ACS Central Science

Scientists at the University of Michigan have developed a new material that can remain liquid far below its predicted freezing temperature and, still in its liquid state, turn into crystal with a light touch.

In a paper published in the journal ACS Central Science, the researchers discuss the properties of one of the derivatives of the diketopyrrolopyrrole (DPP) molecule, called DPP8:  “Upon applying shear force, the dim orange-red fluorescent supercooled liquid transforms to bright greenish yellow fluorescent crystals.”

To keep the molecule liquid in very cold temperatures, the scientists manipulated the molecule until it reached a very specific balance. A University of Michigan press release explains:

“These molecules can be described as a rigid core flanked by two flexible side chains. If the chains are short, the core molecules drive crystallization, but if they are long, the chains interact to form a different kind of crystal. The U-M team found that by varying the lengths of the side chains, they could cause a stalemate between two modes of crystallization.”

In a phone interview, researcher Jinsang Kim explained to Fusion that in this state, the “the DPP8 molecule has balanced intermolecular balanced forces acting opposite directions so that it can’t move either way — it’s stuck there. Because of that, it remains a supercooled liquid.”


That tension prevented the supercooled liquid from changing states. But, the scientists also found that a single touch was enough to turn the liquid to solid. Essentially, agitating the liquid breaks that balance, and prompts the change in state.

Kim said that a single touch can turn the entire liquid into solid crystal when the temperature is high enough to give the molecules mobility to change states — at about 120 degrees. This is what that looks like:


via the American Chemical Society on YouTube.

If you touch the liquid at a lower temperature — around room temperature — just the area agitated by the stylus (or other prodding material) will turn to crystal that glows under UV light. That means you can write crystal messages onto the liquid surface:


Image via ACS Central Science

The researchers also found that the molecule can easily change from liquid to solid, and back. “It’s a completely reversible phenomena,” said Kim.

For now, the researchers are hoping that their findings can be used to develop biomedical sensors. A human cell can trigger the transformation of the liquid into a solid so, Kim said, the “fluorescent marks appear as a footprint of the cell.” Commercially, the findings can one day be used to create material that would be used in optical memory — a method of storing information for computer memory that uses light. But that’s still a long way off.


And if you (like of some of us) are imagining using the material as a mood ring, don’t get your hopes up. “We don’t know the toxicity of the molecule yet,” said Kim. Fair enough.

Danielle Wiener-Bronner is a news reporter.