Optical signatures of no-slip rotation in liquid films: Spiral fringes and frame permittivity effects
Since the discovery of controlled rotation in soap films under electric fields in 2009, the rotation of suspended liquid films has drawn attention across disciplines. While prior studies identified surface charge as a primary driver—particu...
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| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
AIP Publishing LLC
2025-06-01
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| Series: | AIP Advances |
| Online Access: | http://dx.doi.org/10.1063/5.0247527 |
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| Summary: | Since the discovery of controlled rotation in soap films under electric fields in
2009, the rotation of suspended liquid films has drawn attention across
disciplines. While prior studies identified surface charge as a primary
driver—particularly in polar liquids and liquid crystals—our work
introduces a new influencing factor: the permittivity of the supporting frame.
We show that the frame-to-film permittivity ratio governs the transition between
slip and no-slip rotational modes. Using both Particle-Image Velocimetry (PIV)
and visual inspection, we characterize these modes. Although PIV offers precise
measurements, we demonstrate that the two rotational states can be visually
distinguished through their color patterns. Films in no-slip rotation exhibit
vivid, colored spiral fringe patterns, while slip modes display steady central
patterns. These spiral fringes result from white light interference between the
incident wave and the portion transmitted through the film and reflected back
from the second surface. When the film’s thickness is below the temporal
coherence length of white light, and its geometry resembles a hybrid
lens–spiral-phase element, concentric fringes transform into dynamic,
colored spirals. We propose that in the no-slip mode, the rotating film behaves
as a lens-like spiral-phase object. The observed spiral pattern thus serves as
both a qualitative signature and a physical indicator of the no-slip regime.
This finding provides an accessible optical method for identifying rotational
modes and offers new insights into the interplay between electromechanical
forces, geometry, and optical behavior in thin liquid films. |
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| ISSN: | 2158-3226 |