Snapping like a Venus

If you touch me I eat you.

Only water with distilled or rain water. Leave it outside or close to a window. Do not add fertilizers.

These are the gardening advises to grow Dionae Muscipula, also known as the Venus Flytrap. As it is a carnivorous plant, feeding it instead of leaving it hunt, will lead to the growth of a traditional succulent plant, with very small traps. The less it has food, the more it will need to hunt and the bigger the traps will be, the bigger the flower, and the brighter the color of the traps.

VenusPlant-01

Picture of a Dionae Muscipula with open and closed traps.

But the particularity of the Venus Flytrap is more than that. It is the carnivorous plant that has the fastest movement of the fauna kingdom (to the best of our knowledge). In less than 400 ms, it can close and capture its prey for a slow digestion. The mechanism for the closing is complex and still under study but the main lines can be described. When an insect touches several of the tiny hairs present at the surface of the leaf, it activates ions channels that trigger an electrical signal to the cells constituting the plant as well as osmotic pressure changes. Coupled with these biological changes, the internal structure of the leaves is such that it can only adopt two positions: closed or open. Each of these position is a stable state and the transition from one state to the other happens only with a small triggering, the touch of an insect. This is called bistability.

You can also find bistable systems in man-made materials. Typical example is the slap (or snap) bracelet. In industry, bistable systems are used in large aerospace structures like in the wings of airplanes. When the wind changes direction, the wing adopts the second shape to reduce the drag force. In this case, there is no ion channel nor osmotic pressure change but the internal structure of the material does lead to this bistability.

Most common bistable structures are made of long continuous fibre epoxy laminates, known as prepreg, arranged in a bilayer structure. Prepregs consist of fibers, mostly glass or carbon, coated with resin (epoxy). They are manually handled as some kind of big scotch tape with an additional step, the curing of the epoxy matrix. This curing typically is done in an oven between 60 and 200 °C.

bilayer prepreg-01

Bistable structure made of perpendicular alignment of prepreg composite fibers epoxies.

To make bistable structures out of these prepregs, one should incorporate some pre-stress in the bilayer structure. These stresses will give some curvature and the two stable states. Typically, one can stretch the layers in the direction of the fibers prior to curing. After curing under stretching, the bilayer laminate will be released from the stretching force. In that case, the top layer will want to shrink back, as a piece of rubber would do, in the opposite direction to the one it was stretched. Similar with the bottom layer. Since the stretching was in perpendicular directions between the two layers, so is the shrinking. Due to the symmetry, there is no reason why one layer could recover its original shape more than the other one, therefore the structure has to curve towards either the top or the bottom layer. Both shells are stable configurations. The structure is bistable.

Now it is a pity we are working with those prepregs. They are full of uncured epoxy, a very nasty chemical that you should handle with care. Epoxies are composed of a resin and a hardener. In contact with your skin, the hardener might just react with your molecules, mistaking your skin, and whatever is underneath, with the resin. In addition the prepreg sheets are delivered under the form of a roll. Imagine you want to give some curvature to the sheet, it is a hell! This drastically limits the structural designs to square shells. How can we do more funny shapes, starting with the one of the Venus FlyTrap?

Luckily, a method has been developed previously that enables the alignment of short-scale particles using magnetic fields. More specifically, it uses rotating magnets to align vertically and parallel to each other magnetically-responsive platelets of only a few microns in diameter (see previous posts). Using short scale particles and remote-controlled manipulation via magnets enables not only parallel alignment but also more complex type of alignment.

For example, to imitate the curved shape of a venus leaf, we can benefit from the naturally curved lines from a large magnet. But with such a curved alignment, stretching is not possible. Therefore we need another trick to induce the pre-stress. Since our epoxy could be cured at 100°C and that the direction of the reinforcing particles decreases the thermal expansion of the composite material, we simply build up the stresses by letting the structure cool down from 100°C to room temperature.

MagneticSetUps_Curved-01

Cartoon showing a rotating magnet with curved field lines leading to curved vertical alignment of micrometric reinforcement.

And that’s how we obtained a bistable synthetic Venus leaf! Check the references below to see some pictures and cartoons of the results.

References:

Schmied, J.U., Le Ferrand, H., Ermanni, P., Studart, A.R. & Arrieta, A.F., Programmable snapping composites with bio-inspired architecture, Bioinspir. Biomim. 12 (2017)

http://iopscience.iop.org/article/10.1088/1748-3190/aa5efd/meta

Forterre, Y., Skotheim, J.M., Dumais, J. & Mahadevan, L., How the Venus flytrap snaps, Nature 433 (2005)

http://www.nature.com/nature/journal/v433/n7024/abs/nature03185.html

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