Scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a framework for self-excited droplet movement that don’t require any external force. It spontaneously creates and reacts to gradients all by itself.
The framework uses a liquid solvent droplet, such as acetone or nail polish remover—on a thin sheet of material. When the droplet first contacts the surface, some part of the liquid gets consumed into the material, and the material swells. When the material swells, it buckles and creates an incline down which the droplet rolls. Presently, the sheet’s swollen part is presented to the air, and the absorbed liquid evaporates, permitting the sheet to recover its unique shape.
The same process occurs wherever the droplet moves, thus producing an oscillating movement that pushes a liquid droplet back and forth between two spots on the surface. The oscillation continues until the droplet shrinks.
Aditi Chakrabarti, a postdoctoral fellow at SEAS and first author of the paper, said, “This see-saw movement is entirely self-driven by the interaction between these three behaviors—absorption-driven swelling, fluid flow, and evaporation. This type of self-generated motion hasn’t been explored before and could lead to exciting applications.”
Using various solvents and droplet sizes, scientists generated this behavior on thin sheets.
L. Mahadevan, the Lola England de Valpine Professor of Applied Mathematics, of Organismic and Evolutionary Biology, and Physics and senior author of the paper, said, “Harnessing such behaviors and motion in thin-film systems might provide a natural way to drive small-scale engines, oscillators, and pumps. This system could also provide a simple physical model to understand how biological systems, such as protocells, move.”
- Aditi Chakrabarti et al. Self-Excited Motions of Volatile Drops on Swellable Sheets, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.124.258002