Chemistry Reference
In-Depth Information
At a certain critical length,
L
c
, depending on the stream velocity,
U
, the
film stopped growing. It is found that a dimensionless parameter (a ''Weber
number'')
N
w
given by
(8)
U
2
N
{
L
U
w
c
V
V
w
o
is approximately constant with a value of roughly 620 in the experiment.
Here V
w
and V
o
are the surface tensions upstream of the film (pure water)
and at the obstruction end of the film, respectively, i.e., V
w
- V
o
is the total
drop in surface tension. In the experiment this drop was always found to be
around 45 mN m
-1
. This implies that
2
L
c
U
|
constant
It was further found that the surface tension varies linearly along the
film (excluding the front portion with the Reynolds ridge). Using (8) we
get
(9)
2
'
V
K
U
U
x
where 'V is the drop in the surface tension and
x
the distance from a refer-
ence level right behind the Reynolds ridge (Figure 4). Here the dimen-
sionless number
K
# 0.003.
Fig. 4.
Drop in surface tension 'V from a reference level behind the Reynolds
ridge as a function of
U
2
x
where
x
is the distance behind the same reference level
(taken from Mockros and Krone 1968)
