AGGLOMERATE: A group or cluster of the solid primary particles in a dispersion or the internal phase droplets in an emulsion is called an agglomerate. Microscopic examination of a product sample will usually enable one to quickly distinguish between agglomerates and primary particles. Agglomerates are held together mainly by relatively weak electrostatic surface forces between the particles.

BLEND: A blend is a mixture of miscible liquids.

COLLOID MILL: A colloid mill is an in-line processing device that forces the product to travel through a small and accurately controlled gap between a rotor and a stator. The exact nature of the resulting mechanical and hydraulic shear applied to the product is determined by the rotor and stator surface design, the rotor speed and the size of the rotor/stator gap.

DILATANT: Fluids with a viscosity that increases as the level of applied shear increases are called dilatant. The term rheopectic is also used for such fluids. Common examples of such fluids are peanut butter, ceramic slips, high-solid clay dispersions and quicksand.

DISPERSION: A dispersion is a uniform mixture of microscopic solid particles in a liquid, where the solid is insoluble in that liquid. The terms slurry and suspension are also used for such products.

EMULSION: An emulsion is a stable mixture of two immiscible liquids, one of which is uniformly dispersed in the other in the form of microscopic droplets/particles. The two most common types are oil-in-water (o/w), where oil droplets are dispersed in a continuous water phase, and water-in-oil (w/o), where water droplets are dispersed in a continuous oil phase.

HOMOGENIZING: This is a general term for making an emulsion or dispersion more uniform through the application of mechanical and hydraulic shear. Also, the term is used to reference the processing of an emulsion or dispersion with a high-pressure homogenizer.

INSOLUBLE: A solid that remains in the form of solid particles upon immersion in a particular liquid is called insoluble in that liquid.

MISCIBLE: Liquids that do not separate after being combined without the use of significant mechanical shear or stabilizing additives are called miscible.

NEWTONIAN: Fluids with a viscosity that remains constant regardless of the level of shear applied are called Newtonian. Common examples of such fluids are milk, honey, hydrocarbon based oils, fruit juices and clear soups.

RHEOLOGY: As it relates to wet mixing and milling, Rheology is a science the studies the flow properties of fluids when shear is applied. Thus, this is primarily a study of fluid viscosity.

SHEAR: To understand shear, one can picture a volume of a liquid or solid as being a stack of very thin parallel layers. If the bottom layer is fixed in place and one pushes on the edge of the top layer so that the layers remain parallel, the layers will all begin to move in varying amounts and the volume of material will distort in the direction of the applied force. A layer that has other product layers on both sides will experience hydraulic shear. An outer layer that is in direct contact with an external solid object will experience mechanical shear.

SOLUTION: A solution is a mixture of a liquid and a solid that is soluble in that liquid.

SURFACTANT: A surfactant is a chemical additive that facilitates the formation and stabilization of emulsions and dispersions. Surfactants function by chemically or electrically altering the interface between the internal phase liquid or solid and the continuous phase liquid. The name is a contraction of the descriptive term Surface active agent. Related terms that refer to specific types of surfactants include dispersant, emulsifier and wetting agent.

THIXOTROPIC: Fluids with a viscosity that decreases as the level of applied shear increases are called thixotropic. The terms plastic and pseudoplastic are also used for such fluids. Common examples of such fluids are creams, lotions, gum dispersions, salad dressings, mayonnaise and latex emulsions.

This glossary of terms was contributed to Flow Control magazine by David Ekstrom, application specialist at Bematek Systems Inc. (www.bematek.com). Bematek Systems manufactures in-line, hi-shear wet mixers and colloid mills.