West LA Times

Curiosity about an accidental discovery has led to significant advancements in the study of chemical pattern formation. UCLA doctoral student Yilin Wong noticed tiny dots on a germanium wafer with evaporated metal films, which had been left out overnight. Upon closer examination under a microscope, she found beautiful spiral patterns etched into the semiconductor's surface by a chemical reaction.

Wong's unexpected finding revealed that hundreds of near-identical spiral patterns could spontaneously form on a centimeter square germanium chip. Variations in experimental parameters such as metal film thickness resulted in different patterns, including Archimedean spirals and lotus flower shapes.

The discovery was published in Physical Review Materials and occurred when Wong made an error while attempting to bind DNA to the metal film. "I was trying to develop a measurement technique to categorize biomolecules on a surface through breaking and reforming of the chemical bonds," Wong explained.

To understand how these patterns formed, Wong and co-author Giovanni Zocchi, a UCLA physics professor, conducted further experiments. They evaporated layers of chromium and gold onto the germanium wafer and applied an etching solution. The process revealed that stress in the metal films contributed to pattern formation.

"The system basically forms an electrolytic capacitor," Zocchi noted. Over 24-48 hours, catalysis-driven deformations etched remarkable patterns on the germanium surface due to stress-induced wrinkles in the metal film.

Zocchi emphasized that both chemical reactions and mechanical stress influenced pattern development: "The thickness of the metal layer, the initial state of mechanical stress of the sample, and the composition of the etching solution all play a role."

This coupling between catalysis-driven deformations and underlying chemical reactions is rare in laboratory settings but common in nature. Enzymes naturally catalyze growth that deforms cells and tissues into specific shapes similar to those observed by Wong.

"In the biological world, this kind of coupling is actually ubiquitous," Zocchi said. The discovery provides researchers with a non-living system for studying this phenomenon outside natural environments.

Pattern formation studies began with Soviet chemist Boris Belousov's 1951 discovery of oscillating chemical systems and British mathematician Alan Turing's work on reaction-diffusion systems forming spatial patterns like stripes or polka dots. Wong's experiments mirrored Turing's theoretical dynamics.

Despite past interest during the 1980s and 90s, experimental systems for studying chemical pattern formation have remained largely unchanged since their introduction in the 1950s. The work by Wong and Zocchi represents significant progress in this field.