​What is rheology?

​https://youtu.be/eSHH_ssMkSo

​Rheology

​The term “rheology,” from the Greek rheo (“to flow”) and

logos (“science”)

Viscosity- is an expression of

the resistance of a fluid to flow; the higher the viscosity, the

greater is the resistance.

​​Rheology is used to study:

• paints,

• inks,

• doughs,

• road-building materials,

• cosmetics,

• dairy products, and other materials

​Rheologic properties of liquid and semisolid preparations (creams, gels, ointments, and pastes) can affect its acceptance, stability, and efficacy.

​Topical preparation

​ Lotion

​​Ophthalmic preparations

Lubricating eyedrops

• ​increased viscosity could make blinking difficult  

• such preparations should be shear thinning and thixotropic.

​​​Thixotropy

- is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow over time when shaken, agitated, shear-stressed, or otherwise stressed.

Injectable preparation

​Depot injections- uses a liquid that releases the medication slowly, so it lasts a lot longer.

​NEWTONIAN FLUIDS

​A Newtonian fluid's viscosity remains constant, no matter the amount of shear applied for a constant temperature. These fluids have a linear relationship between viscosity and shear stress.

Examples:

• Water

• Mineral oil

• Gasoline

• Alcohol

​​NON-NEWTONIAN FLUIDS

• are the opposite of Newtonian fluids

• when shear is applied, the viscosity of non-Newtonian fluids decreases or increases, depending on the fluid.

The behavior of the fluid can be described in one of four ways:

• ​Dilatant

• ​Pseudoplastic

• ​Rheopectic

• ​Thixotropic

​​Dilatant

 Viscosity of the fluid increases when shear is applied. For example:

• Quicksand

• Cornflour and water

• Silly putty

​​Pseudoplastic

- Pseudoplastic is the opposite of dilatant; the more shear applied, the less viscous it becomes. For example:

• Ketchup

​​Rheopectic

Rheopectic is very similar to dilatant in that when shear is applied, viscosity increases. The difference here is that viscosity increase is time-dependent. For example:

• Gypsum paste

• Cream

​​Thixotropic

Fluids with thixotropic properties decrease in viscosity when shear is applied. This is a time-dependent property as well. For example:

• Paint

• Cosmetics

• Asphalt

• Glue

η​=Coefficient viscosity; ρ = Density

​Kinematic & Relative Viscosity

​U=plastic viscosity (poise)

​F=shearing stress (dynes/cm²)

​f=yield value/ intercept (dynes/cm²)​

​G=rate of shear (sec^-1)

​Plastic Viscosity

​

​Shear Stress & Shear Rate