University of Illinois, Urbana-Champaign
"Sonochemistry In Microgravity"

TEAM CONTACT: Graeme MacDonald
FACULTY ADVISOR: Dr. Henrique Reis

ABSTRACT: Sonochemistry is an emerging field of experimental chemistry that offers great potential for driving high-energy reactions. By focusing high-frequency pressure waves (ultrasound) on a chemical mixture, cavitation is induced in the solvent. Cavitation is the creation of micron-sized bubbles that grow isothermally and then collapse adiabatically (meaning without heat transfer). As the bubble collapses, for about a nanosecond it reaches extremely high temperatures and pressures, causing a local hot spot in the otherwise cold liquid. This hot spot is approximately the temperature of the surface of the sun, the pressure at the bottom of the ocean, and lasts about as long as a lightning strike. This hot spot provides the energies necessary to overcome high chemical reaction energy barriers.

A general effect of ultrasonic irradiation is the considerable increase in chemical reaction rates. Sonochemistry has raised some reaction rates by as much as six orders of magnitude. The presence of buoyancy due to gravity, however, leads to the distortion of cavitation bubbles from a true spherical shape. As a result, they collapse much sooner than they would under ideal circumstances, leading to a smaller energy transfer rate from the bubble collapse to the surrounding fluid. Since the effects of buoyancy are nearly eliminated in microgravity, sonochemistry can benefit from bubbles that can grow to be more spherical than those formed under the influence of a gravitational field. As a result, sonochemistry can achieve higher localized energies and thus higher reaction rates in the fluid medium in microgravity.

The Float'n Illini.has designed an experiment that uses cavitation from ultrasound to create weak nitric and nitrous acids in a water solution. The cavitation breaks water into hydrogen and hydroxyl radicals, which then react with nitrogen gas dissolved in the water to create the weak acids. If, in microgravity, higher temperatures and pressures are achieved in the collapsing bubbles, the rate of formation of the weak acids will increase. The build-up of the weak acids in solution will cause an increase in ion concentration and a measurable drop in pH. The weak acid formation rate is thus expressed through an increase in conductivity and a decrease in pH. The reaction rate should remain constant within 1-g, but proceed slower in 2-gs and faster in 0-gs. Coupling an accelerometer to the system will allow us to correlate the gravitational acceleration with the reaction rates found through the pH and conductivity data.

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Last Modified: Thu Mar 02, 2000