A powder is an assembly of dry particles dispersed in air. If two different powders are mixed perfectly, three theoretical powder mixtures can be obtained: the random mixture, the ordered mixture or the interactive mixture.

Different powder types

A powder

Powder (substance)

A powder is a dry,thick bulk solid composed of a large number of very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms powder and granular are sometimes used to distinguish separate classes of material...

 can be free-flowing if the particles do not stick together or cohesive

Cohesion (chemistry)

Cohesion or cohesive attraction or cohesive force is the action or property of like molecules sticking together, being mutually attractive...

 if the particles cling to one another to form aggregate

Particle aggregation

Particle aggregation in materials science is direct mutual attraction between particles via van der Waals forces or chemical bonding....

s. The likelihood of cohesion increases with decreasing size of the powder particles; particles below 100 µm

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 have generally been found to be cohesive

Random mixture

A random mixture can be obtained if two different free-flowing powders of approximately the same particle size, density

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 and shape

Shape

The shape of an object located in some space is a geometrical description of the part of that space occupied by the object, as determined by its external boundary – abstracting from location and orientation in space, size, and other properties such as colour, content, and material...

 are mixed (see Figure A). Only primary particles are present in this type of mixture, i.e., the particles are not cohesive and do not cling to one another. The mixing time will determine the quality of the random mixture. However, if powders with particles of different size, density or shape are mixed the segregation

Particle segregation

In Particle Segregation particulate solids tend to segregate by virtue of differences in the size, density, shape and other properties of particles of which they are composed. The process of segregation occurs during as well as during subsequent handling of completed mix and it is pronounced with...

 can occur. Segregation will cause separation of the powders as, for example, lighter particles will be prone to travel to the top of the mixture whereas heavier particles are kept at the bottom.

Ordered mixture

The term “ordered mixture” was first introduced to describe a completely homogeneous

Homogeneous (chemistry)

A substance that is uniform in composition is a definition of homogeneous. This is in contrast to a substance that is heterogeneous.The definition of homogeneous strongly depends on the context used. In Chemistry, a homogeneous suspension of material means that when dividing the volume in half, the...

 mixture where the two components adhere to each other to form ordered units . However, a completely homogeneous mixture is only achievable in theory and other denotations were introduced later such as adhesive mixture or interactive mixture.

Interactive mixture

If a free-flowing powder is mixed with a cohesive powder an interactive mixture can be obtained. The cohesive particles adhere to the free-flowing particles (now called carrier particles) to form interactive units as shown in Figure B. An interactive mixture may not contain free aggregates of the cohesive powder, which means that all small particles must be adhered to the larger ones. The difference from an ordered mixture is instead that all carrier particles do not need to be of the same size and a different number of small particles attached to each one. A narrow size range of the carrier particles is preferred to avoid segregation of the interactive units . In practice a combination of a random mixture and an interactive mixture may be obtained which consists of carrier particles, aggregates of the small particles and interactive units .

Formation

The formation of interactive mixtures cannot automatically be assumed, especially if smaller carrier particles or a greater proportion of fine particles are used. If an interactive mixture is to be formed, it is necessary that enough force is exerted by the carrier particles during dry mixing to break up the aggregates formed by the fine particles. Adhesion can then be achieved if the adhesive forces exceed the gravitational forces that otherwise lead to separation of the constituents.

Applications

Interactive mixtures can for example be used in the manufacturing of tablets enhancing the dissolution of poorly soluble drug

Drug

A drug, broadly speaking, is any substance that, when absorbed into the body of a living organism, alters normal bodily function. There is no single, precise definition, as there are different meanings in drug control law, government regulations, medicine, and colloquial usage.In pharmacology, a...

s or for nasal administration

Nasal administration

Nasal administration can be used to deliver drugs for either local or systemic effect. Locally acting drugs are for example decongestants and allergy treatments...

. One common application is for inhalation therapy, where the concept has been used in the development of alternatives to pressurised metered dose inhalers.

The schematic formation of a random mixture (A) and an interactive mixture (B) by dry mixing two powder materials.