Hyperbolic umbilic caustics from oblate water drops with tilted illumination: Observations

- Light scattering observed in the primary rainbow region of water oblate drops.
- The drops are acoustically levitated with a horizontal equatorial plane.
- The illumination may be vertically tilted by a small angle.
- The drop aspect ratio for a hyperbolic-umbilic focal section depends on tilt.
- The location of the V-shaped HUFS caustic depends on tilt.
- A related application of numerical ray-tracing is summarized.

Various groups have reported observations of hyperbolic umbilic diffraction catastrophe patterns in the far-field scattering by oblate acoustically levitated drops with symmetric illumination. In observations of that type the drop's symmetry axis is vertical and the illuminating light beam (typically an expanded laser beam) travels horizontally. In the research summarized here, scattering patterns in the primary rainbow region and drop measurements were recorded with vertically tilted laser beam illumination having a grazing angle as large as 4 degrees. The findings from these observations may be summarized as follows: (a) It remains possible to adjust the drop aspect ratio (diameter/height) = D/H so as to produce a V-shaped hyperbolic umbilic focal section (HUFS) in the far-field scattering. (b) The shift in the required D/H was typically an increase of less than 1% and was quadratic in the tilt. (c) The apex of the V-shaped HUFS was shifted vertically by an amount proportional to the tilt with a coefficient close to unity. The levitated drops had negligible up-down asymmetry. Our method of investigation should be useful for other generalized rainbows with tilted illumination.