WATERLOO — What is soft is strong.

It may sound like a paradox but the ancient Chinese philosopher Laozi purportedly used this phrase to describe water, which is soft and yielding but can eventually erode even the largest of rocks.

Now a team of researchers at the University of Waterloo is poised to take that paradox and make it a reality.

A new polymer developed by UW chemical engineering PhD students is able to switch states between a soft gel and a hard solid within minutes while at the same room temperature, and it could revolutionize the robotics and adhesive industries.

The researchers combined a supercooled salt solution with a super absorbent polymer gel commonly found inside baby diapers to create a material that is soft and flexible, but can be made super strong and rigid on demand.

The polymer, referred to as sal-gel, can achieve the transformation within a wide temperature range, from -90 C to 58 C, and is reversible by simply applying direct heat.

"With traditional materials you only have one property for a given temperature, but now we have two properties," said Kuo Yang, the chemical engineering PhD student who developed the idea. "There is still a lot of work to be done, but this is a very promising first step."

A phenomenon known as supercooling is at the core of the transformation. It involves reducing the temperature of the salt solution below the normal freezing point of water before combining it with the polymer. The material wants to freeze and crystalize, but the salt inside the polymer isn't able to arrange itself into a crystalized form.

Simply touching another salt crystal to the gel provides the necessary road map, however, and activates the reaction. The gel solidifies in just minutes.

"All the molecules in the liquid want to arrange into a crystal, but they can't find a way to align themselves," said Yang. "Once you show them a crystal, they realize 'oh that's how we arrange ourselves'."

The process is called nucleation and it transforms the gel into a rigid, opaque material; it essentially freezes while still at room temperature, Yang said.

The two-in-one polymer could revolutionize robotics by giving machines the soft and flexible touch needed to grip delicate or fragile items, and then the strength and rigidity necessary to pick them up.

Researchers also envision using it as an adhesive by attaching the gel to the contours of a surface, hardening it to create a strong bond, then releasing the bond when necessary. Their tests found the solid state is approximately 10,000 times stronger than the gel form.

They're on the cusp of resolving the paradox that something soft and flexible cannot also be strong, said their professor, Boxin Zhao, a chemical engineer who helped direct their work.

"This material can do both," he said. "That's fundamentally very important."

Aleksander Cholewinski, a PhD student in chemical engineering and member of the research team, said early tests show the process also doesn't appear to weaken the polymer bonds after repeated cycles.

"We've done multiple freeze-melt, freeze-melt cycles and we've actually seen it get slightly stronger in the frozen state," he said. "And when you go back to the melted (form) it isn't damaged."

It took about four months to perfect the process, and about a year overall to publish the results.

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The pair collaborated with post-doctoral fellows Li Yu and Geoffrey Rivers on the research. Their paper, "A hybrid material that reversibly switches between two stable solid states" was recently published in the journal Nature Materials.

jjackson@therecord.com

Twitter: @JamesDEJ