Measurement of liquid film thickness in a micro parallel channel with interferometer and laser focus displacement meter

In the present study, liquid film thicknesses in parallel channels with heights of H = 0.1, 0.3 and 0.5 mm are measured with two different optical methods, i.e., interferometer and laser focus displacement meter. Ethanol is used as a working fluid. Liquid film thicknesses obtained from two optical methods agree very well. At low capillary numbers, dimensionless liquid film thickness is in accordance with Taylor's law. However, as capillary number increases, dimensionless liquid film thickness becomes larger than Taylor's law for larger channel heights. It is attributed to the dominant inertial effect at high capillary numbers. Using channel height H for dimensionless liquid film thickness δ0/H and hydraulic diameter Dh = 2H as the characteristic length for Reynolds and Weber numbers, liquid film thickness in a parallel channel can be predicted well by the circular tube correlation previously proposed by the authors. This is because curvature differences between bubble nose and flat film region are identical in circular tubes and parallel channels.