Intellectual Merit: The generation of droplets and their subsequent motion and evaporation in wind wave systems are important components of the processes of mass, heat and momentum transfer across the air-water interface. Measurements of the distribution and sizes of these droplets have been made in wave tanks and in the field. It is well known that the ocean surface is covered by surfactant mono-layers and that surfactants can have important effects on many free surface phenomena including the ejection of water droplets. The objective of the proposed laboratory experiments is to explore the mechanism of droplet formation by breaking waves and the subsequent motion and evaporation of these droplets with and without surfactants. Experiments with mechanically generated waves, wind waves and mechanically generated waves in the presence of wind will be carried out in clean water and water mixed with various concentrations of several soluble surfactants, and limited experiments in salt water. Cinematic Laser Induced Fluorescence (LIF) measurements of the streamwise and cross stream breaking wave pro le histories, shadowgraph movies of droplet ejections, subsequent motions and size distributions, and Particle Image Velocimetry (PIV) measurements of the 'flow field in the wave crest will be performed from an instrument carriage which will be set to move along the tank with the breaking wave crests or other significant features of the wave system. Measurements of the vertical distribution of droplets and droplet fluxes will also be carried out at fixed locations in the wave tank. Water surface proper-ties including surface pressure isotherms and surface viscoelastic properties will be measured using a Langmuir trough/longitudinal wave propagation device set up in a small tank and using water samples taken from the wave tank. Also, the maximum bubble pressure method will be used in the wave tank in order to determine the in-situ diffusion constants and surface tension. Finally, Second Harmonic Generation (SHG) measurements of instantaneous free surface surfactant concentration will be performed at a fixed location in the tank. All of the above measurements will be used to explore the physics of the droplet ejection, motion and evaporation process.
Broader Impacts: Since droplet production by breaking waves makes an important contribution to air-sea transfer processes, the proposed research will contribute to our understanding of the climate including important phenomena such as global climate change. The results of this study will be applicable to understanding engineering flows (such as flow around ships or in chemical processing) in which free surface and spray generation are involved. The study will contribute to the education of students in the sciences by supporting one graduate student, one undergraduate student and a part-time post doctoral researcher. The PI will also give approximately one talk or host one field visit to his laboratory each semester for local area high school students in order to introduce them to the subject area of this research. It is hoped that this may foster interest in pursuing oceanography in their university studies.
