Photothermal resonance for micro

Feb 04, 2024

Researchers in Japan have exploited resonance to get large amplitude motion from rod-like microcrystals, in a way that could be useful in soft robotics according to Waseda University.

Using UV light, they set out to bend a particular type of 2,4-dinitroanisole crystals – chosen because of their large thermal expansion coefficient. The UV wavelength selected matches an absorption peak in the material, resulting in heating.

Close observation revealed a complicated motion when UV was turned on (or off) suddenly, which turned out to be an exponential rise (or fall) to a 1.2° bend over 100ms and, superimposed over that, a ~±0.05° mechanical ringing at the 390Hz mechanical resonant frequency of the crystal.

“We serendipitously discovered fast and small natural vibration induced by the photothermal effect,” said Waseda scientist Hideko Koshima. “Furthermore, we achieved high-speed and large bending by photothermally resonating the natural vibration.”

The team then exploited this discovery of a fast twitch to get more motion, by pulsing the UV at the resonant frequency – image left, 6mm long 0.15mm thick crystal with extended 390Hz stimulation.

Detailed mathematical modelling revealed that this is purely a thermal effect, and so broadly applicable to other materials, and that it is efficient in converting optical energy to mechanical energy.

“Our findings show that any light-absorbing crystal can exhibit high-speed, versatile actuation through resonated natural vibrations,” said Koshima. “This can open doors to the applications of photothermal crystals, leading eventually to real-life soft robots with high-speed actuation capability.”

How a miniature moving ‘robot’ might be propelled by such crystals, according to Waseda University, which worked with Tokyo Institute of Technology on this project.

The work is described in ‘Photothermally induced natural vibration for versatile and high-speed actuation of crystals‘, published in Nature Communications.

The full paper is available to read without payment, and is worth a look if only to see how accurately the maths matched the measurements.