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Mechanism of Solid Mixing, Liquids Mixing and Semisolids Mixing

A mixer is usually classified according to what type of dosage form it is used for. Following is a summary of mixing processes based on dosage form.

Mechanism of Solid Mixing

A mixer is usually classified according to what type of dosage form it is used for. In the following section, a summary of mixing processes based on dosage form type is provided.

Mixing for solids -
Mixing ease is affected by the following physical properties:
  • Particle shape
  • Stickiness
  • Material density
  • Wettability
  • Particle size and distribution
Mixers with tumble action and fixed shells are the most commonly used for solid mixing. These mixers operate by transporting and shearing bulk material. The speed of rotation of a tumble blender directly affects the mixing efficiency.

The fixed shell mixers contain a stationary container in which the material is held and the mixing is accomplished by moving screws, paddles, or blades. In ribbon blenders, a semicircular bottom surrounds a relatively long, trough-like top with helical blades that open from the top. Using the blades, the charged powder is continuously sifted and shuffled from end to end and rotated around the trough.

Mechanism of Liquid Mixing

Liquid mixing can be categorized into four classes based on its mechanism. These include:

Turbulent Mixing

A turbulent mix occurs when the fluid velocity randomly fluctuates at any given point in a system due to turbulent flow. Fluctuations in fluid velocity at a given point in a turbulent flow occur continuously in three directions, x, y, and z. On the other hand, when the dominant component is in play, the fluid moves in a certain direction. There is a different velocity for a liquid at each location at the same time, in general.

Mixed fluids are highly effective when mixed by turbulent flow. Many eddies of various sizes can be seen as part of the turbulent flow. The term eddy refers to a region of liquid moving as a unit in a particular direction. A large eddy tends to separate into smaller, indistinguishable eddies as it breaks up. The intensity of turbulent flow can also be measured, which depends on the speed of eddies.

Laminar Mixing

By applying shear to an interface between two dissimilar liquids, laminar mixing is the process of mixing dissimilar liquids together. By folding back upon themselves, layers are re-arranged. This increases the number of layers. This increase in interfacial areas also accelerates exponentially over time. Fluid layers can also be simply stretched if there is no significant folding action occurring. The same applies for liquids that need only moderate mixing.

Molecular Diffusion

Thermal motion causes molecules to diffuse at the molecular level, which is called molecular diffusion. As Fick's law relates to concentration gradients, molecular diffusion can be explained by Fick's law. Concentrations decline as mixing is completed, reaching zero when equilibrium is reached. Molecular diffusion reduces the sharp discontinuities between two layers at the interface when laminar flow and molecular diffusion take place simultaneously.

Multiple mechanisms may operate simultaneously during mixing.

Mechanism of Semisolid Mixing

Sigma blade mixer - It consists of two blades moving at different speeds in a mixing vessel having the shape of a double trough. This mixer is suitable for granules and other liquid products.

Triple roller mill - A small volume of material is sheared by the differential speed and narrow clearance between rollers. Roller mills are commonly used to grind ointments and homogenize them.

Planetary mixers - In the mixer, the mixing arm rotates around its own axis and around a common axis, typically the mixing wheel itself. Kneading is provided by the blades, while shearing is accomplished by the narrow passage between them and the wall of the can.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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