P10: Rheology Flashcards
what is rheology
describes flow of liquids and deformation of solids
what is viscosity
expresses resistance of a fluid to flow; the higher the viscosity, the greater the resistance
what is rheology involved in
mixing and flow of materials, their packaging into containers, and their removal before use
what are Newtonian systems
consider a block of layered molecules;
bottom layer fixed in place and top plane of liquid is moved at constant velocity, each lower layer moves with velocity directly proportional to its distance from stationary bottom layer
what is the equation for the rate of shear
dv/dr
dr= height of block
dv= distance between furthest and nearest corner
what happens in non-Newtonian systems
most parmaceutical fluids do not follow Newton’s law of flow
what are examples of non-Newtonian behaviour
colloidal solutions, emulsions, liquid suspensions, ointments
what are the three classifications of flow
plastic, pseudoplastic, dilatant
what is plastic flow
flow does not begin to flow until a shearing stress corresponding to the yield value is exceeded
at stresses below yield value, the substance acts as an elastic material
substances that exhibit a yield value are classified as solids, whereas substances that begin to flow at the smallest shearing stress and show no yield value are defined as liquids
plastic flow is associated with presence of flocculated particles in concentrated suspensions ; more flocculation means higher yield value
what is pseudoplastic flow
exhibited by polymers in solution (unlike plastic systems which are more usually composed of flocculated particles in suspension)
no yield value, viscosity of a pseudoplastic material cannot be expressed by a single value
what are some examples of substances with pseudoplastic flow
natural and synthetic gums
e.g. sodium alginate, methylcellulose
what is dilatant flow
certain suspensions with a high percentage of dispersed solids exhibit an increase in resistance to flow with increasing rates of shear
these systams increase in volume when sheared
what substances are dilatant flow
invariable suspensions containing a high concentration of small, deflocculated particles
these systems will have plastic flow characteristics
what is a rheogram
plot of shear rate as a function of shear stress
simplest rheogram is the linear behaviour shown by Newtonian systems
what is thixotropy
a material which exhibits a reversible time-dependent decrease in apparent viscosity
extent of thixotropy is quantified by the area of hysteresis loop between up and down curves
