Evidence for ubiquitous preferential particle orientation in representative oceanic shear flows

Area of Science:

• Oceanography
• Fluid Dynamics
• Biogeochemistry

Background:

• Oceanic particle fields are complex and influence aquatic optics and remote sensing.
• Previous studies hinted at particle alignment but lacked comprehensive field data.

Purpose of the Study:

• To characterize particle fields and flow in undisturbed oceanic environments.
• To investigate preferential particle alignment and its relationship with flow characteristics.

Main Methods:

• In situ measurements using a submersible holographic imaging system and acoustic Doppler velocimeter.
• Characterization of nearly one million particles (14 μm to 11.6 mm) with diverse shapes and sizes.
• Analysis of particle orientation probability density function (PDF) under varying flow conditions.

Main Results:

• Particles showed preferential horizontal alignment across all sampling stations.
• Alignment was strongest in regions of low velocity shear and weak turbulence.
• Higher particle aspect ratios correlated with increased alignment, matching theoretical models.

Conclusions:

• This is the first comprehensive field study demonstrating nonrandom particle alignment in aquatic systems.
• Findings challenge the assumption of random particle orientation in aquatic optics and remote sensing.
• Phytoplankton may evolve larger aspect ratios for enhanced light harvesting.