Measurement of Bubbles
Bubbles play important roles in oceanography. Bubbles generated by breaking waves, wind, or rain, are responsible for a significant portion of the atmosphere-ocean gas exchange. Bubbles can greatly enhance the exchange of gases across the air-sea interface with the rise of the bubble plume back to the surface and by transfer of gases through the walls of individual bubbles. Bubbles bursting at the surface contribute to aerosol production. Rain impacting the free surface can enhance mixing and generate bubbles during impact. The effect of rain can contribute appreciably to the total exchange in certain regions with high rain rates such as the western equatorial Pacific and Bay of Bengal during the monsoon season. The presence of bubbles and bubble plumes can also affect the transmission of acoustic waves through the ocean.
The ABS Acoustic Bubble Spectrometer®©, can provide data on bubble size distributions and void fractions of bubbles formed during these processes. Passive acoustic methods can only detect the most active bubbles which emit sound during formation and oscillation, but other smaller or stable bubbles were invisible to such a method. In a large distribution of bubbles smaller bubbles are more important than large bubbles due to their higher surface area to volume ratios, and greater residence time in the water column, which provide much higher gas transfer opportunity. The ABS Acoustic Bubble Spectrometer®©, can provide direct measurement of bubble sizes in order to provide fundamental understanding of the gas transfer through bubbles during their formation, stay in the liquid, and migration processes.
The ABS Acoustic Bubble Spectrometer®© has been used in studies of breaking waves, bubble formation in ship’s wakes, and cavitation susceptibility studies.
Figure 1. Beveled ABS hydrophones made to be towed behind a ship.
Rain Droplet Impacts
The impact of liquid droplets of specific size and speed on a body of water can generate bubbles when the envelope of air trapped between the droplet and the bulk solution is pinched off (Figure 2). The formation of entrained bubbles during droplet impact was examined using high-speed video imaging. This study mapped the regions of droplet size and impact velocity conditions where this type of relatively large bubble was entrained. In that study, we found that the drop sizes and impact velocities outside of the drop diameter-impact velocity entrapment region could generate more number of smaller bubbles instead of one large bubble. It was demonstrated that multiple impacts such as an impact on the edge of a crater would increase entrainment and drive the bubble penetration deeper. Figure 3 shows an example of Dynaflow's recent visualization study of droplet impacting on the free surface.
Figure 2. Close-up pictures and numerical simulations of the bubble formation at the bottom of the cavity generated by a rain drop event. Dimensionless time t* (=Ut/R, U=impact speed, R=drop radius) is shown in each figures.
Figure 3. High speed video images of multiple droplet impact (~4 mm diameter) in saltwater conducted at Dynaflow. Scale at left is in mm. In this case, most of the bubbles are formed by the formation and impact of the secondary droplet after the initial impact.
