Rheology Flashcards
Study of flow or deformation of matter under influence of stress
Rheology
Resistance of a fluid to flow
Viscosity (η)
Internal friction in the fluid
Viscosity
Unit of Viscosity
Poise/ Centipoise
Stokes/ Centistokes
Force applied per unit area required to bring about flow
Shear Stress (F)
Unit: dyme/cm2
The velocity gradient between two planes of a body that are separated by a small distance
Rate of Shear (G)
Reciprocal of viscosity
Fluidity
Types of Flow
- Absolute Viscosity
- Kinematic Viscosity
- Relative Viscosity
Fluid in which there is direct proportionality betwen shear stress and shear rate.
Newtonian Flow
Viscosity stays ________ as the applied force changes
Viscosity stays constant as the applied force changes
Constant viscosity with Increasing rate
Kinematic Viscosity
Kinematic Viscosity = Absolute Viscosity / Density
Unit: Stokes
Ratio of viscosity of dispersion to that of solvent
Relative Viscosity
Unitless
Fluids with no direct linear relationship between shear stress and shear rate
Non-Newtonian Flow
Time-Independent / Shear-Dependent
Plastic Flow
Pseudoplastic Flow
Dilatant Flow
Time-Dependent Non-newtonian Flow
Thixotropy
Rheopexy
Aka Bingham Bodies
Plastic Flow
No flow occurs in response to shear stress until a certain transition point is reached
Plastic Flow
Above the Yield Value, they behave as Newtonian Systems (Bingham Flow)
Minumum shear stress required by the system before it deform and begins to flow
Yield Value
Examples of Plastic Flow
Suspensions
Gels, Ointments, Pastes, Creams
Cataplasm cerates
Butter, Margarine
Pseudoplastic Flow
Shear-Thinning System
Liquids become thinner on the application of stress
↑ shear stress; ↓ viscosity
Pseudoplastic flow
Examples of Pseudoplastic Flow
Gums, tragacanth, Na alginate, methyl cellulose
Aqueous Solution of Polymers (CMC, HPMC in Water)
Dilatant Flow
Shear-Thickening Flow
Dilatant Flow
Shear-Thickening Flow
Liquids become thicker, or flow with increased resistance with application of stress
↑ shear stress; ↑ viscosity
Dilatant Flow
Examples of Dilatant Flow
Whipcream
Corn starch Susp
High Solid Content Suspension
Paint Susp
Gradual decrease of the viscosity under constant shear stress followed by gradual recovery when the stress is removed
Thixotropy
Reversible and isotherma transformation of gel to sol
Thixotropy
Shaking: Sol
Standing: Gel
Increased viscosity with time under constant shear rate/stress, followed by gradual recovery when the stress/rate is removed
Rheopexy
Sol-Gel Formation
Shaking: Gel
Standing: Sol
Factors affecting Viscosity
↑ temp, ↑ η (Gases)
↑ MW, ↑ η
+ organic substance, ↑ η
↑ Dispersed Phase Content, ↑ η
Spherical Shape ↓ η
↑ temp, ↓ η (liquids)
+ electrolytes, ↓ η
Measurement of Viscosity
A. Single Shear Rate Instruments - used for Newtonian
1. Capillary Viscometer (Ostwald)
2. Falling Sphere (Hoeppler)
B. Multipoint, Rotational Instruments - both
1. Cup & Bob (Coutte, Mac Michael, Searle ‘Brookefield –rotating spindle/ Bob’ )
2. Plug Flow
3. Cone & Plate (Ferranti-Shirley)
Others:
Penetrometers - Ointment
