Fluorescence and Fiber-Optics Based Real-Time Thickness Sensor for Dynamic Liquid Films

To overcome the limitations/disadvantages of many known liquid film thickness sensing devices (viz. conductivity probes, reflectance based fiber-optics probes, capacitance probes, etc.), a new liquid film thickness sensor that utilizes fluorescence phenomena and fiber-optic technology has been developed and reported here. Measurements from this sensor are expected to facilitate better understanding of liquid film dynamics in various adiabatic, evaporating, and condensing film flows. The sensor accurately measures the instantaneous thickness of a dynamically changing liquid film in such a way that the probe does not perturb the flow dynamics in the proximity of the probe’s tip. This is achieved by having the probe’s exposed surface embedded flush with the surface over which the liquid film flows, and by making arrangements for processing the signals associated with the emission and collection of light (in distinctly different wavelength windows) at the probe’s flush surface. Instantaneous film thickness in the range of 0.5–3.0 mm can accurately (with a resolution that is within ±0.09 mm over 0.5–1.5 mm range and within ±0.18 mm over 1.5–3.0 mm range) be measured by the sensor described in this paper. Although this paper only demonstrates the sensor’s ability for dynamic film thickness measurements carried out for a doped liquid called FC-72 (perfluorohexane or C6F14 from 3M Corporation, Minneapolis, MN), the approach and development/calibration procedure described here can be extended, under similar circumstances, to some other liquid films and other thickness ranges as well.