Viscosity Flashcards
What are the 6 different descriptions of Viscosity?
1) Dynamic Viscosity
2) Kinematic Viscosity
3) Relative Viscosity
4) Specific viscosity
5) Reduced Viscosity
6) Intrinsic Viscosity
Describe Dynamic Viscosity:
This is what is usually referred to when thinking of viscosity – has the symbol (η)
For a Newtonian liquid the rate of flow (ϒ) is directly proportional to the force (σ) we put in = linear graph. It is a coefficient that describes the relationship between rate of flow and force. If not mentioned otherwise, viscosity is “dynamic viscosity”.
What is the equation of Dynamic Viscosity:
η = σ/γ η = dynamic viscosity σ = force (shear stress) ϒ= change in velocity In pharmacy we are working with really small numbers in these equations
What is Kinematic viscosity?
Kinematic talks about the motion of the object but doesn’t care about the forces that caused the motion. Only interested in object’s ability to move.
What does kinematic viscosity involve?
It relates a liquid’s viscosity to its internal force, and its internal force is equivelent to the density. Something with a high density will have a bigger internal force, and therefore a lower viscosity.
The fluid won’t start or stop moving without a force e.g. gravity or friction.
What are the equations of Kinematic viscosity?
density (ρ) = mass (kg)/ volume (m3) ν= η/ρ η = dynamic viscosity ρ = density kinematic viscosity = ν
If something has a dynamic viscosity of 2, and a density of 2000, what is the kinematic viscosity?
0.001 m2s-1
What is relative viscosity?
the ratio of the solution viscosity to the solvent viscosity i.e. compares the viscosity of a solvent without additives, then the viscosity with additives e.g. water on its own then water with sugar.
What is Specific Viscosity?
represents the increase in viscosity caused by the presence of an addative = gap between water on own and water and sugar
What is reduced viscosity?
Relates the concentration of a solute to its effect on viscosity. It can tell us about the degree of solvation and whether there is a strong or weak interaction between the solvent and solute
What is Intrinsic Viscosity?
quantifies the amount by which the additive increases the viscosity of a formulation so, if a little bit of the additive increases the viscosity a lot – it has a HIGH intrinsic viscosity.
What are the three tests to measure viscosity?
1) Capillary method - test how fast the sample flows
2) Rotating method - test how hard it is to stir the sample
3) Falling method - test how long it takes for something tp fall through the sample.
Describe the Capillary method:
Put the liquid into a Capillary viscometer and place pipette filler on the end. Draw the pipette up so that the liquid draws up to the top of the Capillary. Next, time how long it takes for the liquid to flow from the top to the bottom of the capillary. It won’t be too fast due to the narrowing of the capillary
Describe the Rotating viscometer method?
Either have bowl full of liquid that spins and a pressure gage that measures how hard it is to stir, Or have a bowl of liquid with a paddle that spins within it.
Describe the Falling ball/sphere viscometer
Get a tube with funnel in top. Fill with liquid, drop a ball into it. Need to know the size and weight and density of ball. Use a timer to measure how long it takes ball to reach certain point in the viscometers. This relates to stoke’s law – can do it at different temps, with different size balls.
