Non-Intrusive Single Camera Diagnostic For Measurement Of Interfacial Topography In Three-Dimensional Space And Time

Liquid-gas interfaces are complex boundaries that can be of great interest in various fields, from fundamental science to industrial applications. They usually span across multiple spatio-temporal scales and, they can be a key parameter in multiple complex physical processes such as bubble entrainment and interfacial transfer. These make their simulation and measurement challenging but at the same time essential. Although computational fluid dynamic (CFD) models had become very powerful through the years, they still include some degree of empiricism. Experimental high spatio-temporal data for the interfacial topography could not only provide more insights and evidence for the physical process at play but also valuable validation datasets for the CFD models. Non-intrusive interfacial reconstruction, especially in the threedimensional (3D) space is a challenging task and it usually forces to complex experimental configurations. Here, we present a new, non-intrusive, refraction-based diagnostic for reconstruction of liquid-gas interfaces in 3D with one single camera and laser plane. Concisely, a vertical, two-dimensional (2D), laser-induced fluorescent grid is written directly in the liquid phase and captured from the gas phase through a deformed liquid interface. A robust ray tracing algorithm relates the optical deformation of the grid to the interfacial topography. The orientation of the grid provides boundary conditions for the surface reconstruction and minimizes restrictions of existing schemes. These, enable not only to simplify the experimental setup but also to extend the applicability of the refraction-based techniques. The grid generation scheme employed here is Talbot-effect structured illumination (TESI); a flexible method to create a wide range of very fine laser patterns. This allows to easily adapt the technique in a broad range of scales and applications, as well as to combine it with velocimetry. Profilometry and velocimetry can share the same grid and therefore they can be deployed simultaneously enabling the measurement of quantities such as the interfacial shear stress. Finally, the algorithm can treat the data both as time independent and time resolved. Both approaches are elaborated below and preliminary data from a droplet impact are presented and discussed.