Effects Of Internal Structures On Rheology Of Multiple Emulsions

The development of droplet-based microfluidic technology(including the droplet generation, droplet mixing, droplet splitting, droplet coalescence and droplet sorting) makes the precise control, operation and function of droplet become possible. In addition, the phase change within the droplets through polymerization, the package of cells, proteins and DNA as well as the synthesis of micron and nanometer particles could be achieved through droplet-based microfluidic technology. Therefore, the application of microfluidic technology on rheology is huge.Through a two-dimensional(2D) spectral boundary element method, this paper studies the rheology of multiple emulsions containing complex internal structures in confined microchannel and mainly investigates the effects of internal structures on the apparent viscosity of multiple emulsions and film drainage in the head-on collision process of multiple emulsions separately.The study to the apparent viscosity of multiple emulsions is to investigate the asymmetrical deformations of multiple emulsions with different internal structures under rotational shear flows induced by the rotation of the inner cylinder in a two-concentric-cylinder microfluidic system like a rotational viscometer. As the total shear stress(including components and interfacial contribution) of simple emulsions in a shear flow is severely affected by the deformation of the droplets, the deformation parameter D is chosen as the bridge to connect the internal structures of multiple emulsions with their apparent viscosity. Thus, the relations among the internal structures, the deformation of the globule, the torque of the inner cylinder and the apparent viscosity of multiple emulsions are discussed qualitatively. As for the small deformation of multiple emulsions, the inner droplets have both suppressing and enhancing effects on the deformation of the globule, and they will rotate along with the inner circulation which is like a big vortex. Generally speaking, the larger the deformation is, the larger the torque is, which means that the apparent viscosity of multiple emulsions increases along with the increase of the deformation of the globule. As for the large deformation(without breakup) of multiple emulsions, the situations are much more complicated and depend due to the complex interaction between the inner droplets and the outermost interface of the globule.The study to the head-on collision of multiple emulsions is to investigate the approaching process of two multiple emulsions in cross-slot based on film drainage. The film thickness at the centerline and the minimal thickness are chosen to describe the approaching process of multiple emulsions. This paper compared the similarities and differences of the pressure field and flow field during the approaching process of simple droplets and double emulsions and investigated the effects of the volume fraction occupied by the inner droplets and the initial position of the inner droplets on the film drainage in the head-on collision of multiple emulsions. It was found that the existence of inner droplets affected the film drainage in the head-on collision of multiple emulsions. The effects of internal structure on the film drainage increase with the increase of the volume fraction occupied by the inner droplets and the initial eccentric ratio.