Scale-free measurements of oil droplet rising speeds during bacterial bio-degradation*

In marine environments, spilled oil droplets can be transported over kilometers in the water column. As this oil is biodegraded, growing bacteria on the surface of these microscopic droplets give rise to complex emergent phenomena, including interfacial deformations which can generate complex shapes and significantly enlarge droplets. A complete understanding of the fate and transport of spilled oil requires bridging the gap between these length scales and determining how microscale processes affect large-scale transport of oil. Here, we describe experimental observations of rising oil droplets using a scale-free vertical microscope. Droplets of radii 10-100 µm are imaged in a rotating fluid chamber as bacteria attach to and grow on the interface. The rotation rate is matched to the droplet rising speed to allow imaging over 24 hours, corresponding to vertical distances of 3-300 m. The formation of a dense bio-aggregate due to bacterial colonization of the interface significantly alters the rising speed. This effect depends on the droplet radius, and can cause oil to remain in the water column for months longer than previously predicted.