Molecular dynamics study on coalescence phenomena of fully submerged co2 nano-bubbles

Abstract

Nanobubbles have garnered a lot of attention in recent years due to their remarkably longer lifetime and multifaceted applications in numerous practical processes such as nucleate boiling, soft drink manufacturing, carbon capture, surface cleansing etc. The coalescence behavior of these bubbles is of crucial importance as it provides a clear indication of exactly how stable these nanobubbles are, which is vital for most of its use cases. The current study employs classical molecular dynamics simulation to investigate the coalescence dynamics of CO2 nanobubbles that are fully submerged in water. To draw a stark contrast, a comparison with air nanobubble coalescence is presented as well. It is observed that unlike air nanobubbles, CO2 nanobubbles have a greater tendency to obstruct coalescence and do not fully coalesce into a single bubble, in general. State of the entrapped gas inside the nanobubbles, electrostatic repulsion and large negative zeta-potential are the major contributors behind this nature. As the coalescence does not fully progress, two parameters named coalescence neck growth and onset time are used to quantify the degree of partial coalescence. A compilation of findings and analysis for a total of 8 factors and 40 cases has been provided, demonstrating how CO2 nanobubble coalescence is influenced by various parameters such as nanobubble size, surrounding pressure, temperature, the presence of air, and more. These parametric studies show that the degree of coalescence is tunable through adjustment of these influence parameters. Additionally, impact of CO2 nanobubble coalescence progression on the thermal and momentum transport of carbonated water has been investigated using reverse non equilibrium molecular dynamics simulation. The final results show comprehensively that presence of coalescing nanobubble buffers the transport of thermal energy through carbonated water while the average particle velocity increases. Ultimately, this results in a decrement in thermal conductivity and increment in dynamic viscosity during coalescence. Both of these effects start to fade away with the progression of coalescence. This behavioral profiling of CO2 nanobubble coalescence phenomena might serve as control strategies in various nanobubble driven physical processes.