Rheology of liquids Flashcards
difference between rheology and viscosity
study of the flow properties of materials vs resistance of a liquid to flow when it is subjected to stress
formula for viscosity
viscosity (n) = Shear stress/Shear rate
n = F/A divided by dv/dr
what’s the difference between Newtonian and non-newtonian fluid?
newtonian = constant gradient = constant viscosity; always passes through the origin as a straight line
non-newtonian fluid = varying viscosity; curves passing or not passing through the origin
examples of Newtonian fluid
water, organic solvent (chloroform, ethanol), oil, true solutions (saline soln)
some suspensions and emulsions (but have to be very dilute)
3 types of deviation for non-newtonian liquids
plastic (bingham), pseudoplastic, dilatant flow
at low shear stress, plastic flow liquids behave as
elastic solid
when does appreciable flow begins for bingham flow?
shear stress must be equivalent to the yield value (fB)
at stress above yield value, the plastic fluid resembles a ___________
Newtonian system
which two non-newtonian fluid exhibit shear thinning?
plastic and pseudoplastic flow
plastic flow is associated with the presence of ______ in a ____________
presence of flocculated particles in a concentrated suspension
bingham’s equation
U = (F-f)/G U = plastic viscosity, F = shear stress, f = yield value, G = shear rate
for pseudoplastic flow, viscosity ______ with _____________ shear rate
decreases; increasing
at higher shear stress, pseudoplastic flow curve tends towards __________
linearity
pseudoplastic flow equation
log G = N log F - log n’
G = shear rate, F = shear stress, n’ = viscosity coefficient, N = index for pseudoplasticity (N > 1),
A Straight line with limited applications
original eqn: F^N = n’ x G
pseudoplastic flow is associated with
polymers in solution (aqueous dispersions of hydrocolloids such as tragacanth, alginates, methylcellulose and synthetic materials such as polyvinylpyrrolidone.
polymer = long and high MW molecules
for dilatant flow, the viscosity ________ with _________ shear stress
increases, increasing
dilatant flow liquids exhibit
shear-thickening
dilatant flow equation
F^N = n' G G = shear rate, n' = viscosity coefficient, N = index of dilatancy (N<1), F = shear stress
example of dilatant flow
high concentrations of deflocculated particles
non-newtonian liquid consists of
solid component (flocculated/long/deflocculated)
structural changes can happen to a non-newtonian liquid via 3 ways
rate of shear, frequency of shear, duration of shear
most reversible changes for non-newtonian liquids are
not instantaneous
the upcurve and downcurve are usually ________ and form the ______
not superimposable; hysteresis loop
the area of the hysteresis loop indicates
the extent of structural breakdown
which systems can be described as exhibiting thixotropy?
plastic & pseudoplastic
thixotropy is characterised by:
- decrease in apparent viscosity upon application of shearing forces (shear thinning)
- slow recovery of apparent viscosity on standing of the system
(upcurve and downcurve are not superimposable)
what can we do to reduce the recovery time of thixotropy?
gentle rolling or rocking motion helps with reformation of the structure
thixotropy is a desirable property in which type of liquid preparations?
suspensions and emulsions
formula for dynamic viscosity
n = shear stress / shear rate
formula for kinematic viscosity
v = (n) dynamic viscosity / density
where n is the dynamic viscosity
Absolute viscosity applied to what type of fluid
Newtonian fluid
Apparent viscosity applies to what type of fluid
non-Newtonian fluids
what viscometers are used for netwonian fluids? and what are these viscometers alternatively called
capillary viscometers: U-tube, suspended-level
- ‘one point’ instruments
note: U-tube more commonly used and found in BP
viscometers used for non-netwonian fluids? and what are these viscometers alternatively called
Rotational viscometer: concentric cyclinder viscometer, cone and plate viscometer
OR
efflux viscometers (esp for tragacanth = pseudoplastic flow)
these viscometers are alternative called: ‘multiple point’ instruments
precautions to take note for capillary viscometers
- ensure streamline flow by avoiding vibration and maintaining a minimum flow time
- free from air bubbles
- clean viscometer before use
- clamp viscometer vertically
- accurate temperature control
equations used in capillary viscometers (what is the law used?)
