(Nanowerk News) Each material has a characteristic natural vibrational frequency, and when an external periodic force is applied at or near this frequency, the vibrations are significantly amplified. In the world of physics, this phenomenon is referred to as “resonance”. Resonance permeates our daily lives and can be considered beneficial or detrimental, depending on the circumstances. For example, musical instruments such as the guitar rely on resonance for sound amplification, whereas buildings and bridges are more prone to collapse during an earthquake if the frequency of the ground vibrations matches their natural frequency.
Natural vibrations are largely ignored in material actuation, most of which employ mechanically responsive crystals. Versatile actuation technology is highly sought after in the field of soft robotics. Although crystal actuation processes such as photoisomerization and phase transitions have been extensively studied, these methods are less flexible because they require specific crystals.
One approach to increase versatility involves the use of photothermal crystals, which exhibit light-induced bending due to heating. Although they offer the potential for high-speed actuation, the bend angle is usually small (less than 0.5°), making actuation inefficient.
A group of scientists from Waseda University and the Tokyo Institute of Technology (Tokyo Tech) in Japan have managed to circumvent this limitation by exploiting the ancient phenomenon of resonating natural vibrations. The team, which included Professor Junko Morikawa of Tokyo Tech and led by Dr. Hideko Koshima from Waseda University, used 2,4-dinitroanisole β-phase crystals (1β) to demonstrate high-speed resonant large-angle photothermal resonance induced by pulses of UV radiation.
Their research is published in Nature Communications (“Photothermally induced natural vibrations for versatile, high-speed crystal actuation”).
“Initially, the goal of this research was to create crystals that exhibit significant bending due to the photothermal effect. As a result, we chose 2,4-dinitroanisole(1)β-phase(1β) crystals, which have a large thermal expansion coefficient,” Koshima elaborated on the inspiration of the team behind this research. “We by chance discovered a fast and small natural vibration caused by the photothermal effect. In addition, we achieve substantial, high-speed bending by photothermally resonating natural vibrations.”
In their investigation, the team first cooled a methanol solution of commercially available anisol 1 to produce rod-shaped hexagonal single crystals of 1β. They used a pulsed UV laser with a wavelength of 375 nm to shine a light on the crystals and observed the bending response using a high-speed digital microscope. Remarkably, under UV irradiation, the rod-shaped 1β crystals displayed fast natural vibrations at 390 Hz, accompanied by a large photothermal bending of nearly 1°, exceeding the previously reported value of 0.2° in other crystals.
In addition, the bending angle due to natural vibration increases by nearly 4° when exposed to pulsed UV light at 390 Hz (identical to the crystal’s natural frequency). Along with this substantial bending, the team observed a high frequency response of 700 Hz and the highest energy conversion efficiency recorded to date.
The team further substantiated these findings through simulations, which showed a very good agreement with the experimental data.
“Our results show that any light-absorbing crystal can exhibit high-speed, versatile actuation through resonant natural vibrations. This could pave the way for the application of photothermal crystals, eventually leading to the high-speed development of real-life soft robots. actuation abilities and perhaps a society in which humans and robots coexist harmoniously,” Koshima concluded.