Entwicklung eines laseroptischen Messverfahrens zur Quantifizierung der Schichtdicke von Wasserfilmen (English shop)

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Aqueous liquid films occur in countless technical applications as either a desired or an undesired effect. In order to understand and optimise these processes, information on the film thickness as well as on the temporal behaviour of the film is indispensable. A measurement technique that allows liquid films to be characterised with respect to their thickness and shape would contribute substantially to achieving these goals. The quantitative technique for measuring water film thicknesses on surfaces developed and applied in the present work is based, on the one hand, on laser-induced fluorescence and, on the other, on spontaneous Raman scattering by water molecules. Compared with existing measurement methods, these offer the advantage of two-dimensional resolution, also for the detection of evaporating water films. The fluorescence originates from added tracer substances whose fluorescence intensity was investigated as a function of concentration and temperature. For application in evaporating systems, in which the tracer is required to exhibit co-evaporative behaviour, equilibrium calculations were carried out in order to select fluorescence markers matched to the evaporation properties of water. Ethyl acetoacetate fulfils the required properties and was used as the tracer for the investigations presented here.

Calibration measurements at known film thicknesses in a calibration cell provide the relationship between the detected signal and the film thickness for a thickness range from 5 to 1000 μm. While for initial fundamental investigations the films were applied to a transparent surface using a pipette and an injector, in a second step the transfer to a technical system is carried out, namely a flow channel with the possibility of injecting the tracer–water solutions. Here, film thicknesses are determined on a metallic surface. Phase-resolved measurements provide information on the temporal development of the films during the injection process. In addition to the contour, they also provide information on the deposited liquid mass, while time-resolved investigations examine the process of film evaporation on a heated surface. Single-shot film thickness measurements offer the possibility of visualising movements of the film surface.