  Inflow control is essential for most fluid mechanics experiments. Although
vertically falling soap film flows have been extensively used in the last four
decades to study two-dimensional flows, its inflow stability has not yet been
discussed in detail. In this article, aiming to improve the inflow stability of
the system, we discuss how flow driving systems dominate the inflow states and
the statistics of soap film flow properties. We report experimental
measurements of inflow rates using different flow driving methods, followed by
soap film velocity measurements by Laser Doppler Velocimetry (LDV). The
widely-used method of a constant-pressure-head reservoir exhibits a continuous
drop of inflow rate. In addition, even when the flow is not disturbed, the soap
film displays high magnitudes of velocity fluctuations. The mean square of
velocity fluctuation was measured at 4.6% of the mean. We also test other
methods without the reservoir, where the flow is directly controlled by a pump;
two staggered syringe pumps are able to maintain inflow stability over days and
reduce the velocity fluctuation to 0.5% of the mean. From these measurements,
we conclude that the drop of inflow rate might be caused by micro/milli-scale
air bubbles, which cannot be completely removed in the system. A method to
control the inflow actively is necessary in a soap film setup, when a stable
and long standing film flow is needed.
