Bouncing Liquid Jets James Bomber, Nick Brewer, and Dr. Thomas Lockhart Department of Physics and Astronomy, University of Wisconsin - Eau Claire Abstract When a jet of canola oil strikes a bath of the same oil within the correct range of speeds and angles, the jet will rebound off the bath back into the air. This project studied the air film which separates the jet from the bath making this phenomenon possible. Using high-speed video, lasers, and flow-tracing with dyes and smoke, visual evidence of the existence of the air film is presented. The processes by which the air film and bounce are maintained are also discussed. Acknowledgements The funding for our project was provided by the Blugold Fellowship Program and Summer Research Experience for Undergraduates at the University of Wisconsin - Eau Claire. We would also like to thank the Physics and Astronomy Department, University of Wisconsin - Eau Claire, for allowing us to use their digital camera. Fig. 1. An incoming jet reflects off the surface of a bath of canola oil. Bouncing Jet Mechanics As the incoming jet plunges into the bath, it captures some of the surrounding air. The jet remains separated from the oil bath by this thin air layer which is being constantly replenished by the incoming jet. The air layer provides a very low friction surface. The upward force from the surface tension of the bath deflects the incoming jet causing it to be reflected upwards. The bounce will continue until the air layer is disturbed. (1) (2) The bounce will only occur for a certain range of incident angles and speeds. Near the middle range of angles and speeds, the bounce is stable and can continue for minutes at a time. Near the edges of the range it becomes increasingly unstable until the air layer breaks and the bounce stops. If the incoming speed or angle is too large, the jet plunges into the bath because the surface tension does not provide a large enough force to reflect the jet. If the incoming speed or angle is too small, the jet will ride along the surface of the bath. Laser Jet In the photo shown below a laser beam has been placed so that the laser light travels thorough the jet. This is similar to a fiber optic cable where the light is totally internally reflected and travels the length of the cable. As can be seen from the laser light in the photo, the bouncing jet is separated from the oil bath. The jet is illuminated by light that is scattered due to the laser striking bubbles and other particles in the jet. After the bounce, the jet merges with the bath. The particles that were in the jet can be seen exiting into the bath. Data Collection Our experimental setup allowed us to take high quality digital photos and 600 fps videos from close range using a Casio EX-F1. ImageJ was used to process the videos of the bounce and track the dye tracers (Fig. 5-6). (3) ImageJ calculated the velocity of each dye tracker and the results were imported to Excel and calibrated using known measurements. For each time interval the respective velocities of each track were averaged and fit with a linear trend line. Results and Conclusion It was determined that the fractional energy loss for the reflected portion of the bounce ranged between approximately.30 and.45. It was also determined that the velocity of the jet decreased linearly along the length of the dimple. It was shown using smoke tracers and lasers that the oil in the jet and the bath remain separated by the air film during the bounce. The rotating oil drop at the end of the incoming jet also provided evidence for this separation. Objectives To find experimental evidence that the air film is being continually replenished by the incoming jet. To show that the oil in the jet remains separate from the bath throughout the bounce. To study the change in velocity and energy loss of the jet during the bounce. References (1)Thrasher, Matthew, et al. "Bouncing Jet: A Newtonian liquid rebounding off a free surface." Physical Review E 76, (2007). (2) Dell’Aversana, Pasquale, et al. “Suppression of coalescence by shear and temperature gradients.” Physics of Fluids 8 (1), 15 (1996). (3) All image processing was done using ImageJ, a public domain Java image processing program available from Smoke Tracer To show that the air film is being replenished by the jet during the bounce, a smoke tracer was produced near the incoming jet. When the smoke was close enough to the jet it was pulled into the dimple. Figures 2-4 show the smoke emerging on the other side of the dimple. This shows that the jet pulls nearby air into the dimple, continually replenishing the air film. If the air film was not being replenished, it would leak out causing the jet and bath to come in direct contact therefore stopping the bounce. Fig. 7. Fractional energy loss is plotted according to incident angle. Fig. 6. Average velocity of the jet is plotted corresponding to each video frame. Calculations Using the standard equations of motion, (1) (2) a formula for the path of the bounce was derived. (3) Taking the coefficients of x and x 2 from an Excel curve fit of experimental data, we were able to first solve for theta and then for the velocity. The incident angle used in the formula is the angle between the bath and the jet. The fractional energy loss of the reflected bounce was computed using the formula (4) Fig. 8. A laser beam traces through the jet like a fiber optic cable. Rotating Drop A drop of oil placed in front of the jet does not coalesce but rather remains separate from the bath and the jet. This separation is maintained by an air film that the jet is continually dragging underneath the drop. The drop stays in place because there is a small dimple created where the jet enters the bath. The evidence that the air film is moving is in the fact that drop rotates in place. The frictional force between the moving air film and the drop causes the rotation. Fig. 9. An oil drop rotates in place at the end of an incoming jet. Fig. 5. Dye tracers being tracked by ImageJ. Fig.4. Fig. 2. Fig. 3. Fig.2-4. Image sequence showing a smoke tracer being pulled through the dimple by the jet.