Strain-controlled diffraction of light from stretchable liquid metal micro-components

This paper elucidates and characterizes the origin of the spectrum of colors that appear on the surface of polydimethylsiloxane (PDMS) microchannels filled with liquid metal. The use of an oxygen plasma to seal the PDMS microchannels results in a thin oxide layer on the walls of the channels that buckles under compression to create diffractive corrugations. The liquid metal reflects the light from these corrugations. The thin layer of gallium oxide that forms on the metal improves the adhesion of the metal to the walls and thereby keeps it conformal with the corrugations during the buckling process. The conformal coating of metal makes the diffraction pattern more evident than if the metal was placed directly onto a pre-corrugated surface. This approach represents a simple method of fabricating soft diffractive elements with lithographically defined shapes that can be switched between a metallic and a colored state in response to compression; this feature may be used to sense compressive forces optically or for inherently aligned diffractive elements for optofluidics.