Surface deep: Light-responsive top layer of plastic film induces
movement

Date:
August 22, 2020
Source:
Nagoya University
Summary:
Scientists show that only a thin, topmost layer of the
light-dependent azobenzene-containing plastic film needs to be
light-sensitive, rather than the entire film, opening up new ways
to potentially reduce production costs and revolutionize its use.



FULL STORY ========================================================================== Azobenzene-containing plastic film is a peculiar material; its surface
can change shape when exposed to light, making it a valuable component in modern technologies/devices like TV screens and solar cells. Scientists
now show that only a thin, topmost layer of the light-dependent azobenzene-containing plastic film needs to be light-sensitive, rather
than the entire film, opening up new ways to potentially reduce production costs and revolutionize its use.


==========================================================================
So far, it had been widely accepted that the light-sensitive nature of
this material extends throughout the whole film, but scientists did
not understand what was causing the shape-shifting movement. A group
of scientists led by Dr Takahiro Seki of Nagoya University, Japan, set
out to figure out exactly how this happens; they have published their
findings in the journal Scientific Reports.

They cite a well-studied phenomenon called Marangoni flow as their
inspiration: owing to this phenomenon, differences in "surface tension"
(the property by which the particles in the outermost layer of liquids
are always attracted inwards, creating a boundary for the liquid) cause
many soft, plastic films to move in a peculiar pattern. The most famous
example of this phenomenon is the formation of "wine legs" or droplets
of liquid evaporating and streaking down the surfaces of wine glasses.

They decided to test whether ultraviolet light triggered changes in the
surface tension of azobenzene plastic film, and whether those changes
resulted in the film moving. They chose to first cover azobenzene film
with a very thin top layer that was light-sensitive, then exposed this
film to UV radiation. Next, they did the same with film that was covered
in a top layer unresponsive to light. To their excitement, the scientists
found surface structural changes in the film with a light-sensitive top
layer, but not in the film with a "light- insensitive" top layer. "This is
the first time anyone has demonstrated that only the light responsiveness
of a very thin 'nanometer' level layer is needed for azobenzene-containing
film to alter its surface morphology under UV," said Dr Seki.

An important observation of this study is that the movement of the
material isn't dependent on "light polarization," or the direction in
which light waves travel. If it were, that would suggest that there is
another force on the molecular level affecting the whole film. Instead,
Dr Seki concludes that it is probably the changes in chemical structure
at the surface induced by the UV radiation that changes surface tension, inducing movement to the top of the film.

Describing the wider ramifications of their results, Dr Seki states:
"We are only at the cusp of developing this discovery onto an industrial
scale, but you can imagine how needing only a very small amount of light-sensitive material can reduce costs. Many optical devices like photocopiers, printers, and monitors depend on the light-based surface
change in azobenzene polymer film.

Based on our findings, azobenzene film can also act as an "actuator"
(that part in a device that moves other parts) in nanomachinery."
These newly discovered properties have vast implications, from improving
the economics of production and lowering material prices, to advancing
the field of nanotechnology itself.


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


========================================================================== Journal Reference:
1. Issei Kitamura, Keisuke Kato, Rafael Benjamin Berk, Takashi Nakai,
Mitsuo
Hara, Shusaku Nagano, Takahiro Seki. Photo-triggered
large mass transport driven only by a photoresponsive
surface skin layer. Scientific Reports, 2020; 10 (1) DOI:
10.1038/s41598-020-69605-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200822085716.htm

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