NUMERICAL STUDY OF SOLUTE GRADIENT-INDUCED SELF-PROPULSION OF SUSPENDED DROPLETS

In the presence of solute concentration gradients, suspended droplets undergo spontaneous motion. The reason is that the non-uniform distribution of solutes at the droplet interface can cause an interfacial tension gradient at the fluid interface, inducing interfacial flow. This process involves the interface movement of the self-propelled droplet, the evolution of the near-interface flow field, the solute concentration field, and the coupling effects of multiple physical fields. Understanding this complex dynamic process holds significance. This paper constructs a multiphase-multicomponent fluid numerical model to describe solute-induced droplet migration phenomena by combining the conservation-type Allen-Cahn equation, incompressible Navier-Stokes equation, and the advection-diffusion equation for solute. The accuracy of the numerical model is validated through case studies and theoretical comparisons (Laplace pressure difference of stationary droplets, buoyancy driven bubble rise, and solute concentration driven droplets migration). The simulation investigates solute Marangonii effects under different Marangoni numbers, including phenomena of coalescence and separation of two droplets in different sizes. Results indicate that larger droplets exhibit faster movement, and an increased Marangoni number shifts the self-propelled droplet interface mass transfer from diffusion-dominated to advection-dominated, enhancing the impact of droplet movement on the ambient solute field. Meanwhile, the solute gradient at the interface is reduced, which weakens the Marangoni effect, and decreases the droplet migration speed. The larger the size of the droplet, the more significant decrease in its velocity. This study provides a reliable numerical model for solving physical problems in multiphase multi-component fluid systems in the future and provides reference data for the manipulation of multi-component micro-droplets.