What the study found
The study found that a scallop-like swimmer with reciprocally flapping wings can move in a nearly frictionless, cohesive granular medium made of hydrogel spheres when the flapping frequency matches the material’s inverse relaxation time. The swimmer moved in the opposite direction from its motion in a cohesion-free granular material of hard plastic spheres.
Why the authors say this matters
The findings indicate that time-dependent interactions in the medium can produce locomotion even for reciprocal swimming motions, which the authors present as relevant to understanding movement in viscoelastic granular materials. The study suggests that drag and propulsion can depend on the material’s relaxation behavior and the swimmer’s inertia.
What the researchers tested
The researchers experimentally studied a scallop-like swimmer with reciprocally flapping wings in a nearly frictionless, cohesive granular medium consisting 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 only at intermediate frequencies, when the flapping frequency matched the inverse relaxation time of the material. At higher or lower frequencies, the swimmer showed no motion except for a short initial transient phase. X-ray radiograms showed that the wing motions created low-density zones, which produced a hysteresis in drag and propulsion forces; together with the swimmer’s inertia, this accounted for locomotion at intermediate frequencies.
What to keep in mind
The abstract describes one experimental swimmer in one type of cohesive granular hydrogel, so the scope is limited to that system. Limitations beyond this are not described in the available summary.
Key points
- A scallop-like swimmer moved in a cohesive granular hydrogel only at intermediate flapping frequencies.
- The best motion occurred when the flapping frequency matched the material’s inverse relaxation time.
- The swimmer moved in the opposite direction from the same swimmer in cohesion-free hard plastic spheres.
- At higher or lower frequencies, the swimmer did not move beyond a short initial transient phase.
- X-ray radiograms showed low-density zones and force hysteresis linked to the motion.
Disclosure
- Research title:
- Reciprocating swimmer moves in viscoelastic granular hydrogel
- Authors:
- Hongyi Xiao, Jing Wang, Achim Sack, Ralf Stannarius, Thorsten Pöschel
- Institutions:
- Brandenburg University of Applied Sciences, Friedrich-Alexander-Universität Erlangen-Nürnberg, Friedrich-Alexander-Universität Erlangen-Nürnberg, Friedrich-Alexander-Universität Erlangen-Nürnberg, Otto-von-Guericke-Universität Magdeburg, Otto-von-Guericke-Universität Magdeburg, University of Michigan
- Publication date:
- 2026-04-27
- OpenAlex record:
- View
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