Versatile, high-speed, and efficient crystal actuation with photothermally resonated natural vibrations

(Nanowerk News) Each material possesses a distinctive natural vibrational frequency, and when an external periodic force is applied at or near this frequency, the vibrations are significantly amplified. In the realm of physics, this phenomenon is referred to as "resonance." Resonance permeates our everyday lives and can be considered either beneficial or detrimental, depending on the circumstances. For example, musical instruments like guitars rely on resonance for sound amplification, while buildings and bridges are more susceptible to collapse during an earthquake if the ground vibration frequency corresponds to their natural frequency.
Natural vibration has been largely overlooked in material actuation, which predominantly utilizes mechanically responsive crystals. Versatile actuation technologies are highly sought after in the field of soft robotics. Although crystal actuation processes such as photoisomerization and phase transitions have been extensively researched, these methods lack versatility as they necessitate specific crystals.
One approach to enhance versatility involves the use of photothermal crystals, which exhibit bending due to light-induced heating. Despite offering the potential for high-speed actuation, the bending angle is typically small (less than 0.5°), rendering the actuation inefficient.
A group of scientists from Waseda University and Tokyo Institute of Technology (Tokyo Tech) in Japan have successfully circumvented this limitation by harnessing the ancient phenomenon of resonated natural vibration. The team, which included Professor Junko Morikawa of Tokyo Tech and was led by Dr. Hideko Koshima of Waseda University, employed 2,4-dinitroanisole β-phase crystals (1β) to demonstrate large-angle photothermally resonated high-speed bending induced by pulsed UV irradiation.
Their research was published in Nature Communications ("Photothermally induced natural vibration for versatile and high-speed actuation of crystals").
"Initially, the objective of this research was to create crystals that exhibit significant bending due to the photothermal effect. As a result, we selected 2,4-dinitroanisole (1) β-phase crystal (1β), which has a large thermal expansion coefficient," Koshima elaborates on the team's inspiration behind the study. "We fortuitously discovered rapid and minor natural vibration induced by the photothermal effect. Furthermore, we achieved high-speed and substantial bending by photothermally resonating the natural vibration."
In their investigation, the team first cooled a methanol solution of commercially available anisole 1 to produce hexagonal, rod-shaped 1β single crystals. They utilized a pulsed UV laser with a wavelength of 375 nm to irradiate the crystals and observed the bending response using a digital high-speed microscope. Remarkably, under UV irradiation, the rod-shaped 1β crystals displayed a rapid natural vibration at 390 Hz, accompanied by a large photothermal bending of nearly 1°, surpassing the previously reported value of 0.2° in other crystals.
Moreover, the bending angle due to natural vibration increased to almost 4° when exposed to pulsed UV light at 390 Hz (identical to the crystal's natural frequency). Alongside this substantial bending, the team observed a high response frequency of 700 Hz and the highest energy conversion efficiency recorded to date.
The team further substantiated these findings through simulations, which exhibited excellent concordance with experimental data.
"Our results demonstrate that any light-absorbing crystal can exhibit high-speed, versatile actuation through resonated natural vibrations. This can pave the way for the application of photothermal crystals, ultimately leading to the development of real-life soft robots with high-speed actuation capabilities and perhaps a society where humans and robots coexist harmoniously," Koshima concludes.
Source: Tokyo Tech (Note: Content may be edited for style and length)
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