Fundamental Study of Bubble Coalescence in Solutions and on Surfaces

• Author / Creator

  Liu, Bo

• The coalescence of air bubbles is an elementary process influencing the performance of various industrial processes such as oil extraction, water purification, and mineral flotation. The possible coalescence between two colliding bubbles is significantly influenced by the liquid drainage rate from the thin film trapped between the bubbles.
  The focus of this thesis is to investigate the film dynamics between fast colliding air bubbles in aqueous solutions. A unique method is developed based on a custom-designed Dynamic Force Apparatus (DFA). Two bubbles, one generated at the orifice of a capillary tube, the other immobilized on a transparent hydrophobic glass (surface microbubble), are brought together for the collision at controlled speeds. During the bubble collision, the interaction force and the interference fringes are obtained simultaneously. Experimental parameters can be flexibly adjusted, including the surface microbubble size, solution concentration, and collision speed.

  Under aged/contaminated conditions, the air-water interface is immobile. The interaction force and the initial formation of the dimple profile during the collision, agree well with the prediction from the Stokes-Reynolds-Young-Laplace model with the tangentially immobile boundary condition at the air-liquid interface. However, an ‘express exit’ was observed during bubble collision, leading to the unexpected rapid drainage of the trapped liquid. This phenomenon partially explains the shorter coalescence times from experiments as compared with model predictions. The film rupture thickness was consistently observed at 25±15 nm.

  In contrast, when clean water was used, the film thinning rate was almost equal to the collision speed, resulting in bubble coalescence within milliseconds. The experimental data are explained by a theoretical model assuming mobile boundary condition at the air-water interface. Changing the interfacial tension by 10-4 N/m, by adding a small amount of surfactant, would be sufficient to immobilize the air-water interface. The surface mobility is determined by the competition of fluid shear stress on the film surface and the Marangoni stress that arises from the uneven distribution of surface-active components at the air-water interface. The above finding proved the existence of fully mobile air-water interfaces in bubble coalescence, and also explained why this boundary condition is difficult to be achieved experimentally in previous research.

  The following exploration was conducted in surfactant solutions at the concentration of up to 2 mM. A simple experimental technique was developed, in which the freshly generated bubbles can collide after staying in bulk for a very short period (~10 ms or ~50 ms). From the bounce or coalescence results, we found that the mobile air-water interface is achievable even in surfactant solutions similar to those used in industrial processes. The freshly generated bubbles can be mobile and may switch to immobile after staying in bulk for tens of milliseconds. The surface mobility is jointly determined by the aging time and bulk surfactant concentration, corresponding to the dynamic adsorption of the surfactant onto the air-water interface. This work bridges the fundamental understanding of surface mobility to real applications.

• Subjects / Keywords

  • thin film drainage
  • Bubble coalescence
  • lubrication
  • surface mobility

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-myfy-gb73

• License

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