Experimental investigations of the effects of surfactants on droplet production by bubbles rising to a free surface.

Abstract

Air bubbles traveling upwards from within a liquid, burst at the free surface and eject droplets into the air. The liquid droplets produced from the bursting of air bubbles at the free surface of the ocean have long been considered as constituting a significant fraction of the marine aerosol production. This process plays an important role in the transfer of mass and energy between the ocean and atmosphere. The droplets are generated by two distinct mechanisms which characterize the drops as either `jet droplets' or `film droplets.' The goal of this study is to obtain statistical data about the size of these droplets produced by air bubbles bursting on a non-quiescent free surface and study effects of surfactant on the droplet distribution. Two different surface conditions are created by using "clean" water and a 0.4% v/v aqueous solution of Triton X-100 surfactant. A bubble field is created with air injected through an array of hypodermic needles arranged in a grid pattern and five different bubble production rates are studied for each surface condition. Measurements of the bubble diameters as they approach the free surface are obtained with diffuse light shadowgraph images. A laser-light high-speed cinematic shadowgraph system is employed to record and measure the diameters and motions of the droplets. The movies are processed on a custom-built MATLAB code which measures droplets as small as 100 μm to within 7% error. Droplets with diameters between 50 and 100 μm are also measured, but with lower accuracy. The droplet size distributions as a function of the production rate and surfactant conditions are reported. The results show a clear distinction between the droplet distributions obtained in clean water and the surfactant solution. A bimodal droplet distribution is observed for clean water with at least two dominating diameter peaks. For the surfactant solution, a single distribution peak is seen.