Double Emulsions Using a Microcapillary Device
Original entry by Hyerim Hwang, AP 225, Fall 2011.
A. S. Utada, E. Lorenceau, D. R. Link, P. D. Kaplan, H. A. Stone, and D. A. Weitz, "Monodisperse Double Emulsions Generated from a Microcapillary Device", Science '2005 308, 537-541
Highly structured fluids consisting of emulsion drops that contain smaller droplets inside are called double emulsions. This research demonstrates the preparation of anisotropic colloidal particles with well-coordinated patches using particle-stabilized oil-in-water emulsions. Fabrication of these double emulsion droplets are not easy, so traditional steps lead to very ill-controlled structuring. In this study, a microcapillary device was fabricated to produce double emulsions that contained a single internal droplet in a coreshell geometry. Droplet size can be controlled from the flow profiles of the fluids.
A microfluidic device which is composed of cylindrical glass capillary tubes and can generate double emulsions in a single step is fabricated and it allows precise control of the outer and inner drop sizes as well as the number of droplets encapsulated in each larger drop. As shown in Figure 1, the inner fluid is pumped through a tapered cylindrical capillary tube, and the middle fluid is pumped through the outer coaxial region. The outer fluid is pumped through the outer coaxial region from the opposite direction, and all fluids are forced through the exit orifice formed by the remaining inner tube. The size distribution of the emulsion droplets is determined by the breakup mechanism, whereas the number of innermost droplets depends on the relative rates of drop formation of the inner and middle fluids. When the rates are same, the annulus and core of the coaxial jet break simultaneously, generating double emulsion with a single internal drop. These types of double emulsions can be generated when both fluids are simultaneously dripping or simultaneously jetting as shown in Figure 2A and Figure 2B. In Figure 3, the solid line represents the results of the model that predicts the drop size in the dripping regime, and the dashed line represents the results of the model that predicts drop size in the jetting regime. The dotted line is the jet radius predicted for a flat velocity profile, and dashed-dotted line is the jet radius predicted for a parabolic velocity profile.
Double emulsions were used to generate encapsulation structures by manipulating the properties of the fluid that makes up the shell. This technique is promosing in that the high degree of control can be afforded by this and the completely separate fluid streams. The microcapillary fluidic device is three dimensional, perfectly shielding the inner fluid from the outer fluid. Double emulsions can be generated which are dispersed in either hydrophilic or hydrophobic fluids. Increasing the production rate requires the operation of parallel devices and an operational device requires control of the wetting properties of the inner channels.