Three-Dimensional Detection Of A Gas-Liquid Interface By Means Of Oblique Colored Light Sources

The present work introduces an extension of the shadowgraphy method by differently colored oblique light sources for the observation of the three-dimensional dynamics during drop impingement. The wetting of a flat or structured surface by an impinging droplet is relevant for many technical processes and therefore a further understanding of the drop impact dynamics is essential for an optimization of these processes. The most widely used optical measurement technique for drop impact experiments is the shadowgraphy method, which produces a clear two-dimensional representation of the gas-liquid interface. However, drop impact dynamics – especially on structured surfaces – are inherently three-dimensional. Therefore, two lateral light sources are introduced, in order to produce glare points on the gas-liquid interface that reveal additional three-dimensional information of the droplet shape. Narrow-banded LEDs with distinct spectra and maxima in the visible light illuminate the droplet from different angles in red, green and blue light, respectively, while a high-speed RGB-camera captures the images produced by each light source in the corresponding image channel, therefore creating three unique views of the droplet. Thereby the shadowgraphy image resulting from the blue backlight illumination is recorded in the blue image channel, while the glare points from the green and red lateral light sources are captured in the accordingly remaining channels. In order to compensate for the mutual perturbation of the images resulting from cross-talk between the channels and the polychromatic light of the LEDs, a color correction is introduced, which is based on the transfer function between the light sources and the channels of the RGB-camera. The transfer function is determined by a spectral integration over the power distribution of the individual light sources and sensitivities of the single camera channels likewise, with the assumptions of fully specular reflection and that the spectral distribution of the reflected light equals the incoming light. In experiments with the proposed measurement setup of a water droplet impinging on a flat substrate it is successfully demonstrated, that three unique and independent grayscale images can be reconstructed with this color correction function, which successfully removes the mutual perturbations. The presented method is a pure addition to the shadowgraphy method, as it produces a clear shadowgraphy image and optionally provides two further unique views by glare points on the gas-liquid interface within a single RGB-camera setup.