# rheology

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Category: Education

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## Presentation Transcript

### Slide 1:

Rheology –concept of viscosity factors influencing the viscosity

### Rheology :

Rheology Flow of liquids Deformation of solids Study of flow properties of liquids is important for several dosage forms, e.g. :- Simple liquids, gels, ointments, creams and pastes

### Concept Of Viscosity :

Concept Of Viscosity Flow property of a simple liquid is expressed in terms of VISCOSITY Viscosity is an index of resistance of a liquid to flow. Higher the viscosity of a liquid, greater is the resistance to flow. For example, honey, syrup, groundnut oil, all resist the flow more in comparison to water or alcohol

### Explanation to understand Concept of Viscosity :

Explanation to understand Concept of Viscosity Liquid at rest Stress induced changes Shear stress applied on A A B C D E N A B C D E N Consider a ‘block of liquids’ consisting of parallel layers of molecules ,similar to a deck of cards

### Mathematical Treatment- Viscosity :

Mathematical Treatment- Viscosity Shear Stress – the force per unit area, F,/A, which is applied to bring about the flow. F, A Velocity gradient or rate of shear rate of shear ,G = Shear Stress F = dv (change in velocity) dr (change in distance)

### Higher the velocity of the liquids, greater is the force per unit area required to produce a certain rate of shear. relationship b\w shear stress and rate of shear :

Higher the velocity of the liquids, greater is the force per unit area required to produce a certain rate of shear. relationship b\w shear stress and rate of shear Shear stress α Rate of shear F, dv A dr F, dv A dr F = G Coefficient of viscosity α =

### Slide 7:

Coefficient of Viscosity – force per unit area required to maintain unit difference in velocity b\w two parallel layers in the liquid, 1 centimeter apart. Fluidity –Ф reciprocal of viscosity Ф = Kinematic Viscosity – viscosity divided by density of liquid η ρ 1 η =

### Slide 8:

Relative viscosity - (a synonym of "viscosity ratio") is the ratio of the viscosity of a solution η to the viscosity of the solvent η0 used.  Specific viscosity defined as relative increase in the viscosity of the solution over that of the solvent alone. Reduced viscosity ratio of specific viscosity to the concentration (c) = η η0 η0 ηr = ηsp = η-η0 η0 ηsp C

### Factors Influencing The Viscosity :

Factors Influencing The Viscosity Intrinsic Factors Extrinsic factors

### Intrinsic Factors :

Intrinsic Factors Chemical nature – molecular size, shape and intermolecular forces influences the viscosity

### Extrinsic factors :

Extrinsic factors Pressure ,temperature and added substances, influences the viscosity increase in pressure enhances the cohesive forces of interaction, leading to increase in viscosity. Non electrolytes like sucrose , glycerin ,the Solution exhibits increased viscosity.

### Temperature :

Temperature Increases temperature, the system acquires thermal energy which facilitates the breaking of the cohesive forces. viscosity of liquid decreases In cases gases, temp increases, increases viscosity owing to the increased collisions and interactions. 