Eindhoven version of famous Huygens experiment shows thin plastics can synchronize their swing

Researchers at the Eindhoven University of Technology (TU/e) have modestly followed in the footsteps of the famous Dutch physicist Christiaan Huygens by studying synchronization in plastics. They demonstrate for the first time that two plastic films can communicate with each other through a piece of plastic of the same material – oscillating in-phase and out-of-phase during light irradiation. This coupled deformation is relevant for the development of new soft robotic systems. The researchers published their results this week in the journal Nature Materials.

Legend has it that in 1665 Christiaan Huygens was lying ill in bed and he observed the swing of two pendulum clocks mounted next to each other on a common wooden beam placed at the top of two chairs. To his surprise, he noticed the two pendulums often swinging towards and away from each other in perfect harmony.

That was the first observation of synchronization – the state of two or more events happening at the same time – of pairs of moving pendulums. Since the 1700s, synchronization has been observed everywhere in nature: from the regularity of the circadian rhythm to the synchronized firing of pacemaker cells giving the regular impulse for our hearts to beat.

Eindhoven version of the famous experiment

The story continues today in the laboratories of the Institute of Complex Molecular Systems (ICMS) at TU/e, in a collaboration between the departments of Chemical Engineering and Chemistry and Mechanical Engineering. A team of researchers led by prof. Dick Broer now show that not only metronomes and pendulum clocks can synchronize their swing but plastics as well.

In the newer Eindhoven version of the famous experiment, the oscillators are flexible plastic films that are brought into a swinging motion by means of a focused ultraviolet (UV) light-emitting LED lamp. The plastic is built from a well-organized set of molecules connected in their liquid crystal state, in the same way as it occurs within our LCD monitors or televisions.

The UV irradiation heats the film and the resulting stress bends the film at the location of the light spot. The film then starts to oscillate as a result of self-shadowing: the film bends until the tip blocks the light and shadows the ‘hinge’. The hinge, now in the shadow, cools down and the film unbends. As a result, the light beam hits the film at the same spot again and the film bends back.

The repetition of this sequence gives rise to the oscillations. In a way, this can be compared with a children’s swing which is pushed each time it comes back to the parent. In this case, the parent is replaced by the LED lamp. The frequency of the swing is constant and well defined by the swing dimensions and its mechanical properties.

Ghislaine Vantomme, first author of the paper in Nature Materials.

New robotic systems

Now, following Huygens’ experiment again, when two of these plastic swings are connected through a plastic film of the same material, their swinging motions, although initially being random and independent from each other, become coupled and synchronized. The synchronization can be either in-phase or out-of-phase or can even take more complex synchronized figures. New to Huygens’ version is that the pendulums are made from flexible, bendable plastic and that the swing is initiated and maintained through a light source.

The importance of these findings, published in Nature Materials, is that they demonstrate that two plastic films can communicate with each other through a piece of plastic consisting of the same material. Coupled motion is relevant for the development of new ‘soft robotic systems’ that are made from soft and compliable materials, and could possibly be used in medicine or manufacturing. They can perform functions like transporting themselves by synchronized kicks, performing coordinated actions like gripping and releasing of objects, and carry out a series of collective actions initiated by a single external or internal trigger.

Ghislaine Vantomme, Lars C. M. Elands, Anne Helene Gelebart, E. W. Meijer, Alexander Y. Pogromsky, Henk Nijmeijer and Dirk J. Broer, “Coupled liquid crystalline oscillators in Huygens’ synchrony.”

https://www.nature.com/articles/s41563-021-00931-6

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