Effects of impurities on the bursting dynamics of large bubbles
Multiple-holes formation during bubble bursting
Background
The bubble bursting process is of crucial importance in many chemical, environmental, and medical science and engineering settings. The microscopic droplets formed during bubble bursting can remain suspended in air and are known to play an important role in the mass, momentum and energy transport between the ocean and the atmosphere. Except for the ubiquitous surfactant in nature, in most practical situations, the aqueous phase usually consists of impurities such as microorganisms, contaminant droplets, and microbubbles with varying sizes. These microscopic impurities can be accumulated in bubble film and thus in the aerosolized droplets due to bursting, causing airborne transmission of pollutants and infectious diseases, raising serious public health concerns for communities near contaminated ocean, rain puddles and wastewater treatment plant.
Figure 1. Origins of the different kinds of sea spray droplets. Andreas et al. (1995), H. Lhuissier and E. Villermaux (2012)
Experiment
To investigate the impurity effects on the bursting process of a bubble, immiscible solid particle/oil droplet types and their volumetric concentrations in water are varied. Three types of immiscible particles or droplets have been used to investigate the effects of surface/interfacial tension. The impurities types include one solid particle that cannot spread at the air/water interface and two oil droplets with varying non-zero spreading coefficients, we establish three test cases, namely, non-spreading, low and high spreading coefficient cases.
Figure 2. (a) Bubble film retraction velocity, film thickness and the number of droplets were quantified using high-speed visualization. (b) Droplet size and velocity distribution were measured using digital in-line holography.
Figure 3. Sample time series of bubble bursting process for (a) pure water, (b) SiO2 particle-in-water suspension, (c) silicone oil-in-water emulsion and (d-e) sunflower oil-in-water emulsion.
Figure 4. Schematic for water bubble cap retraction and fragmentation, rim, rim thickness, ligament, ligament spacing and film droplets.
Figure 5. Enhanced sample holograms after background subtraction, three consecutive frames sample of depth compressed minimum intensity image used for segmentation for (a-b) water only case and (c-d) sunflower seed oil case at 0.05% volumetric concentration.