8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P6.23Measurement of drop-wise condensation on a planesubstrate using confocal scanning microscopyHelmut Kühnelt, 1 Christian Jungreuthmayer, 1 and Christoph Körber 11 AIT Austrian Institute of Technology GmbH, Giefinggasse 2, 1210, Vienna, AustriaConfocal scanning microscopy (CSM) was applied to measure drop-wise condensation on a cooledpolycarbonate plate. A reflected light confocal microscope designed for technical surface metrologywas used for three-dimensional profiling of the droplets. CSM has an increased lateral resolutionis increased compared to wide field microscopy. The true benefit of CSM is found in theoptical sectioning capabilities. In practice vertical resolutions of a few nanometers can be achievedfor microscopic objectives with high numerical aperture NA. The main limiting factor remains themaximum surface slope which can be measured since it depends on the half angle of NA. Waterdroplets on polycarbonate show a contact angle of approximately 80◦. Thus the cap of the dropletdown to the maximum surface slope can be measured accurately by CSM as well as the plane substrateand the drop contact line of the drop. In order to evaluate the properties of the single dropslike volume, eccentricity and their size distributions the missing data has to be reconstructed. Theapplied procedure for data reconstruction and droplet evaluation was programmed using MATLABand it allows fully automatic analysis of series-measurements. The experimental setup used untreatedplane test samples of polycarbonate originating from industrial injection molding to studydrop-wise condensation under surface conditions found in the final product. Large sets of statisticaldata could be obtained. This data will further be used for validating a drop-wise condensationmodel for CFD as well as a fast condensate measurement method for testing during product development.23