Capillary windlass: Exploiting spider thread properties to engineer bioinspired micron-sized capillary motors – Capillary_Windlass

Our project is to study a natural mechanism taking place in spider webs and to reproduce this mechanism with synthetic materials in order to develop industrial applications at the microscale. A spider web comprises three kind of threads: radial, frame, and capture threads. Our object of interest is the capture thread with which the web spiral is constructed. The biological function of these capture threads is to hold the web together and to catch flying insects, that is to act as tension cables and shock absorbers. A key feature of the capture thread is that it is decorated by regularly spaced liquid droplets, thought to have the role of a glue that would stick to the incoming prey. It appears that these glue droplets have yet another function. In 1989 it was proposed by one of the project participant that the glue droplets would draw the capture thread inside them, thereby holding the thread in tension. During a shock event the spare amount of fiber in the droplet would be used to provide extensibility to the system. Moreover the dissipation generated by relative motion of the fluid and the fiber would absorb energy. This hypothesis has been coined the windlass mechanism, a windlass being a winch used to pull heavy weights. For different reasons this mechanism is only partly acknowledged and other scenario to explain the extensional properties of capture threads have been proposed.

We want to mechanically investigate this windlass mechanism, prove it to be the main cause of the great extensibility of capture threads, and draw biological implications. Furthermore, using a biomimetic approach, we want to reproduce the mechanism on synthetic fibers and droplets, and to explore possible applications of this mechanism in micro-systems. Finally, as the liquid droplets are the key component in the system, we will develop new contactless tools to measure their physical properties. More precisely we will have to be able to measure surface tension, elasticity, and viscosity of the droplet in-situ, that is when they lie on the fiber. To this end we will design contactless measurement techniques, using laser or acoustic radiation pressures to poke the droplet and monitor its subsequent relaxation.

This project has its source in a biological system that we will study mechanically and physically with the goal to develop new micro-systems, sensors or actuators, and new measurement techniques.