What the study found
A scallop-like swimmer with reciprocally flapping wings could move through a nearly frictionless, cohesive granular medium made of hydrogel spheres when its flapping frequency matched the material's inverse relaxation time. The swimmer moved in the opposite direction from how it moved in a cohesion-free granular material of hard plastic spheres.
Why the authors say this matters
The authors conclude that the locomotion depends on a time-dependent effect in the material, together with the swimmer's inertia. The study suggests that the drag and propulsion behavior in this kind of granular hydrogel medium is shaped by how the medium relaxes over time.
What the researchers tested
The researchers experimentally studied a swimmer with reciprocal wing flapping in a granular medium of hydrogel spheres. They varied the flapping frequency and used X-ray radiograms to observe how the wing motions affected the surrounding material.
What worked and what didn't
Significant locomotion was observed at intermediate frequencies, specifically when the flapping frequency matched the inverse relaxation time of the material. At higher or lower frequencies, the swimmer did not move except for a short initial transient phase. X-ray radiograms showed that the wing motions created low-density zones, and these were associated with a hysteresis in drag and propulsion forces.
What to keep in mind
The abstract describes a specific experimental system: a scallop-like swimmer in a cohesive granular medium of hydrogel spheres. It does not provide broader limits, quantitative measures, or information about other swimmer designs beyond the comparison with hard plastic spheres.
Key points
- The swimmer moved in a cohesive granular medium only at intermediate flapping frequencies.
- Maximum motion occurred when the flapping frequency matched the material's inverse relaxation time.
- The direction of motion was opposite to that seen in cohesion-free granular material of hard plastic spheres.
- X-ray radiograms showed low-density zones created by wing motions.
- The authors link locomotion to hysteresis in drag and propulsion forces plus swimmer inertia.
Disclosure
- Research title:
- Reciprocal flapping produces motion in viscoelastic granular hydrogels
- Authors:
- Hongyi Xiao, Jing Wang, Achim Sack, Ralf Stannarius, Thorsten Pöschel
- Institutions:
- Friedrich-Alexander-Universität Erlangen-Nürnberg, University of Michigan, Otto-von-Guericke-Universität Magdeburg, Brandenburg University of Applied Sciences
- Publication date:
- 2026-04-27
- OpenAlex record:
- View
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