equation relating viscosity and time
Poiseuille’s Law: n = (pi x r4 x t x P)/ 8 x l x V
r = radius of capillary tube t = time of flow P = pressure diff across capillary tube ends l = length of capillary tube V= vol of liquid
–> n = K t p
where k is a constant, p here refers to density
n1/n2 = (t1 x p1) / (t2 / p2)
OR
V1 / V2 = t1/ t2
v = kinematic viscosity
n = (dynamic) viscosity
1 and 2 refers to test and reference liquids respectively
flow time requirement for U-tube (Ostwald) viscometer
at least 300s for size A,
at least 200s for all other sizes (B to H)
- ensures less possibility of turbulence
- also ensure a streamline flow is formed to follow the Poiseuille’s law eqn
flow time requirement for suspended-level (ubbelohde) viscometer
at least 300s for size 1,
at least 200s for all other sizes (1A to 5)
- ensures less possibility of turbulence
- also ensure a streamline flow is formed to follow the Poiseuille’s law eqn
2 types of concentric cylinder viscometers
Searle type = inner cylinder is rotated
Couette type = outer cylinder is rotated
the inner cylinder is also called as what for the concentric cylinder viscometers
spindle
what do rotational viscometers measure
measurement based on viscous drag exerted on a body when it its rotated in a liquid
what do capillary viscometers measure
time of flow by gravity
what do we measure using the couette type?
torque transmitted to the inner cylinder
stress exerted on the inner cylinder translate to the angular deflection of the point = equal to the viscosity
equation relating viscosity with angular deflection
n = (K x tetha) / w
where k is a constant, tetha = angular deflection
w = angular velocity of outer cylinder
n = (dynamic) viscosity
original eqn:
C x tetha = ( 4 x pi x h x w x n)/ (1/r1^2 - 1/r2^2)
r1 = radius of inner cylinder r2 = radius of outer cylinder h = height of inner cylinder covered by test liquid
diff sizes of spindle can cause diff defraction, give examples
larger spindle for lower viscosity = larger area, greater defraction (i think it’s deflection)
smaller spindle for greater viscosity = lower deflection
example of a type of concentric cylinder viscometer
brookfield viscometer (i think its a searle type: inner cylinder is rotated)
problems of concentric cylinder viscometers
- shear rate of test liquid across the gap is not constant
- end effects can be significant
- frictional heating significant at high shear rate
- large vol of test liquid required if gap is big
equation connecting viscosity and torque on cone + radial velocity of plate for cone and plate viscometer
n = (K x G) / ohm
G = torque on cone
K is a constant
ohm = radial velocity of plate
important point for cone and plate viscometer
angle should be small to avoid edge effect (referring to angle c in diagram)
advantages of cone and plate viscometer compared to concentric cylinder
- vol of test liquid needed is small (unlike the concentric cylinder)
- filling and cleaning is easy
- shear rate is constant throughout the test liquid (unlike the concentric cylinder)
what does efflux viscometers measure
time taken for liquid to flow though a small orifice
longer the efflux time = more viscous the liquid
examples of efflux viscometers
redwood viscometer, flow cups
what is the main use of redwood viscometer
determines viscosity of standard solution of tragacanth relative to that of water
diff bet u-tube (ostwald) viscometer and suspended-level (ubbelohde) viscometer
suspended-level (ubbelohde) viscometer have a U-tube + side arm
advantages of suspended-level (ubbelohde) viscometer (two adv)
(1) no need to fill viscometer with precise vol of liquid
2) can be used for very viscous liquids (10x higher than U-tube viscometer
why non-newtonian liquids uses multiple point instrument
shear rate is not directly proportional with shear stress
viscosity is not constant
multiple point instruments allow to find apparent viscosities
why newtonian liquids uses single point instrument
shear rate is directly proportional with shear stress
viscosity is constant
single point determination at a specific shear rate is adequate
