Fluid mechanics of grab/release and volume scavenging instabilities:

Cornell University, Ithaca NY

Investigators

Abstract

CBET-0653831 P. H. Steen, Cornell University - Endowed Surface tension at a liquid/gas interface is known to be cohesive and liquid capillarity at small scales exerts an attraction between adjacent solids. The super-adhesion exhibited by the palm beetle is capillarity-based. Its ability to grab and release depends on manipulating a large number N of small volumes of liquid held by surface tension. This study of the fluid mechanics of release has a focus on the volume-scavenging instabilities that can occur when two or more capillary surfaces (e.g. droplets) connect, enabling volume exchange. Modeling and computation will provide the scientific basis for the target task -- to experimentally realize a man-made adhesive pad inspired by the beetle -- and to better understand the beetle's remarkable capability. Intellectual merit. The normally weak force of surface tension can be effectively amplified by parallel action. The palm beetle, an insect native to Florida, exhibits extra-ordinary adhesion ('super-adhesion') as a defense mechanism using just this strategy. Its stickiness derives from making contact with 120,000 tiny droplets of oil. Together, these little liquid bridges make a bond that can withstand a force up to 100 times the beetle's body weight. Perhaps, most surprisingly, this bond can be released in less than a second. How does the beetle release itself? Provided volume exchange between liquid bridges is possible, one bridge can scavenge volume from its neighbors. When does this instability occur and, if it does occur, can it ease detachment? Can scavenging be avoided or enhanced in a man-made device? It is proposed to answer these questions. They belong to a study of the stability of a system of N coupled nonlinear liquid components. Statics gives the energy landscape and the dynamics of volume transfer is important when scavenging competes with the detachment time-scale. Broader Impacts. Nature provides a variety of controllable adhesion examples (house-fly, gecko, etc). The beetle is distinguished by its control of pico-liter volumes of liquid oil. Understanding the beetle's mechanism(s) of release is (are) important not only to the naturalist but to the engineer. The feasibility of man-made adhesives, of strength per unit weight comparable to what the beetle exhibits and that can be turned on/off (grab/release) on the order of a second, is addressed by the proposed study. Hence, immediate impact will be on man's capability to build a switchable super-adhesive pad for a breadth of applications (ceiling-hanging robotic devices and so forth). More broadly, it is well-accepted that adhesion between two solids and spreading at a liquid-solid interface are closely related to the issue of friction between moving parts and dissipation in the liquid coating of solids. More efficient coating processes would impact both industrial production and daily human activities. The highest level impact will be on the science base for understanding the stability of systems of N-coupled capillary elements. Finally, an impact through education of students will accrue from the study, as well as outreach to the public. Numerous accounts of nature-inspired technology in the popular press strike a positive chord, as they should.