Orientational relaxation times of rhodamine 700 in glycerol-water mixtures

Observation of wall wettability under imposed flow by fluorescence depolarization: dependence on surface oxygen content and degree of polymer branching

This work was concerned with the dependence of the interfacial tension (Gamma(SL)) on surface degree of oxygen content and on polymer branching degree. The static Gamma(SL) was evaluated by contact angle (theta;(c)) and the dynamic Gamma(SL) by fluorescence depolarization of molecular probes seeded in induced flows of monoethylene glycol. The latter results were interpreted using statistical covariant analysis. Two different systems of flowing films were studied: free films flowing on the surfaces on which they impinge and films flowing inside 1-mm-thick microflow cells. The solid surfaces were polyethylene of low density, medium density, high density, and linear with low density, polypropylene, vinyl acetate co-polymer with oxygen content of 15% and 28%, borosilicate, and tin dioxide. Increase in oxygen content of the surface decreased both the static and the dynamic Gamma(SL), which demonstrated that the presence of oxygen atoms hindered wetting. Only the dynamical Gamma(SL) was sensitive to polymer branching, and it increased as branching degree decreased. This was attributed to the higher hydrogen-atom density at the surface, which favored temporary intermolecular bonds between the surface and the flowing liquid.

Detection of chemical alterations at internal walls of microchannel flow cells by nondestructive fluorescence depolarization

A recent trend is the production of workable microchannel flow cells (MF cells). The nondestructive methods used to assess their reliability are based mainly on output monitoring and do not evaluate internal chemical interactions. We investigate a nondestructive method for evaluating changes in the chemical composition of the inner walls based on evaluation of the extent of alignment of a fluorescent probe in a liquid flowing within MF cells. Two MF cells were built with a 10-microm inner spacing. Their inner walls had four parallel SnO(2) strips, 2.00 mm wide, separated by 0.50-mm-wide glass strips. One cell had strips parallel to the flow and the other perpendicular. Flow-induced intermolecular alignment of rhodamine B in monoethylene glycol was scanned with 28-microm precision by fluorescence depolarization, using polarized-laser-induced fluorescence within induced flows (PLF-FI). No changes of polarization were seen when the flow was stopped. Under flowing conditions, polarization was always 4% lower in the glass region as compared to SnO(2). Glass had a higher solid-liquid interfacial tension (determined by contact angle measurements), thus being more wettable and increasing the drag, which propagates into the liquid flow, decreasing polarization. PLF-FI can thus identify regions with different chemical constitutions.