MEASURING THE TEMPERATURE FIELD OF AN OPTICAL THERMOCAVITATION BUBBLE USING PLANAR LASER-INDUCED FLUORESCENCE
Vicente Robles Jr.1, Darren Banks2.
1Pomona College, Santa Ana, CA, 2University of California, Riverside, Riverside, CA.
Cavitation has developed from an undesirable, damaging phenomenon toward one with applications in the biomedical and fluid-control fields. Optical thermocavitation is a process in which a laser is focused into an absorptive liquid, causing it to superheat. This leads to the creation, growth, and collapse of a vapor bubble. Applications such as laser-assisted surface cooling enhancement and skin-poration by optical cavitation are being developed in the lab of Dr. Aguilar (University of California, Riverside). The former seeks to achieve cooling through induced mixing while the latter is a drug delivery method that punctures the outer layer of skin. To explore the dynamics of bubble formation and effects on surroundings, a technique is being developed to measure the temperature field around a cavitation bubble. The non-intrusive technique is called planar laser-induced fluorescence. It uses rhodamine B in an aqueous copper nitrate (CuNO4, 24% by mass in water) solution. Rhodamine B fluoresces when exposed to a 440 nm wavelength laser. We developed a calibration curve relating the measured intensity with temperature. The fluorescence intensity was measured against temperature in 10 ℃increments from 25 ℃ to 75 ℃, and a linear relationship was found. Fluorescent intensity decreases by approximately 10% for each interval. Then, the optimal concentration of rhodamine B in the solution was determined. At room temperature, we found that, as the concentration went from 1 droplet to 5 droplets per 10 mL of aqueous CuNO4, the average fluorescence increased by about 50 (RGB-pixel average). The next step is implementation of a carbon-nanotube solution as the absorption medium and perfecting the image analysis algorithm.