Liquid film formation during bubble collision: A direct numerical simulation study

The liquid film behavior is of utmost importance in understanding bubble coalescence. However, the initial stage of bubble liquid film formation remains unclear and lack of investigation. In this work, liquid film dynamics is investigated via direct numerical simulations of collision of bubble pairs using a Volume of Fluid method. The detailed microscopic evolution of the liquid film (before rupture) is investigated with extremely high resolution with a minimum mesh size of 40 nm. A customized adaptive mesh refinement is deployed to ensure affordable computation of such high-resolution simulations. The simulation results reveal two interesting instants, one when two bubble interfaces start to ‘feel’ each other (characterized by the initial effect distance s₀) and the other instant when onset of a liquid film formation takes place (characterized by the initial film thickness h₀). These two characteristic dimensions are significantly different, with s₀ always several times larger than h₀. They might however be challenging to differentiate in experimental measurement depending on the spatial resolution. These two characteristic dimensions comprise important parameters to describe the initial stage of liquid film formation. They both increase with an increase in contact velocity and bubble diameter, but with a decrease in surface tension. In contrast, the liquid viscosity has little effect on s₀ and h₀, thus not important for the initial stage of bubble liquid film formation. Finally, descriptive expressions for the initial effect distance and initial liquid film thickness are derived based on the DNS results via dimensional analysis.