A large variety of oil in water emulsification methods have been described ranging from simple set-up with a beaker and stirrer to, for instance, methods based on static micromixers, where the particle size can be controlled by the flow rates of the oil and water phase in the micromixer, or surface liquid spraying, where the oil phase is sprayed on the surface of the stirred water phase. Also, a "jet excitation method" has been described to achieve size-uniform microparticles by feeding the drug/polymer solution through a glass nozzle, which is equipped with an ultrasonic transducer, into a stream of aqueous carrier, followed by solvent removal in a standard evaporation procedure from the oil in water emulsion. Droplet formation by Rayleigh-Plateau instability, a process driven by interfacial tension that results in axisymmetric undulation of the liquid jet (perturbations with wavelengths larger than the jet radius) and the break-up of the cylindrical fluid thread into droplets, is the basic principle of numerous micromixers. This classic mechanism of droplet formation is superimposed by ultrasonic high-frequency oscillation of the nozzle in the "jet excitation method", which allows prediction and control of the droplet size by the applied frequency and flow rate.
In order to obtain injectable microparticles for long-acting depot applications, a polydisperse particle size range of 20-100 µm is usually desired. Smaller particles,