Experimental And Numerical Simulation Study On Deformation Characteristics Of Double Emulsion In Shear Flow Field

Double emulsion droplet has the unique structure of "double films-triple phases", which possesses the functions of material segregation, targeted release, component seal. Therefore, it has a good application prospect in biology, medicine, food, chemical engineering, and so on.An deep insight into the deformation of double emulsion droplet in shear flow plays an important role is of significant scientific value and technical supports to the development of theory and technology for precise control of rheological behavior of double emulsion droplet.Currently, the rheological behavior of the double emulsion droplet in shear flow has not been fully understood, especially the deformation characteristics of the double emulsion droplet in shear flow needs to be further explored. Accordingly, in this paper, based on the VOF (Volume of Fluid) method for tracking the liquid/liquid interface, the theoretical models of a incompressible double emulsion droplet was established and numerically analyzed. A visualization experimental platform is also set up to observe the deformation behaviors of double emulsion droplet under shear, which is used to validate the current theoretical model. By the combination of numerical simulation and experimental observation, the steady and unsteady deformation of double emulsion droplet within single and two inner droplet under shear are investigated, respectively. The deformation mechanism of these two kinds of double emulsion droplet are revealed. In addition, the effects of dimensionless shear strength, the radius ratio and the viscosity ratio on the deformation behaviors of double emulsion droplet are discussed. Consequently, a serial of important results and conclusions are obtained as flows:(1) The viscous shear force is the main factor causing droplet deformation, and the surface tension and viscous forces are the main factors to hold the droplet in sphere. The deformation degree of droplet increases with the increasing Caout and the deformation degree of the inner droplet is more smaller than the overall double emulsion droplet under the same Caout.(2) Different from the single-phase droplet, the double emulsion droplet appears to stretch-relax back, and repeat again in a damped oscillatory fashion. The deformation coefficient and deformation angle experienced a fluctuation process with gradual damping until the droplet finally achieves its steady deformation. (3) An obvious difference between the rheological behaviors of the above mentioned two kinds of double emulsion droplet is observed. Owing to the entrainment effect of the closed vortical flow developed interior to the outer drop, two inner droplets are approached and huddled together in the central region of the outer droplet. Then, they rotate as a whole along with the vortical flow within the outer drop periodically. Meanwhile, the shape of the outer droplet exhibits the corresponding periodic change with two rotating inner droplets. In addition, the initial position of two inner droplets and Caout also have obvious functions on the deformation of double emulsion droplet and the huddled inner droplets’ rotating speed. Furthermore, the deformation of outer droplet with two inner droplets is also increased with the increase of Caout under the same conditions.(4) For the double emulsion within single inner droplet, the inner droplet promotes the retraction of outer droplet in the process of transient deformation oscillation and lead to more intense transient deformation oscillation of double emulsion droplet than the single-phase one. In the same condition, the double emulsion droplet can exhibit both larger and smaller steady deformation than the single-phase droplet, which is attributed to the competition between the coexisting enhancing and suppressing effects by the inner droplet on the deformation. These two mechanisms are caused by the competition between the effect of high pressure inside the inner droplet and the accelerating fluid inside the area of throat. A corresponding regime diagram is plotted to quantitatively recognize the respective dominant regime of these two effects over a range of capillary number and radius ratio of the inner droplet to the outer one.