Chemistry Reference
In-Depth Information
Profile of a Pendant Drop
DS
DE
DE
FIGure 2.11
Pendant drop of liquid (shape analysis).
However, the advantage of the former over the latter method consists in that it
makes it possible to choose the most convenient form and size of the body (platinum
rod, ring, or plate) so as to enable the measurement to be carried out rapidly but
without any detriment to its accuracy. The “detachment” method has found an appli-
cation in the case of liquids whose surface tensions change with time.
Shape of the liquid drop (Pendant drop method):
The liquid drop forms as it flows
through a tubing (Figure 2.11). At a stage just before it breaks off, the shape of the
pendant drop
has been used to estimate γ. The drop shape is photographed and, from
the diameter of the shape, γ can be accurately determined.
The parameters needed are as follows. A quantity pertaining to the ratio of two
significant diameters are
S
= d
s
/d
e
(2.33)
where d
e
is the equatorial diameter, and d
s
is the diameter at a distance d
e
from the tip
of the drop (Figure 2.11). The relation between γ and d
e
and S is found as
γ = ρ
L
g
d
e
2
/
H
(2.34)
where ρ
L
is the density of the liquid, and
H
is related to
S
, but the values of 1/
H
for
varying
S
were obtained from experimental data. For example, when
S
= 0.3, 1/
H
=
7.09837, whereas, when
S
= 0.6, then 1/
H
= 1.20399. Accurate mathematical func-
tions have been used to estimate 1/
H
for a given d
e
value (Adamson and Gast, 1997;
Birdi, 2002). The accuracy (0.1%) is satisfactory for most of the systems, especially
when experiments are carried out under extreme conditions (such as high tempera-
tures and pressures).
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