Researchers from Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a technique for controlling the movement of liquid droplets to naturally drive small-scale engines, oscillators and pumps.

The so-called system of self-excited motion does not demand an outside force such as gravity, temperature gradients or hydrophobic chemical coatings to propel fluid droplets across surfaces to generate heat, according to researchers. Instead, the technique reportedly creates the gradients independently.

Liquid solvent oscillating on a thin substrate. Source: Aditi Chakrabarti/Harvard SEASLiquid solvent oscillating on a thin substrate. Source: Aditi Chakrabarti/Harvard SEAS

Using a liquid solvent droplet like acetone or nail polish remover on a sheet of material, the researchers determined that once the fluid droplet contacted the surface, a portion of liquid is absorbed into the material, encouraging it to swell. Once swollen, the surface warps, resulting in an angle that the droplet can roll down. Following the swollen region’s exposure to the air, the absorbed liquid will evaporate, which enables the sheet to achieve its previous shape.

According to the team of researchers, the process is repeated each time the droplet moves, which results in an oscillating motion that forces the droplet to travel back and forth between two locations on the material’s surface — a process that is ongoing until the droplet is depleted.

"This see-saw movement is entirely self-driven by the interaction between these three behaviors —absorption-driven swelling, fluid flow and evaporation," said Aditi Chakrabarti, first author of the paper. "This type of self-generated motion hasn't been explored before and could lead to exciting applications."

The research appears in the journal Physical Review Letters.

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