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Professor James Duncan Discovers Effects of Surfactant in Ocean Waves

Professor James Duncan Discovers Effects of Surfactant in Ocean Waves

University of Maryland Professor of Mechanical Engineering James H. Duncan and Post Doctoral Researcher Xinan Liu have made a discovery concerning the effects of surfactants on spilling breaking waves. They have determined in laboratory simulations that surfactants, like those found in the ocean, can cause the formation of small jets in these waves. These jets are likely to affect the transfer of heat, mass and energy across the air-sea interface. Their findings were published in the January 30, 2003 issue of the journal Nature.

Background

Wind conditions affect all bodies of water. In high wind conditions, the crests of the breaking waves turn white, signaling the entrainment of air bubbles and the ejection of water droplets. These breakers are known as white caps. At even higher wind conditions, plunging breakers are formed. In these breakers, a jet ejects forward from the crest of the wave and plunges into the wave face, entrapping large quantities of air and ejecting large numbers of water droplets into the atmosphere.

Plunging breakers at the shoreline are the subjects of numerous photographs, such as surfer photos. Under light to moderate winds in open water, weak breakers with short wavelength occur. These breakers do not entrain a significant amount of air because the surface motion does not have enough kinetic energy to overcome surface tension. These are called micro-scale breakers. Since there is no white cap they are difficult to see.

The research conducted by Duncan and Liu is important in understanding the natural effects of the oceans on the earth's climate. The rates of transfer of mass, heat, momentum, and energy between the ocean and the atmosphere are critical components in the control of our climate. For example, a large portion of the carbon dioxide discharged into the atmosphere ends up in the oceans. The turbulence, water droplets, and air bubbles created by wind-generated breaking waves dramatically increase these transfer rates.

Surfactants are substances that reduce surface tension and create surface elasticity and viscosity. Soaps and detergents are examples of surfactants. Each surfactant molecule has a hydrophobic (water hating) end and a hydrophilic (water loving) end. They are attracted to the water-free surface where they form a layer that is a single molecule thick. In any natural body of water, surfactants are ubiquitous. Surfactants in the oceans are created as by-products of the respiration of organisms such as plankton, and are found in the highest concentrations in coastal regions.

Research

In their research, Duncan and Liu studied breaking waves in the Department of Mechanical Engineering's wave tank. The tank is 50 feet long, 4 feet wide and 3 feet deep. The waves were generated with a mechanical wave maker instead of with wind.

The behavior of the waves was measured photographically. The camera was set to take 300 pictures per second and was positioned to view the wave from the side. The wave was illuminated with a 1-mm-thick light sheet from a Nd:YAG laser. The light sheet was oriented parallel to the direction of motion of the wave. The water was mixed with a fluorescent dye and an optical filter was placed in front of the camera. With this system, the light source for the pictures is the fluorescing dye. The entire photographic system was mounted on an instrument carriage that was set to move along the tank with the crest of the breaking wave. The combination of the high frame rate and the crest-fixed camera reference frame resulted in a movie of the breaking process that is dramatically slowed down in time, allowing for observations and measurements that are impossible with the human eye or a standard video camera.

For a given wave maker motion, Duncan and Liu observed the breaker in water with surfactant levels ranging from near zero (clean water) to high enough to reduce the surface tension to one-half its value in clean water. In the clean water case, breaking was initiated by the appearance a small bulge on the wave crest and a train of capillary waves growing on the front wave face. The crest became turbulent without overturning of the water free surface. For most surfactant conditions, the breaking behavior was qualitatively similar to the clean water case, though the capillary waves decreased and the wave's crest bulge diminished in size. In these cases, the surface motion during breaking seemed less energetic than in clean water. However, at the highest surfactant concentration, the wave changed dramatically. A tiny jet formed on the bulge near the toe. This jet was ejected into the air and fell to the water surface upstream of the toe. At impact the jet was about 10 mm long and 5 mm thick, and entrapped a small tube of air. The surface motion in this case was more energetic than in the case without surfactants. These experiments mark the first time a jet of this type had been observed. The very high temporal and spatial resolution of their measurement system offered the capability to discover this type of breaker. This phenomenon may be important since the entrained air is likely to increase air-water transfer rates.

This work is funded by the Ocean Sciences Division of the National Science Foundation and is done in collaboration with Prof. Gerold Korenowski of the Department of Chemistry at Rensselaer Polytechnic Institute. In this work, Prof. Korenowski is performing additional measurements on breaking waves.

For more information, please contact Dr. Duncan at duncan@eng.umd.edu.

February 15, 2003


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