rheology Flashcards
rheology
the study of the deformation and flow of matter
stress
the force required (per unit area) that leads to deformation
strain
the resulting deformation
elasticity
if the material fully recovers when the stress is removed it is elastic
compliance
the ease of deformation
dynamic viscosity=
F/(dv/dr) = shearing stress/rate of shear
viscosity
The viscosity of a fluid is the internal resistance
(or friction) involved in the relative motion of
one layer of molecules with respect to the next
relationship between attractions and viscosity
strong attractions between liquids=high dynamic viscosity
weak attractions=low dynamic viscosity
effect of temperature on dynamic viscosity
gases>^temp, ^interactions=increased contact-higher viscosity
liquids>^temp, decreased reactions=decreased contact-lower viscosity
types of viscometers
Ostwald viscometer. ubbelhode viscometer
different types of viscosity
specific, reduced, relative, intrinsic, apparent
what can flow properties be characterised as
Newtonian or non-Newtonian
4 different flow types
Newtonian, plastic, pseudo plastic, dilatant
Newtonian
exhibited by many simple liquids and true solutions eg caster oil, water, chloroform
plastic
will not start to floe until a stress corresponding to the lower yield value is applied, exhibited by some ointments, pastes and creams
pseudo plastic
substance will start to flow immediately as a stress is applied.
apparent viscosity effect on flow properties
Newtonian fluids= same apparent viscosity
non-Newtonian fluids= invariable changes in apparent viscosity
rotational viscometers
concentric cylinder (‘cup and bob’)
sample is sheared in the space between the cup and the bob.
couette- cup rotates
Searle-bob rotates
the torque (stress) on the rotating bob required to maintain a constant speed (shear rate) of rotation against the viscous drag is measured
rotational viscometers
geometry is of a flat circular plate with a wide-angled cone placed centrally above. the sample under test is placed in the gap between the cone and plate. continuous measurement of shear stress vs shear rate produces viscosity profile of sample/fluid/solution
continuous measurement of napp necessary to determine behaviour
falling sphere viscometer
stokes law
single point viscometer
only one viscosity an be measured at one point, can measure the viscosity of Newtonian fluids, usually as a quality control measure for non-Newtonian fluids
thixotropy
‘to change by touch’ the presence of a hysteresis loop indicated that a breakdown in structure has occurred. the area within the loop may be used as a measure of the degree of this breakdown (usually recoverable)
pharmaceutical areas in which rheology is significant
mixing, particle size, pumping, stability (fluids)
spreading and adherence to skin, removal from jars, mixing of solids and liquids, release of drug from base (semisolids)
powder flow (solids)
processing efficiency (processing)
