Abstract

Microbubbles in the ocean may be coated by a thin film of surfactant since such substances can be abundant in natural waters. We investigated theoretical scattering patterns for a spherical gas bubble (of radius a) coated by a film of uniform thickness h and refractive index n_f surrounded by water of refractive index n_w = 4/3. The patterns were computed from the partial-wave series of Aden and Kerker for ka ranging from 100 to 2500 where 2π/k is the optical wavelength in water. The corresponding range of a is 7.5 to 189 μm; h ranged from 0 to 3 μm and n_f was typically real and equal to 1.5. Noncoated bubbles are known¹ to exhibit coarse irradiance oscillations as the scattering angle θ decreases below a critical value for total reflection θ_C = 82.8°. Anticipated coatings remove neither this coarse structure nor the superimposed finely spaced structure. The magnitude of the latter can be enhanced by the coating. The coarse oscillations are perceivably shifted toward larger θ when n_f > n_w. Ray optics predicts a shift in θ_C ∝ (n_f − n_w)h/a when h≪ a and |n_f − n_w| ≪ n_w − 1. When using the irradiance near 82.8° to size bubbles, this shift in θ_c is negligible for anticipated coating parameters.

© 1986 Optical Society of America