Emulsions are essential materials in many chemically based products, including pharmaceutical, adhesives—such as wood and paper adhesives—food, and personal care. The characteristics and utility of emulsion products depend on the product’s formulation and the processing method, which are interdependent. Processing parameters such as mixing time, energy, and thermal history impact the fi nal product. For example, the stability of an emulsion typically depends on both its chemistry and the size distribution of the discontinuous phase material—usually controlled by the method and energy of phase dispersion. In addition, for an identical chemical formulation, product results can differ when the emulsion is prepared at different scales because of changes in the processing physics. The formulation of stable emulsions is not a trivial process, and cosmetic emulsion preparation and scale-up require at least a thorough understanding of the art and science of lotions and creams.
Emulsions are usually formed under high-shear conditions in conventional mixers, and the agitator confi gurations can vary (see sidebar). The complexity and challenges of emulsion formation have also been addressed by specialized equipment such as colloid mills, homogenizers, Pohlman whistles—in which a fl uid stream causes a vibrator to break the stream into droplets, just as a whistle causes resonant waves in an air stream—ultrasonic applicators, rotor-stator mixers, and turbines.