No abstract

The high-speed impact of a droplet on a bulk fluid at high Weber number (We) is not well understood but is relevant to the production of marine aerosol by raindrop impact on the sea surface. These splashes produce a subsurface cavity and a crown which closes into a bubble canopy, but a floating layer of immiscible oil, such as a crude oil slick, alters the splash dynamics. The effects of oil layer fluid properties and thickness, droplet size and impact speed are examined by high-speed visualization. Oil layer rupture and crown behaviour are classified by dimensional scaling. The subsurface cavity volume for impact on thick layers is shown to depend on the Reynolds number (Re), although canopy formation at high Re introduces a competing We effect since rapid canopy closure is found to retard cavity expansion. Time-resolved kinematic measurements show that thin crude oil slicks similarly alter crown closure and cavity growth. The size and spatial distributions of airborne droplets are examined using high-speed holographic microscopy. The droplets have a bimodal distribution with peaks at 50 and 225 µm and are clustered by size at different elevation angles. Small droplets (50 µm) are ejected primarily at shallow angles, indicating production by splashing within the first 100 µs and by breakup of microligaments. Larger droplets (225 µm) are found at steeper elevation angles, indicating later production by capillary instability acting on large ligaments protruding upward from the crown. Intermittent droplet release while the ligaments grow and sweep upward is thought to contribute to the size-dependent spatial ordering. Greater numbers of small droplets are produced at high elevation angles when a crude oil layer is present, indicating satellite droplet formation from ligament breakup. A crude oil layer also increases the target fluid Ohnesorge number, leading to creation of an intact ejecta sheet, which then ruptures to form aerosolized oil droplets.

Krill are aquatic crustaceans that engage in long distance migrations, either vertically in the water column or horizontally for 10 km (over 200 000 body lengths) per day. Hence efficient locomotory performance is crucial for their survival. We study the swimming kinematics of krill using a combination of experiment and analysis. We quantify the propulsor kinematics for tethered and freely swimming krill in experiments, and find kinematics that are very nearly metachronal. We then formulate a drag coefficient model which compares metachronal, synchronous and intermediate motions for a freely swimming body with two legs. With fixed leg velocity amplitude, metachronal kinematics give the highest average body speed for both linear and quadratic drag laws. The same result holds for five legs with the quadratic drag law. When metachronal kinematics is perturbed towards synchronous kinematics, an analysis shows that the velocity increase on the power stroke is outweighed by the velocity decrease on the recovery stroke. With fixed time-averaged work done by the legs, metachronal kinematics again gives the highest average body speed, although the advantage over synchronous kinematics is reduced.

Lay Abstract Antarctic krill, one of the most important species of the Southern Ocean ecosystem, are denser than water and must swim continuously to avoid sinking. They swim by beating their five pairs of swimming legs in a sequential pattern from back to front. Hovering by continuous swimming is costly in energy, and we hypothesize that the observed sequential stroking pattern provides an efficient means for krill to remain in the water column. Our goal was to measure the flow around a swimming Antarctic krill to understand the swimming mode and the induced water motion. We used four high‐speed cameras filming at 400 frames per second to measure the three‐dimensional flow produced by a krill hovering in an aquarium, allowing us to estimate the energy required. An additional estimate was made based on a theoretical model that is usually applied to helicopter hovering. The detailed velocity measurements provided information about the flow induced by the motion of the individual legs and the added benefit of the leg moving into water that was already flowing due to the motion of the previous leg. The water motion underneath the krill appeared as a time‐varying jet consistent with that observed for other multilegged swimming animals that use the sequential stroking pattern. This suggested that Antarctic krill are operating in a similar regime of high energy efficiency.