To measure the thickness of liquid films from 10 to 60 μm thick, we used photochromic dye activation of the non-intrusive methods. Photochromic dye activation method is being used as one of the method to measure the liquid velocity distribution in the flow field. The method utilizes the change of the dye from a colorless form to a colored form when irradiated with a specific wavelength. Based on the characteristics of photochromic dye, when a thicker liquid film is irradiated with the beam of a fixed intensity, the color will be darker because the number of dye molecules increases at a fixed dye mass fraction. For the activation method, we used silicone oil with a viscosity of 10 centi-Stokes and commercial SO (spironaphthoxazine) and SO-ANTH (spiroanthraoxazine) dyes from Sigma-Aldrich Inc. To make liquid films with exact and known thicknesses, we used two glass wafers. A film formed between the two wafers within a few seconds after placing a drop of the liquid of known volume on one wafer and covering it with the second. The film thickness could be estimated from the diameter of the wafer and the dropped liquid volume. To quantitatively evaluate the color intensity of dyes, we captured the images using digital camera then analyzed the images using the Paint shop pro. The gray scale color intensity using the captured images of the activated dye with thickness from 10 to 60 μm showed the repeatability below ±1.0% when measured with a silicone oil solution containing 0.1% SO and SO-ANTH dyes. And the color intensity with our liquid film thickness ranges linearly changed. So, this photochromic dye activation method can be used to measure the thickness of thin liquid films. © 2006 Elsevier Ltd. All rights reserved.
Kim, J., & Kim, M. H. (2006). A photochromic dye activation method for measuring the thickness of liquid films. Measurement: Journal of the International Measurement Confederation, 39(6), 497–504. https://doi.org/10.1016/j.measurement.2005.12.013