One-Step Emulsification of Multiple Concentric Shells with Capillary Microfluidic Devices

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Started by Lauren Hartle, Fall 2011.

"One-Step Emulsification of Multiple Concentric Shells with Capillary Microfluidic Devices" S.-H. Kim, and D. A. Weitz. Angewandte Chemie International Edition, DOI: 10.1002/anie.201102946 (2011).

Keywords

Capillary, Carbon Black, Drop, Drug Delivery, Emulsion, Encapsulation, Fluid Jet, Hydrophilic, Hydrophobic, Magnetic, Microfluidics, Polymer

Introduction

Encapsulation by emulsion is an efficient and effective method used in numerous industries. Methods for producing single and double emulsions are well-understood, but the production of higher order encapsulations has proven difficult to streamline. Typically, single drop makers are put in series, requiring sensitive protocols. In particular, flow rate must be precisely controlled at each stage to ensure a favorable outcome, and the thickness of each layer is difficult to control. Weitz and Kim present a one step emulsification method to make homogeneous higher order emulsion drops.

Methods

Figure 1 shows a) a schematic for a Water/Oil/Water/Oil triple emulsion, b) the formation of said droplets, and c) droplets collecting downstream. Three capillary tubes are necessary: one large square capillary treated along the inner surface for hydrophilicity. Two smaller tapered capillaries (inserted in either end of the square capillary, for collection and injection purposes) are treated for hydrophobicity. The injection capillary, injects the substance destined for the innermost drop. In this system, water is injected via the injection capillary, and oil is simultaneously injected through the square capillary on the same side. On the collection side, water and oil are simultaneously injected into the square capillary (the oil is injected through a small secondary capillary). Water remains along the inner surface of the square capillary and oil remains near the collection tube surface. In this method, coaxial interfaces form when these multiple phases are pushed through a single orifice (the collection tube). Higher order emulsion drops are produced by initiating "dripping" or "jetting" modes in the various phases. A triple emulsion is formed. A plot of the diameters of each phase vs the volume flow rate of the fluid for innermost drops is shown in Figure 2, along with images of droplets of varying sizes.

D5-1.png
D5-2.png

The flow rate of the inner most jet determines the size of the final droplet. The innermost droplet size remains fairly constant, but the drop creation rate increases, hence resulting in more rapid break up of the outer jets. The following equations, based on mass-balance, relate the drop diameter D of the different phases to the volumetric flow rate Q. <math> D_2 = D_1 \left( 1+ \frac{Q_2}{Q_1} \right)^{\frac{1}{3}} </math> <math> D_3 = D_1 \left( 1+ \frac{Q_2+Q_3}{Q_1} \right) ^{\frac{1}{3}}</math>

Other parameters affecting droplet properties are as follows:

  • An increase in <math>Q_2</math> decreases <math>D_1</math>. At large <math>Q_2</math> , "plug-like drops" are formed with more than one inner drop.
  • The opposite triple emulsion Oil/Water/Oil/Water, can be formed by reversing the surface treatments on the capillary tubes. Images of the reverse of the original emulsion, in single and multiple enclosed inner drops is included in Figure 3 a-c.
  • UV curing of the oil phase can produce permanent capsules.
  • Magnetoactive inks can be made by enclosing magnetic material and carbon black in the innermost drop. The magnetic response of the drops are shown in Figure 3 d. If the innermost drop is pulled to the front of the particle, the ink surface appears black. Otherwise, the ink takes on the cloudy white appearance of polystyrene, or the polymer phase of choice.
D5-3.png

To produce a quadruple emulsion, the schematic in Figure 4a is used: the left side remains the same as in Figure 3 a, but the square capillary on the injection side is hydrophilic and the injection capillary is hydrophobic. Figure 4 b-c shows droplets in production, and downstream.

D5-4.png

Conclusions/Soft Matter Connection

The Weitz group demonstrates a robust method for multi-layer drop encapsulation, with applications in magnetoactive ink, drug delivery, high throughput "micro reactors". The paper demonstrates the power of manipulating surface interactions via surfactants and surface functionalization to produce highly regular, small scale 3-D structures.