Mixing is a process in which two or more than two separated components/phages are placed/lie as close as possible to each other.
Powder segregation (demixing) is the opposite process of mixing, which means the separation of the drug from the powder components.
Objectives of mixing:-
- To achieve uniform/homogeneous mixture- uniform drug distribution in the preparation.
- Maximize the contact surface area of component (promote interfacial physical and chemical processes)
- To obtain dispersion
- Promotion of chemical reaction
Types of mixture
POSITIVE MIXTURE
- Materials mix spontaneously without external action/energy and Irreversible
- Tends to approach a perfect mixing
- E.g. Mixing of gases, miscible liquids
NEGATIVE MIXER
- Eg. Suspension (insoluble solids are mixed with a vehicles) and emulsion (two immiscible liquids)
- High energy needs to be supplied to keep component adequately dispersed
- After mixing, the negative mixture has a tendency to separate out from each other I.E. Reversible
NEUTRAL MIXTURE
- Neither mixing Nor de-mixing occurs unless acted upon by an external system of force/energy
- Static in behavior
- E.G. mixing of powder, paste, ointments
Mechanism of Mixing/Homogenization
- Mixing of particulate solids (powders)
- Mixing of Miscible liquids & Suspensions
- Mixing of Semi-SOLIDS
Mixing of particulate solids (powders)
There are three main mechanisms involved in the mixing process, related to the different kinds of particle motion.
- Convection
- Diffusive
- Shear
1. Convection
Relatively large mass (group of particles) of materials is moved from one part of the Powder bed to another (driven by the rotational force provide by revolving a mixer vessel, paddle, blade, ribbon, * gas.
2. Diffusive
Random movement of individual particles among other particles within the Powder.
3. Shear
One layer of powder particles moves relative to another layer of particles (slippery Planes are formed/set up- give rise to laminar flow).
Shear mixing: Movement of a layer of particles, rather than the movement of individual particles.
Mixing equipment’s For Powders
1. Tumbling mixers/blenders
2. Fluidized bed mixers
3. Agitator mixers
4. Planetary mixture for powder & semi-solid
Tumbling mixers/blenders 
Agitator (ribbon) mixers
Tumbling mixers/blenders:
Mostly use to mix powders with good flowability and granules, rather than cohesive or powders with poor flowability, because shear force provided by these mixers is not enough to separate the individual particles from agglomerates.
The ribbon mixer can mix powders with poor flowability.
Nauta mixer: composed of a bottom discharger with a rotating screw-fastened to the upper end of the rotating arm. The screw conveys the product to the top, where it can flow back to the powder feed. Hence the vertical impeller and horizontal rotating arm are combined together to induce a combination of convection, shear, and diffusive mixing.
Mixing of Miscible liquids & Suspensions
Mechanism of Liquids & Suspensions mixing
There are four mechanism involved into in the mixing of Miscible liquids & Suspensions.
1. Bulk transport
2. Turbulent
3. Molecular diffusion
4. Laminar Mixing/Streamline
Mixing equipment’s For Liquids
1. Shaker mixers
2. Paddle/ Propeller mixers (used for mixing of relatively low viscosity dispersion)
3. Turbine mixers (effective for high viscous liquid)
4. In-Line mixers
5. Sonic & Ultrasonic
Propeller Mixture
During mixing of liquids air get entrapped and leads to the formation of vortex . These can be avoided by making the following changes in the position of propeller shaft Offset from the centre Mounted at angle Enter the side of the vessel Using push pull propeller By the use of baffles.
Mixing of Semi-SOLIDS
Mechanism of Mixing of Semisolids
1. Bulk transport
2. Turbulent
3. Molecular diffusion
Equipment’s for Mixing of Semi-SOLIDS
- Triple roller mill
- Agitator/ Sigma mixer
- Planetary mixer
1. Triple roller mill
2. Agitator/ Sigma mixer
- 2 blade (sigma shaped), move at different speed toward each other;
- Speed: Twice than other
- Wet Granulation
