Scientists have found a way to bend thick molecular crystals at high speeds by using the photothermal effect, allowing for wider applications for light-activated mechanical crystals.
Flexible is not a word normally associated with crystals. That is, if we are talking about crystal gemstones like diamonds and rubies. In the microscopic world of atoms and molecules, crystals are flexible, bendable, and even twistable. Rather than being hard and rigid, “molecular crystals” can go as far as to jump when excited with light or heat.
Molecular crystals are a blanket term for various substances that have relatively weak intermolecular forces, ranging from dry ice to solid forms of noble gases like argon and krypton. What makes these crystals so mechanically responsive is a process known as photoisomerisation, where molecules can take on different structures by absorbing light. However, this UV-dependent process comes with its own limitations for crystal actuation, like slow mechanical response and a narrow wavelength range. Moreover, only very thin crystals (up to 20 microns) show demonstrable mechanical response.
Now, a recent study by scientists from Waseda University, Japan, have shown that it is possible to rapidly bend thick crystals. They reported fast bending motions in a 40-micron thick crystal and attributed their results to the “photothermal effect,” which refers to when heat is generated by exciting materials with light.
Researcher Hideko Koshima, who led the study, explained, "Last year, our team accidentally discovered that the photothermal effect causes a crystal to bend fast, but we couldn't explain why. Against this backdrop, we sought to create a new, faster bending crystal and clarify the underlying mechanism."
Early on their investigation, the team first experimented on a promising mechanical crystal candidate, known as salicylideneaniline. They exposed a thin salicylideneaniline derivative crystal to UV light and saw that the crystal could significantly bend within approximately one minute. But as the scientists began to use increasingly thicker crystals, they noticed that the bend angle dropped rapidly. This inverse relationship between crystal thickness and angle of bending is characteristic of photoisomerisation. Therefore, the bending motions were attributed to photoisomerisation.
However, a peculiar observation was made when the team illuminated a crystal of over 40 microns thick with UV light. They found that the crystal could bend very rapidly within several milliseconds. Furthermore, when they experimented with pulsed UV laser light, they could make the crystals bend very quickly, at a frequency of 500 Hz (cycles/second).
To explain these results, Dr. Koshima and colleagues speculated that there could be another mechanism responsible for the bending, particularly to do with the photothermal effect. They proposed a mechanism in which heat conduction near the crystal surface resulted in a non-steady temperature gradient along its thickness, thus causing the crystal to bend. Their proposal was later validated upon calculating the temperature gradient numerically and successfully simulating the photothermal bending motion.
Thrilled about the implications of their study, the researchers believe that creating and designing various mechanically responsive crystals will now be possible via the photothermal effect. Expanding the versatility of crystal as actuation materials can pave the way for widespread applications in actuators, artificial muscles, and soft robotics.
"As the photothermal effect occurs in almost all crystals that absorb light, any light may move any crystal at high speeds. Further, the bending motion can now be simulated, providing the basis for practical applications such as in light-driven actuators. What's more, these light-activated mechanical crystals can be used to create novel soft robotic structures that ensure safe human-robot interaction,” said Koshima.
Source: Hasebe et al. (2021). Photothermally Driven High-Speed Crystal Actuation and Its Simulation. Journal of the American Chemical Society, 143(23), 8866–8877.