The role of free-surface turbulence and surfactants in air–water gas transfer

Laboratory measurements of gas-transfer velocity and free-surface hydrodynamics were made for oscillating grid-stirred turbulence. Air–water gas-transfer velocities were determined from the waterside dissolved oxygen mass balance, and an innovative digital particle image velocimetry technique was used to measure the two-dimensional free-surface flow field. The impact of adventitious and purposely introduced surfactant films on free-surface turbulence and gas-transfer rates was explored quantitatively. As expected, bulk turbulence was unable to provide a unique relationship for the gas-transfer rates observed, owing predominantly to surfactant effects. However, the surface divergence computed from the free-surface velocity field was capable of reconciling the gas-transfer data. This is possible evidence confirming that the free-surface divergence is an important process involved in interfacial gas transport. A relationship for the gas-transfer velocity in terms of the surface divergence and the surface pressure is presented that successfully relates the interactions between surface renewal, surfactants, and gas transfer.