2023/02/27 by Chen, Yibo, Chong, Kai Leong, Liu, Haoran +2
#FOS: Physical sciences #Fluid Dynamics (physics.flu-dyn)
paper · doi:10.48550/arxiv.2302.14008
Active oil droplets in a liquid are believed to repel due to the Marangoni effect, while buoyancy effects caused by the density difference between the droplets, diffusing product, and ambient fluid are usually overlooked. Recent experiments have observed active droplet clustering phenomena due to buoyancy-driven convection (Kruger et al. Eur. Phys. J. E, vol. 39, 2016, pp.1-9). In this study, we numerically analyze the buoyancy effect in addition to Marangoni flow, characterized by Peclet number Pe. The buoyancy effects originate from (i) the density difference between the droplet and the ambient liquid, which is characterized by Galileo number Ga, and (ii) the density difference between the diffusing product (i.e. filled micelles) and the ambient liquid, characterized by a solutal Rayleigh number Ra. We analyze how the attracting and repulsing behavior depends on the control parameters Pe, Ga, and Ra. We find that while Marangoni flow causes repulsion, the buoyancy effect leads to attraction, and even collisions can take place at high Ra. We also observe a delayed collision as Ga increases. Moreover, we derive that the attracting velocity, characterized by a Reynolds number Red, is proportional to Ra1/4/(l/R), where l/R is the normalized distance by radius between neighboring droplets. Finally, we obtain repulsive velocity, characterized by Rerep, as proportional to PeRa-0.38. The balance of attractive and repulsive effects results in Pe ∼ Ra0.63, which agrees with the transition curve between regimes with and without collision.