“…The actual holding time of the liquid inside tube depends on its size. For example, for a 500 µL flowmeter, 0.3 second was determined to be a sufficient residence time for the soap liquid in the tube to make a thin, yet strong layer of soap at the gas entrance (Lashkari and Kruczek, 2008A). It is important to note that such a short residence time would not be attainable if the bubble were made manually by squeezing a rubber bulb.…”