Dancing with the light: A new way to make crystals bend by shining light


Date:
July 29, 2021
Source:
Waseda University
Summary:
Generating mechanical motion in crystals using light or heat has
increasingly become the focus of materials scientists. However,
the conventional mechanism employed for the purpose produces slow
responses and is ineffective for thick crystals. Now, in a new
study, scientists report and validate a new mechanism for generating
fast bending motion in thick crystals with light-induced heating,
opening doors to light-driven mechanics with more versatile
crystals.



FULL STORY ==========================================================================
The word "crystal" typically conjures up images of gemstones like
diamonds in our minds. Generally associating words like "hardness" and "rigidity" with crystals, we would most likely not consider them as
flexible. However, certain "molecular crystals" can bend, twist, and
even jump when excited with light or heat and have garnered attention
from chemists, materials scientists, and engineers alike owing to
their potential applications in actuators, artificial muscles, and
soft robotics.


==========================================================================
The secret to the remarkable mechanical properties of molecular crystals
lies in "photoisomerization," a process in which a molecule takes on
a different structure by absorbing light. However, several drawbacks,
such as slow mechanical response and limited choice of wavelength
for inducing photoisomerization, limit the applicability of molecular
crystals. Moreover, only very thin crystals (up to 20 microns) can show appreciable mechanical response.

Now, in a recent study published in the Journal of the American Chemical Society, scientists from Japan have taken things to the next level by
making thick crystals bend rapidly with UV light using what is called
the "photothermal effect," a phenomenon in which heat is generated by
exciting materials with light. Adjunct Researcher Hideko Koshima from
Waseda University, Japan, who led the study, lays out their motivation:
"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." Scientists first exposed a
thin salicylideneaniline derivative crystal, a promising mechanical
crystal candidate, to UV light and obtained substantial bending within approximately 1 second. However, the bend angle dropped rapidly with
increasing crystal thickness, revealing that the bending was caused by photoisomerization. Things took an interesting turn when on illuminating
a thick (>40 microns) crystal with UV light, they observed an extremely
rapid bending within several milliseconds, a distinct signature of the photothermal effect. Furthermore, by using a pulsed UV laser light, they
could make the crystals bend at a frequency of 500 Hz (cycles/second).

The scientists next determined heat conduction in the crystal using
temperature wave analysis and suggested a potential bending mechanism
in which a non-steady temperature gradient in the thickness direction
generated the high-speed bending. By calculating the temperature gradient numerically, they simulated the bending motion to validate the proposed mechanism.

The team is thrilled about the implications of their findings. "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,"speculates Koshima, excited.

So, is a future where a friendly and safe robot companion such as Baymax
from Big Hero 6 within our grasp? The findings by Koshima's team have
certainly put us one step forward in that direction.

========================================================================== Story Source: Materials provided by Waseda_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Shodai Hasebe, Yuki Hagiwara, Jun Komiya, Meguya Ryu, Hiroki
Fujisawa,
Junko Morikawa, Tetsuro Katayama, Daiki Yamanaka, Akihiro Furube,
Hiroyasu Sato, Toru Asahi, Hideko Koshima. Photothermally Driven
High- Speed Crystal Actuation and Its Simulation. Journal
of the American Chemical Society, 2021; 143 (23): 8866 DOI:
10.1021/jacs.1c03588 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210729122137.htm

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