Whisky-Inspired Active Matter

Whisky is a valuable commodity around the world, partly due to its rich heritage and complex taste and scent. Analyses of the final product show a complex symphony of chemicals ranging from sulfides, phenols, and tannins to esters, serving as an imprint of the manufacturing and aging processes that each whisky goes through. A well-known reactivity between copper and sulfides during whisky making inspires the determination of whether this reactivity can be harnessed on the microscale. Our system uses spherical Cu@SiO₂ Janus particles in aqueous solutions of dimethyl sulfide to determine the capacity of DMS to serve as a chemical fuel for the propulsion of microswimmers. Other sulfur-containing molecules, e.g., aliphatic mono- and disulfides, thiophene, thiazole, and thiols, are tested as well to determine their capacity for Cu@SiO₂ Janus particle actuation. Our investigation identifies that water-soluble aliphatic monosulfides successfully actuate the microswimmers with velocities reaching up to 30 μm·s^-1. We propose a possible underlying reaction mechanism for these fuels and discuss alternatives, e.g., DMSO and Cu₂O are formed when DMS is used as a chemical fuel. To further reflect the whisky-inspired type of fuel, ethanol-water mixtures as the swimming medium are examined, and their effects on the swimming behavior of Cu@SiO₂ Janus particles are evaluated. This work expands the array of fuels that can be used to propel microswimmers.