High-shear mixer

A high-shear mixer disperses, or transports, one phase or ingredient (liquid, solid, gas) into a main continuous phase (liquid), with which it would normally be immiscible. A rotor or impeller, together with a stationary component known as a stator, or an array of rotors and stators, is used either in a tank containing the solution to be mixed, or in a pipe through which the solution passes, to create shear. A high-shear mixer can be used to create emulsions, suspensions, lyosols (gas dispersed in liquid), and granular products. It is used in the adhesives, chemical, cosmetic, food, pharmaceutical, and plastics industries for emulsification, homogenization, particle size reduction, and dispersion. Fluid undergoes shear when one area of fluid travels with a different velocity relative to an adjacent area. A high-shear mixer uses a rotating impeller or high-speed rotor, or a series of such impellers or inline rotors, usually powered by an electric motor, to "work" the fluid, creating flow and shear. The tip velocity, or speed of the fluid at the outside diameter of the rotor, will be higher than the velocity at the center of the rotor, and it is this velocity difference that creates shear. A stationary component may be used in combination with the rotor, and is referred to as the stator. The stator creates a close-clearance gap between the rotor and itself and forms an extremely high-shear zone for the material as it exits the rotor. The rotor and stator combined are often referred to as the mixing head, or generator. A large high-shear rotor–stator mixer may contain a number of generators. Key design factors include the diameter of the rotor and its rotational speed, the distance between the rotor and the stator, the time in the mixer, and the number of generators in the series. Variables include the number of rows of teeth, their angle, and the width of the openings between teeth. In a batch high-shear mixer, the components to be mixed (whether immiscible liquids or powder in liquid) are fed from the top into a mixing tank containing the mixer on a rotating shaft at the bottom of the tank.