Jianlin Liu

Curvature-driven bubbles or droplets on the spiral surface

Directional motion of droplets or bubbles can often be observed in nature and our daily life, and this phenomenon holds great potential in many engineering areas. The study shows that droplets or bubbles can be driven to migrate perpetually on some special substrates, such as the Archimedean spiral, the logarithmic spiral and a cantilever sheet in large deflection. It is found that a bubble approaches or deviates from the position with highest curvature of the substrate, when it is on the concave or convex side. This fact is helpful to explain the repelling water capability of Nepenthes alata. Based on the force and energy analysis, the mechanism of the bubble migration is well addressed. These findings pave a new way to accurately manipulate droplet or bubble movement, which bring inspirations to the design of microfluidic and water harvesting devices, as well as oil displacement and ore filtration.

Hard to be killed: Load-bearing capacity of the leech Hirudo nipponia

With the evolution for several millions of years, leeches have developed a perfect capability to resist mechanical loads, which provides many inspirations to engineer new materials and new devices. To uncover the mechanism of its strong survival ability, several mechanical approaches, such as compression, tension, adhesion, impact and blood suction experiments were tried. Our experimental results show that a leech (Hirudo nipponia) can surprisingly withstand a compressive force of nearly 106 times its body weight. In tension, this animal demonstrates large deformation and its strain can reach a value bigger than 3. To avoid being removed from the host skin, it produces an adhesion force superior to 118 times its body weight, and it can endure an impact force at least 1500 times its weight. Also the leech skin can bear an internal fluid pressure of around 6 times the atmospheric pressure. These data show that the leech cannot be killed easily through normal mechanical loading approaches. All these amazing performances lie in hierarchical structures and ductility of the skin with highly developed and compact annuluses, and this feature is beneficial to leechs survival.

Nonlinear Vibration of an Elastic Soft String: Large Amplitude and Large Curvature

Mechanical nonlinear vibration of slender structures, such as beams, strings, rods, plates, and even shells occurs extensively in a variety of areas, spanning from aerospace, automobile, cranes, ships, offshore platforms, and bridges to MEMS/NEMS. In the present study, the nonlinear vibration of an elastic string with large amplitude and large curvature has been systematically investigated. Firstly, the mechanics model of the string undergoing strong geometric deformation is built based on the Hamilton principle. The nonlinear mode shape function was used to discretize the partial differential equation into ordinary differential equation. The modified complex normal form method (CNFM) and the finite difference scheme are used to calculate the critical parameters of the string vibration, including the time history diagram, configuration, total length, and fundamental frequency. It is shown that the calculation results from these two methods are close, which are different with those from the linear equation model. The numerical results are also validated by our experiment, and they take excellent agreement. These analyses may be helpful to engineer some soft materials and can also provide insight into the design of elementary structures in sensors, actuators and resonators, etc.

Insights into adhesion of abalone: A mechanical approach

Many living creatures possess extremely strong capability of adhesion, which has aroused great attention of many scientists and engineers. Based on the self-developed equipment, we measured the normal and shear adhesion strength of the abalone underwater and out of water on different contact surfaces. It is found that the adhesion force of the abalone can amount to 200 or 300 times its body weight. The effects of wettability and roughness of the surface, and the frictional coefficient of mucus on the adhesion strength have been discussed. The theoretical calculation manifests that the normal adhesion force mainly stems from the suction pressure, van der Waals force and capillary force of the pedal, and their limit values are given. These findings may provide some inspirations to engineer new-typed materials, micro-devices, adhesives and medicine.