what is rheology
study of flow properties of materials
what is viscosity
resistance of a liquid to flow when it is subjected to stress; usually mentioned as dynamic viscosity
how to count shear stress
force/ area
how to count shear rate
= velocity gradient = dv/dr
how to count viscosity
shear stress/shear rate
what are newtonian liquids
liquids that have constant viscosity
- flow curve (rheogram) shows straight line passing through origin
plastic flow
bingham flow:
- behaves as an elastic solid at low shear stress
- a certain shear stress equivalent to yield value must be exerted before appreciable flow begins
- at shear value above yield value- resembles newtonian system
- apparent viscosity dependent on shear rate
- liquid experiencing plastic flow are shear thinning
how to count plastic flow
plastic viscosity= (shear stress - yield value ) / shear rate
psuedoplastic flow
- liquid flow as soon as shear stress is applied
- viscosity decreases when increasing shear rate
- flow curve tends otwards linearity at higher shear stress
- liquid exhibiting psuedoplastic flow are shear thinning
- apparent viscosity dependent on shear rate
what is thixotropy
decrease in apparent viscosity upon application of shearing forces (shear thinning), slow recovery of apparent viscosity on standing
- desirable property to have in suspensions and emulsions
type of viscometer for netwonian liquids
capillary viscometers- utube, suspend level
type of viscometer for non-newtonian liquids
rotational viscometer - concentric cylinder, cone and plate
advantage of suspend level (ubbelohde) viscometer
- no need fill viscometer with precise volume of liquid
2. can be used for very viscous liquids (x10 more than utube)
precaution of capillary viscometer
- ensure streamline flow by avoiding vibration and maintaining a minimum flow time
- liquid should be free from air bubble
- ensure viscometer is clean before use
- ensure viscometer is clamped vertically
- ensure accurate temperature control
problems of concentric cylinder viscometeres
- shear rate of test liquid across the gap is not constant
- end effects can be significant
- frictional heating is significant at high shear rate
- filing is difficult if gap is small
- large volume of test liquid is required if the gap is big
main advantage of cone and plate viscometer
- volume of test liquid needed is small
- filling and cleaning is easy
- shear rate is constant throughout the test liquid
