Viscosity describes how a liquid resists the laminar movement of two
neighboring layers. This resistance to flow can be seen with gases, liquids and
even solids.
Stoke's law:
In laminar flow, a ball with radius R moving through a liquid with viscosity η
has a speed proportional to the frictional force.
Fr = 3 π η d v
Or:
where:




Fd is the frictional force acting on the interface between the fluid and the
particle (in N),
η is the fluid's viscosity (in [kg m-1 s-1]),
R is the radius of the spherical object (in m), and
v is the particle's velocity (in m/s).
Intrinsic viscosity
The intrinsic viscosity can be extrapolated by measuring several polymer
solutions with different concentrations. It corresponds to the viscosity of a
solution at concentration c=0. From this result, the molecular weight average
of a polymer can be determined using the Mark Houwink equation.
The Mark–Houwink equation gives a relation between intrinsic viscosity [η] and
molecular weight M:
[η] = KMa
Viscosity can be readily followed using an
 Ostwald viscometer (Fig1).
 Falling ball viscometer (Fig2).
Ostwald viscometer: This, essentially, consists of a capillary tube down which
a known volume of protein solution is allowed to flow under gravity. The time
taken for this flow is measured (t1 and also that of the solvent (t0); the relative
viscosity is then given by:
Relative viscosity: ηrel =η1/η0 = ( t1/t0 ) X ( ρ1/ρ0)
Where η1 is the viscosity of the protein solution of density ρ1 and η0 the
viscosity of the solvent of density ρ0. If the densities are taken to be the same
then the expression = ( t1/t0 )
Falling ball viscometer 2
Figure 1:Ostwald viscometer
Figure 2: Digital Falling ball viscometer (Brookfield)
Ostwald Viscometer
Two markers to measure the time during
this period
Photed by Demonstrator Nouf alshareef
Add certain volume of liquid
Add certain volume of liquid
(Measure the time as the liquid
passes through this length)
Capillary tube
Photed by Demonstrator Nouf alshareef
MATERIALS
1. Ostwald viscometer
2. Falling ball viscometer
3. Water bath at 20°C
4. Glucose 1,3,5,7g%
5. Dextran 1,3,5,7g%
6. Glycerine
7. Stop watch accurate to at least 0.1 s
250 ml
METHOD
Viscosity is very sensitive to temperature, so all solutions and the
viscometer must be kept at 30°C in the water bath.
Always handle the viscometer by one limb only and never squeeze the
two arms together.
1. Rinse the viscometer with water and place it in position in water
bath by carefully clamping one limb. Check that it is vertical using
a plumb line
2. Introduce exactly 20 ml (or the volume marked on the viscometer)
of water at 20°C into the bulb A with a syringe or pipette.
3. Leave for 5 min to equilibrate, then either apply positive pressure
to the wide limb (I) or gentle suction to the other limb (II) until the
meniscus rises above the upper graduation mark B.
4. Release the pressure and measure the time (to the nearest 0.1 s) for
the liquid to flow between the two graduation marks B and C.
Repeat the experiment until the flow times agree within 0.2 s
5. Calculate the average flow time: (t0), solvent, and then with the
glucose dissolved in the water (t1).
6. Calculate the relative viscosities (t1/t0 ) using the values from the
curves.
7. Draw standard curve of relative viscosities (t1/t0 ) vs. con g%
8. Calculate intrinsic viscosity (intercept with y access).
References:
D.T.Plummer, An introduction to Practical. Bioc. MC Grow Hill
http:/ www./canadawide.ca/
http://en.wikipedia.org/wiki/File:Stokes_sphere.svg
http://en.wikipedia.org/wiki/Mark%E2%80%93Houwink_equation
Name
Computer no.
Group
date
/
Substance
T1
T0
/
Rel. viscosity