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.

Walking crystals may lead to new field of crystal robotics

Researchers have demonstrated that tiny micrometer-sized crystals—just barely visible to the human eye—can "walk" inchworm-style across the slide of a microscope. Other crystals are capable of different modes of locomotion such as rolling, flipping, bending, twisting, and jumping. In the future, these moving crystals may open the doors to the development of crystal-based robots.

The researchers, led by Hideko Koshima at Waseda University in Tokyo, Japan, have published a paper on walking and rolling crystals in a recent issue of Nature Communications.

"We believe that this finding opens the doors to a new field of crystal robotics," Koshima told Phys.org. "Currently, robots made from metals are rigid and heavy, making them unsuitable for daily interaction with humans. Our goal is to make symbiotic soft robots using mechanical crystals."

In their work, the researchers investigated asymmetric crystals derived from chiral azobenzene. In experiments, they showed that exposing the crystals to alternating hot and cold temperatures (changing between 120° and 160° C over the course of approximately 2 minutes) causes changes in the crystals' shapes.