Rheology, Kinematic and Newtonian Viscosity
Please read the laboratory assigned for this week. In addition, you will find the information in your text on page 259 to 262 of interest. Also page 278.
The information in the two chapters associated with heterogenious systems should be reread in light of what we discus in class and laboratory.
The term Rheology comes from the greek rheo - to flow and logos - science. Thus, Rheology is the science of the flow of matter.
Newtonian Flow
Let's consider a deck of cards. As some of you know, I enjoy a good game of Bridge. This is played with a standard deck of cards. I hope you all have seen a standard deck of cards. If not ask Buckley to show you what it looks like. Pure liquids or solutions can be thought of as a stack of layers of molecules, not unlike our deck of cards. If we apply a force to the top most layer of molecules it will begin to move. It in turn will cause the next layer of molecules to move etc. Each subsequent layer moves a little slower than the one above until we get to the last layer which is attached to the surface and does not move at all. The force used to cause the liquid to move is called the shearing stress and is represented by "F". It is considered to have been applied to a given area of surface(F'/A). The change in velocity of the layers of liquid, as you move from the surface that was pushed by the Shearing Stress to the layer that is fixed in place by its interaction with the surface is called the rate of shear and is written as dv/dr, change in velocity with respect to the change in radius.
F'/A = dv/dr : We sometimes simplify the equation by using G for dv/dr and F for F'/A. This gives us:
F = (Viscosity)G or Viscosity = F/G
Newton looked at this problem with respect to pure liquids and he found that there was a direct relationship between the shearing stress applied and the the rate of shear. The constant of proportionality is called the viscosity or coefficient of viscosity. If you were to plot shearing stress vs rate of shear, you would get a straight line with a slope equal to the viscosity.
In laboratory we will use a capillary viscometer. Since the force is due to hydrostatic pressure the farce is dependent on the density of the liquid. Thus, we can develop a simple relationship between different Newtonian liquids which looks like this;
Viscosity1 / Viscosity2 = density1 X t1/density2 X t2
Where the t value is the time needed for a fixed volume of fluid to flow through the viscometer.
Non-Newtonian Flow
While pure liquids show a linear relationship between rate of flow and force applied or shearing stress heterogeneous systems often do not.
Systems which do not move when you first add force or shearing stress but then begin to move after a minimum amount of force is added are called plastic. Once these systems move, the addition of force causes a linear increase in rate of shear. We often use the term plastic viscosity "U" to describe these systems where:
U = (F-f)/G; U is the plastic viscosity, F is the shearing force, G is the rate of shear and f is the minimum force needed to get any movement commonly called a yield value.
There are some systems which have no yield value but which do not show a linear relationship between F & G. These are called pseudoplastic.
Systems which get more viscous as more force is added are called Dilatant
Heterogeneous systems can also exhibit a time sensitve change in viscosity. If a system changes its viscosity with time it is call a thixotropic system. It is possible to be plastic and thixotropic or pseudoplastic and thixotropic.
What would be the advantage of each system?
Which type of heterogeneous system would most likely fit each rheology pattern?
If you wished to design a liquid pediatric dosage form which rheology pattern would you attempt to create?