AskNature

“To jump out of water onto sea ice, emperor penguins must achieve sufficient underwater speed to overcome the influence of gravity when they leave the water. The relevant combination of density and kinematic viscosity of air is much lower than for water… Analysis of published and unpublished underwater film leads us to present a hypothesis that free-ranging emperor penguins employ air lubrication in achieving high, probably maximal, underwater speeds (mean ± SD: 5.3 ± 1.01 m s–1), prior to jumps. Here we show evidence that penguins dive to 15 to 20 m with air in their plumage and that this compressed air is released as the birds subsequently ascend whilst maintaining depressed feathers. Fine bubbles emerge continuously from the entire plumage, forming a smooth layer over the body and generating bubbly wakes behind the penguins… From simple physical models and calculations presented, we hypothesize that a significant proportion of the enhanced ascent speed is due to air lubrication reducing frictional and form drag, that buoyancy forces alone cannot explain the observed speeds…” (Davenport et al. 2011: 171)

“Before jumping out of the water onto ice, the penguins swim at the surface and then dive on inspiration (Kooyman et al. 1971). We believe they dive with plenty of air in the plumage, with erected feathers making room for an air layer about 25 mm thick (following Du et al. 2007). Kooyman et al. (1971) described the grooming behaviour by which surface swimming emperor penguins load their plumage with air and we confirmed this by observation… They subsequently dive to ~15 to 20 m (by which depth the air volume will have decreased by a substantial amount…). During the dive, or when achieving that depth, they depress the feathers (to fix the plumage volume at the new, decreased level). When the birds swim quickly upwards, the decompressing air will flow out by virtue of the available fixed plumage volume being substantially less than the initial volume. Plumage consists of a fine, multi-layered mesh over the whole of the body surface comparable to a porous medium with an estimated pore size of <20 μm (Du et al. 2007), so the expanding air will automatically issue as small bubbles. This arrangement resembles the flat-plate experiments of Sanders et al. (2006), who used a 40 μm pore size sintered stainless steel strip for microbubble air injection. The ‘active’ part of the process consists solely of maintenance of depressed feathers during the near vertical phase of the ascent in order to regulate expulsion of air driven by decompression. As bubbles continue to enter the boundary layer along the plumage, they are swept downstream and move outwards, thus increasing the void fraction in the boundary layer downstream to finally leave in the wake behind the bird; or they coalesce with other bubbles to form rather large bubbles at the outer edge of the boundary layer…” (Davenport et al. 2011: 175-176)

“…Emperor penguins ascending rapidly in the water column to jump onto ice shelves emit bubble clouds into the turbulent boundary layer over most of the body surface throughout their ascent. Emission does not diminish as a penguin approaches the surface, but increases. Because the bubbles are produced over most of the body surface, their drag-reducing function should exceed the performance of marine engineering plate/ship models described so far, in which maintaining sufficient bubble coverage within the turbulent boundary layer is a major problem.” (Davenport et al. 2011: 180)