Multiphase flows

In general, the study of fluid mechanics systems is complex due to the broad range of scales of motion that they present. Bubbles are a particular type of two-phase flow characterized by the dispersion of a gas phase into a liquid phase, in which the bubble’s small scale can have a significant impact on the overall flow dynamics. Multiphase flow differential features with respect to single-phase flows are the presence of capillary forces, appearing at the interface; and the difference in physical properties of the phases, namely density. These induce different physical mechanisms that can be leveraged for industrial applications.
Bubble lifecycle: a) nucleation b) growth c) detachment d) dispersion
Bubble formation starts with the release of gas molecules from the electrode into the liquid in dissolved form. However, the formation of a bubble is not immediate. Instead, the liquid has to attain a high enough supersaturation before a bubble can nucleate by agglomerating gas molecules into a separate phase from the liquid. This process is thermodynamically favored at surfaces, edges, and cavities. Once the bubble has nucleated, it grows by sucking in gas molecules from their surrounding supersaturated liquid. This is opposed by the capillary force, which acts at the interface of the bubble surface. At the contact line between gas, liquid and solid, surface tension results in an adhesive force, which keeps the bubble attached. Bubbles are finally detached when the adhesive force is overcome by other forces, such as inertia, shear, and buoyancy. At this point, the bubble is dispersed into the liquid. Owing to the large differences in density, bubbles experience a significant buoyancy force, which accelerates them with respect to the liquid phase, producing a significant stirring mechanism. In the vicinity of the electrode, this accelerates the arrival of fresh reactants to the active sites.

Surface tension opposes the deformation of the bubble, conferring it with an elastic behavior. Similarly to a spring, surface tension tries to restore the spherical shape, accelerating the fluid around it. The resulting capillary force introduces extra terms to the evolution of vorticity, which is known to enhance mixing. In the context of water electrolysis, the additional mixing mechanisms stated above have a very significant effect on the intensification of mass transfer processes. Simultaneously, the adherence of bubbles to the surface also blocks active sites from reacting further, such that a quick release is beneficial. However, this will result in smaller bubbles. My research tries to ulitmatelly answer the following questions: What is the most optimal bubble departure to enhance mass transfer? How can we optimize the cell geometry to control such detachment? The control of these processes requires a comprehensive understanding of all the governing physical mechanisms. My research tries to better understand bubble detachment dynamics in the context of water electrolysis. The same mechanisms are also relevant in the context of turbulent bubbly flows. In the vicinity of surfaces, they are can reduce skin friction very significantly. The reduction of drag via the introduction of bubbles is known as “air lubrication”, which is a hot topic in the maritime industry.