Researchers create morphing crystals powered by water evaporation

Shape-shifting crystals that directly convert evaporation energy into powerful motions have been created in research co-led at the University of Strathclyde.

Water evaporation, as observed when a puddle of water disappears on a summer day, is a remarkably powerful process. If it were harnessed, the process could provide a clean source of energy to power mechanical machines and devices.

The water-responsive materials were created by using simple variants of biological building blocks, known as tripeptides, to create crystals that are simultaneously stiff and morphable. The materials are composed of three-dimensional patterns of nanoscale pores where water tightly binds, and these pores are interspersed with a molecular network of stiff and flexible regions.

When humidity is lowered and reaches a critical value, the water escapes from the pores, leading to a powerful contraction of the interconnected network. This results in the crystals temporarily losing their ordered patterns until humidity is restored and the crystals regain their original shape. This newly designed process can be repeated over and over and gives rise to a remarkably efficient method of harvesting evaporation energy to perform mechanical work.

The research was co-led by research groups at the University of Strathclyde and the Advanced Science Research Center at the Graduate Center, CUNY (City University of New York). It has been published in Nature Materials.

Professor Tell Tuttle, of Strathclyde’s Department of Pure and Applied Chemistry, who co-led the research, said: “In this work, we have demonstrated that it is possible to use minimalistic building blocks to create evaporation-driven actuators, but equally importantly, we have also been able to show how they work through a complex interplay of exchanging networks.

“This understanding allows further development and tuning of this technology for optimisation towards different application areas.”

By using a combination of laboratory-based experiments and computer simulations, the researchers were able to identify and study the factors that control the actuation of these crystals. This approach resulted in new insights that inform the design of more efficient ways to use evaporation for a variety of applications, which may include robotic components or mechanical micro- and nano-machines that are powered by water evaporation.