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In the maritime industry, the injection of air bubbles into the turbulentboundary layer under the ship hull is seen as one of the most promisingtechniques to reduce the overall fuel consumption. However, the exact mechanismbehind bubble drag reduction is unknown. Here we show that bubble dragreduction in turbulent flow dramatically depends on the bubble size. By addingminute concentrations (6 ppm) of the surfactant Triton X-100 into otherwisecompletely unchanged strongly turbulent Taylor-Couette flow containing bubbles,we dramatically reduce the drag reduction from more than 40% to about 4%,corresponding to the trivial effect of the bubbles on the density and viscosityof the liquid. The reason for this striking behavior is that the addition ofsurfactants prevents bubble coalescence, leading to much smaller bubbles. Ourresult demonstrates that bubble deformability is crucial for bubble dragreduction in turbulent flow and opens the door for an optimization of theprocess.

Bubble drag reduction requires large bubbles