Acoustic backscattering observations from non-spherical gas bubbles with ka between 0.03 and 4.4

The study of gas bubbles in liquid media is of importance in many areas of research. Gas bubbles are often studied using in situ measurement techniques; however, acoustic inversion techniques have also been used to extract physical properties of gas bubbles. These inversion techniques rely on existing analytical scattering models; however, these models often assume that the gas bubbles are spherical in shape and have an equivalent bubble radius, a, that is small compared to the incident acoustic wavelength (ka ≪ 1), which is not always valid. This study aims to understand how the departure from these assumptions affects the acoustic backscattering cross section, σ_bs, of non-spherical gas bubbles. Experimental estimates of σ_bs of non-spherical gas bubbles of different sizes, with ka values ranging between 0.03 and 4.4, were compared to four commonly known analytical σ_bs models. All models performed equally at predicting σ_bs for ka smaller than 0.5; however, there was no model that better predicted the experimental estimates of σ_bs for ka larger than 0.5, regardless of bubble shape. Large variabilities in the experimental estimates of σ_bs are observed for ka larger than 0.5, which are caused by the variability in bubble shape and size, as well as the bubble's orientation.