Elucidate the mechanism behind spiral-wave angle switching on spinning discs

Elucidate the physical mechanisms that cause spiral waves on thin liquid films flowing over spinning discs to switch their propagation angle β, including the observed reorientation of wave sections, during spiral wavefront destabilization and breakaway.

Background

Spiral waves in thin-film flow over spinning discs typically unwind in the direction of rotation due to the Coriolis effect, yielding negative angles β. The authors observed that during wavefront destabilization and breakup into 3D waves or wavelets, spiral segments can switch to positive angles β and detach, a phenomenon also reported in recent high-speed imaging studies.

While the paper documents the occurrence of such angle switching and provides qualitative observations about asymmetry and breakaway dynamics, it explicitly states that the underlying mechanism has not yet been elucidated, identifying a targeted open problem for further investigation.

References

Although the unwinding of spiral waves in the direction of rotation is expected from the Coriolis effect, the mechanisms behind this switching of wave angle have yet to be elucidated and have also been observed in other high-speed imaging experiments recently .

— Thin film flow over a spinning disc: Experiments and direct numerical simulations  (2412.12730 - Stafford et al., 2024) in Section 4.3 (Wave evolution and internal flow mechanisms), near Figure 13