Three-dimensional encoding of a gas-liquid interface by means of color-coded glare points

The present work introduces an extension of the shadowgraphy method by differently colored oblique light sources for the observation of the three-dimensional spatio-temporal dynamics of gas-liquid interfaces. The proposed expanded approach is tested and elaborated with the example of a droplet during impingement. Particularly, it is elaborated in a combined experimental/theoretical approach, how well glare points from differently colored oblique light sources can be used to encode additional 3D information of the droplet shape within a single shadowgraph image. 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. 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. In experiments with the proposed measurement setup of a water droplet impinging onto a flat substrate it is successfully demonstrated that three unique and independent grayscale images can be reconstructed with this color correction function. The optimal illumination angles for the lateral light sources are determined experimentally, which lead to consistent glare points on the deforming gas-liquid interface throughout the dynamic process of the drop impact. An ellipsoidal droplet is considered to derive information on orientation and three-dimensional shape of a non-axisymmetrical droplet from the relative positions of the glare points and the shadowgraph contour. Thereby it is successfully demonstrated that the additional three-dimensional information encoded in the glare points can lay the groundwork for the volumetric reconstruction of the deforming gas-liquid interface during the impingement of a droplet.