How chemistry and force etch mysterious spiral patterns on solid surfaces

Amazing stuff! Maybe just a curiosity?

"Key takeaways

• Curiosity about a mistake that left tiny dots on a germanium wafer with evaporated metal films led to the discovery of beautiful spiral patterns etched on the surface of the semiconductor by a chemical reaction.
• Further experiments showed that the patterns arise from chemical reactions that are coupled to mechanical forces through the deformation of a catalyzing agent.
• The new system is the first major advance in experimental methods to study chemical pattern formation since the 1950s. Studying these complex systems will help scientists understand other natural processes, from crack formation in materials to how stress influences biological growth. ...

.Over the course of 24-48 hours, a chemical reaction catalyzed by the metal film etched remarkable patterns on the germanium surface. Investigation of the process revealed that the chromium and gold films were under stress and had delaminated from the germanium as the catalytic reaction proceeded. The resulting stress created wrinkles in the metal film that, under further catalysis, etched the amazing patterns the researchers had seen. ...

One of the most exciting findings in this study is that the patterns are not purely chemical but are influenced by residual stress in the metal film. The research suggests that the metal’s preexisting tension or compression determines the shapes that emerge. Thus, two processes, one chemical and one mechanical, worked together to yield the patterns. ...

The study of pattern formation in chemical reactions began in 1951 when the Soviet chemist Boris Belousov accidentally discovered a chemical system that could spontaneously oscillate in time, which inaugurated the new fields of chemical pattern formation and nonequilibrium thermodynamics. At the same time and independently, the British mathematician Alan Turing discovered that chemical systems, later termed “reaction-diffusion systems,” could spontaneously form patterns in space, such as stripes or polka dots. The reaction-diffusion dynamics observed in Wong’s experiments mirrored the theoretical ones posited by Turing. ..."

From the abstract:

"We present a solid state system which spontaneously generates remarkable engraving patterns on the surface of Ge. The layered construction, with a metal film on the Ge surface, results in coupling of the metal catalyzed etching reaction with the long range stress field at the Ge-metal interface. The etching patterns generated have similarities with Turing patterns, hydrodynamic patterns, crack propagation, and biological form. We describe spirals, radial patterns, and more disordered structures. Euler buckling of the metal layer generates a characteristic wavelength for some patterns."

How chemistry and force etch mysterious spiral patterns on solid surfaces | UCLA "Hundreds of regular patterns spontaneously form on a small germanium chip"

Metal-assisted chemical etching patterns at a Ge/Cr/Au interface modulated by the Euler instability (no public acccess)

A logarithmic spiral with a diameter of 500 μm, approximately half the diameter of a sewing needle.

A lotus-shaped pattern on the scale of 2000 μm.