooling, cleaning, and so on. The oil starvation in and around the contacts is harmful to the performance and fatigue life of rolling bearings. Therefore, it is of necessity to understand the behaviors of oil transfer and the patterns of air–oil two-phase flow in bearings, especially with the influence of different capillary properties. This work established a transient air–oil two-phase flow model in a ball bearing based on computational fluid dynamics (CFD). Groups of cases are implemented to investigate the behaviors of oil transfer and air–oil flow under different capillary conditions with speed, surface tension, and viscosity. Flow patterns are classified by the morphological features of the air–oil flow. Staged phenomena are analyzed with flow patterns and reach good agreements with the observations from experiments. It is found that the oil distribution and air–oil flow behaviors in a ball bearing are strongly related to the speed and the ratio of oil viscosity and air–oil surface tension (μ_oil/σ). The flow maps imply that the levels of capillary number (Ca) may be the boundaries and the critical points of flow pattern transition between the different flow patterns in bearing.

There are few experimental results available on film thickness at speeds above 5 m/s and they are almost all based on the optical ball-on-disc test rig. In contrast to the contacts in a rolling bearing, in which the lubricant in the oil reservoir distributes symmetrically, ball-on-disc contact shows asymmetry of lubricant distribution due to centrifugal effects. In order to closely imitate the contact occurring between the ball and the outer ring of a ball bearing, this study proposes an experimental model based on ball-on-glass ring contact. An optical matrix method is used to analyze the optical system, which is composed of a steel ball-lubricant-chromium-coated glass ring. Based on the optical analysis, the measurement system is improved in order to obtain a high quality interference image, which makes it possible to measure the film thickness at high-speeds conditions.