Microencapsulation is a process in which small particles of solid, liquid or gas are packaged within a secondary material to form a capsule. There are many cases where it is advantageous or even necessary for a particular material or a mixture of materials to be encapsulated, primarily to control the rate of mass transfer to and from, and/or reaction with, its intended environment. In pharmacy, for example, encapsulation of a bioactive (drug) within a particulate delivery system lends a means of control over the release of the bioactive to achieve a time profile which yields the desired physiological response. In food industry, encapsulation has also been used to reduce the reactivity of the core materials with the outside environment (e.g. light, oxygen, and moisture) thus prolonging the shelf-life of the products. Furthermore, encapsulation allows for liquids to be handled as solids; toxic materials to be handled with much reduced safety risks.

Many techniques have been developed for encapsulation. These include spray-drying, prilling or spray-chilling, fluidized coating, interfacial polymerisation, coacervation, freeze drying, phase separation, and multiple emulsions. A serious limitation with most of these conventional encapsulation techniques is that, in addition to the low entrapment efficiency problem, they produce particles/capsules with little control over the particle size and size distribution, resulting in product particles with inconsistent particle sizes and broad size distributions.

In many of the applications, it is desirable or even necessary to have a narrow size distribution of the capsules so that the total amount as well as the release rate can be manipulated. As such, conventional technologies for encapsulation render the capsules unsuitable for many applications, in particular, as particulate drug delivery systems where tight particle size distribution is necessary for the control of drug release.

This paper presents a novel technique for the production of relatively uniform-sized water-in-oil compound droplets, which can be converted into polymer capsules with water as their cores by evapouration or solvent extraction methods. The technique is based on the break-up of a laminar water-in-oil compound jet in a bulk continuous water phase. The compound jet is formed using an in-house design and fabricated macroscopic flow-focusing cell where an inner water phase from a tube inside the cell is forced to flow through the exit orifice in the bottom of the cell together with an immiscible outer oil phase. The compound jet eventually breaks up downstream due to interfacial instabilities, resulting in the formation of individual water-in-oil compound droplets, which is dispersed in the continuous water phase.

Initial results show that, in certain range of flow conditions, total entrapment of the inner water phase can be achieved in both the dripping and laminar jet break-up regions. The size and number of water drops inside each of the compound droplet can be manipulated by controlling the total flow and flow rate ratios of the two immiscible phases through the exit orifice of the flow cell. Photography by a high speed camera at a rate of 1000 frames per second also reveals some interesting flow dynamics and drop formation processes.