Observation of the Effect of Wetting Efficiency on a Flow of Liquids Impinging on Solid Substrates by Fluorescence Depolarization

Steady-state fluorescence depolarization has proven, in the past few years, to be a method sensitive to adhesion of thin liquid films flowing without boundaries on solid substrates of different chemical constitutions. This work extends our prior preliminary assessment of the efficiency of the method and the influence of experimental variables. Ten thin films of ethylene glycol (MEG) were mapped over 102 mm2 by polarized laser-induced fluorescence (PLF−FI) while flowing on substrates. The interfacial tension, ΓSL, was varied either by changing the chemical constitution of the substrate (borosilicate, tin dioxide, poly(vinyl alcohol) (PVA), and linear alkylbenzene sulfonate (LAS)) or by addition of tensoactives to MEG (sodium dodecyl sulfate (SDS) and poly(ethylene oxide) (PEO)). Contact angle measurements were employed to classify interfaces according to their static ΓSL, that is, their wetting efficiencies. PLF−FI experimental variables, such as laser power, film thickness, and impinging velocity profile, did not alter the data within the range studied, except for flow rates over 220 cm s-1. Average polarization (P av) increased from 10.5% to 13.5% upon decreasing adhesion by varying the chemical constitution of the solid. When surfactants were dissolved in the liquid flow, polarization ranged from 5.4% to 3.8% while for surfactant as substrate it increased. Simple multivariate principal component analysis (PCA) was applied to downstream polarization averages (P down) for each map. The two principal components accounted for 95.6% of the variance, ordering the maps according to decreasing adhesion and gathering them into three groups: dissolved surfactants, liquid films flowing at the highest flow rate, and solid surfaces of different chemical composition. Hierarchic cluster analysis (HCA) showed that, as adhesion decreased, similarity among the polarization maps increased.