NUMERICAL STUDY OF MIXING IN A DROPLET MOVING THROUGH A WINDING CHANNEL

Direct numerical simulations have been conducted for chaotic mixing inside an immiscible droplet moving through a winding channel or tube in order to clarify the detailed mechanism of the enhancement of mixing, which is useful for the design of efficient micro devices. An immersed-boundary method is used for simulating the flow inside the bending channel, while the PLIC-VOF algorithm is used for tracking the droplet interface. The molecular mixing is ignored and only the mixing due to the chaotic advection is considered by using passive tracer particles. By considering the relation between the flow structure and the mixing inside the droplet in two-dimensional computations, it is found that the quality of mixing strongly depends on the activity of the dipole structure of the flow inside the droplet, and that the activity depends on various parameters including the dimension and the shape of the winding channel. In a winding channel with a locally narrowed region, for example, circulating flows associated with the dipole are off-centered to reduce the efficiency of mixing. A three-dimensional computation shows that the quality of mixing is decreased in the edge regions of the droplet.