A Field Study of Particle Orientations in Shear Flows

Oceanic waters are populated by a myriad of particles of various shapes and sizes which play a critical role in the propagation and scattering of light. Ocean optics theory and models (e.g., radiative transfer) inherently assume that the particles are randomly oriented in the water column. Increasingly, this critical assumption has been challenged by a handful of recent theoretical and experimental studies. Thus, it is imperative to thoroughly assess the prevalence of any non-random particle orientation by characterizing the biophysical interactions through in situ measurements in natural, undisturbed oceanic flows.

To achieve this objective, a suite of optical and acoustic instruments were concurrently deployed during field measurements recently conducted at East Sound, WA. The platform consisted of the following instrumentation: (i) a submersible holographic microscopy system (HOLOCAM) capable of acquiring video images at 15 Hz, while maintaining adequate resolution to characterize particles ranging in size over three orders of magnitude, i.e. 1 micron to ~ 5 mm; (ii) an acoustic Doppler velocimeter and a high resolution Doppler profiler to provide simultaneous measurements of the turbulence and shear in the water column, and (iii) an optical package consisting of several instruments at different orientations measuring polarized and unpolarized volume scattering functions and path attenuation. Over a span of 10 days, nearly 40 runs, each spanning 10-15 minutes were obtained by slowly profiling the platform with a free fall velocity of 4-10 cm/s through the water column under varying environmental conditions. In this presentation, we explore whether the local small scale flow structure plays an important role in preferentially orienting the particle field.