Chemistry Reference
In-Depth Information
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U-tube capillary viscometers
U-tube capillary viscometer is a simple and widely used method for
qualitative estimation of liquid viscosity. The essence of this method
is to measure the resistance of liquid to flow through a 'U'-shaped
capillary channel. The flow time of a liquid through a certain length
of the channel is recorded as the efflux time and is used to calculate
apparent viscosity. Several types of U-tube capillary viscometers, such
as Ostwald, Canon-Fenske, Ubbelohde, and Kimax
R
, have been used
to measure the apparent viscosity of milks. These instruments differ
in geometry of the glass tube and are all designed as gravity-operated
devices. U-tube capillary viscometers are only suitable for Newtonian
fluids because shear rate varies during discharge of the fluid. Griffin
et al
. (1989) found that concentrated skimmed milk showed Newtonian
behaviour at stresses below 16 mN
m
−
2
and measured viscosity using
an Ostwald U-tube viscometer. The flow behaviour of milk can be
assumed as Newtonian during discharge in a U-tube capillary viscometer
because the shear rate range of this process is very low.
U-tube capillary viscometers are mostly used to measure relative
viscosity (Equation 7.10) of milk samples:
·
=
η
solution
η
solvent
Relative viscosity
η
(7.10)
rel
where
η
solvent
correspond to the apparent viscosities of so-
lution (milk) and solvent (water). The apparent viscosity of skimmed
milk was also used as
η
solution
and
solvent
when studying the effect of fat content
to viscosity (Kyazze and Starov, 2004). The apparent viscosity is pro-
portional to the efflux time of fluid flowing through the capillary tube.
Therefore, the time required for the solution to flow through a capillary
viscometer is measured, divided by the time required for the solvent
to flow through the same viscometer, and reported as relative viscosity.
Relative viscosity is positively associated with total solids or specific
components, such as fat, representing the contributions of dispersed or
dissolved components to viscosity.
Relative viscosity has been used not only to determine the effects
of milk composition on viscosity but also the effects of other factors
such as heat treatment. Jeurnink and de Kruif (1993) studied the effect
of heat treatment on skim milk viscosity using an Ubbelohde capillary
viscometer. The U-tube viscometer is a convenient and inexpensive
method to measure and compare the apparent viscosity of milk products.
However, the data collected is only at a certain range of shear rates, which
depends on the physical properties of fluid and the tube geometry and
cannot be controlled. The fluid is assumed to be Newtonian within this
η
