Chongfan Technology
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14
2025
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03
Light | Meta-surface Enabled Visualization of Two-Dimensional Transparent Flow Field
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Introduction
“The wind is known by the falling leaves, and the current by the unruffled water.”The flow of nature is ubiquitous, yet often hidden from view. Transparent flow field visualization technology plays a crucial role in engineering and scientific applications, while achieving high-dimensional visualization of transparent flow fields under non-invasive conditions remains a significant challenge.
Recently, a research team from Nanjing University has made new progress in this field. By introducing multifunctional metasurfaces, the team members have achieved real-time, full two-dimensional visualization imaging of transparent flow fields. This achievement was published inLight: Science & Applications，titled “Non-invasive and fully two-dimensional quantitative visualization of transparent flow fields enabled by photonic spin-decoupled metasurfaces.” Nanjing University School of Modern Engineering and Applied SciencesProfessor Xu Ting、Professor Lu Yanqingand the School of Electronic Science and EngineeringProfessor Yan Fengare the corresponding authors of this paper.
Flow fields (including liquids and gases) are usually transparent and invisible to the naked eye or traditional cameras. Therefore, transparent flow field visualization is a key technology in many scientific and engineering disciplines, such as the study of shock wave structures, combustion chamber flows, wind tunnels, and boundary layers. Visualizing flow fields not only enhances understanding of fundamental physical phenomena but also greatly facilitates the development of efficient engineering systems.
In the field of flow field information detection, researchers have developed various techniques to acquire information about transparent flow fields. For example, probe methods can provide high temporal resolution for measurements at specific points; particle image velocimetry (PIV) can effectively track flow patterns and measure velocity fields; laser-induced fluorescence (LIF) can visualize complex flow patterns, turbulent structures, and chemical reactions; traditional shadowgraphy techniques can obtain one-dimensional flow field information through optical methods; and infrared thermal imaging can non-invasively detect temperature, which is very useful for studying the thermal effects of flow surfaces. However, although existing techniques each have unique advantages in the study of transparent flow fields, they also have obvious limitations, such as invasiveness, the need for specific tracers, reliance on complex post-processing algorithms, or only presenting one-dimensional information.
The research team proposed a non-invasive, real-time two-dimensional flow field visualization method. This method, by introducing a multifunctional metasurface array, achieves two-dimensional visualization images of transparent flow fields with only a single exposure, including flow field information of horizontal and vertical density gradients (Figure 1a). In this design, the metasurface needs to undertake two key functions: First, the metasurface acts as a spatial filter. For left-circularly polarized (LCP) incident light, the metasurface behaves as a step function in the horizontal direction, thereby extracting the horizontal dimension information of the flow field; while for right-circularly polarized (RCP) incident light, the metasurface performs spatial filtering in the vertical direction to capture the vertical dimension information of the flow field. Therefore, this device establishes a mapping relationship between the polarization state and the density gradient vector in two orthogonal dimensions. Second, this metasurface device can effectively decouple the spatial overlapping information of the flow field in two dimensions.
Figure 1: Schematic diagram of transparent flow field visualization based on metasurface
According to the functions that the metasurface needs to accomplish, the research team made full use of the design freedom and flexibility of nanostructures to develop a photonic device composed of multiple metasurface sub-arrays (Figure 1b), each sub-array can be described by a different Jones matrix. By solving the eigenvalues and eigenvectors of the matrix, the specific design of the nanostructure can be obtained. Each metasurface sub-array has a different function, and their synergistic effect achieves a function that is difficult to achieve with traditional optical techniques, namely non-invasive, real-time, full two-dimensional visualization of transparent flow fields. This technology not only provides an excellent platform for fluid physics research but also shows broad application potential in industrial design and machine vision fields.
As a proof of concept, the team researchers experimentally demonstrated the application potential of the proposed visualization technology in different scenarios, including temperature field visualization (Figure 2), gas leak detection, visualization of various fluid physical phenomena (Figure 3) and 3D morphology reconstruction of transparent solids. In an axisymmetric flow field, Abel inversion combined with standard photometry can also be used to achieve quantitative calculation of flow field-related information, such as temperature and density. In addition, since the system can present two-dimensional flow field images in real time, a high-speed camera can be used to capture the transient changes of the flow field. Figure 3e shows the spatiotemporal evolution process of a butane plume recorded at a rate of 500 frames per second (FPS).
Figure 2: Visualization and quantitative analysis of the temperature field
Figure 3: Visualization of various fluid physical phenomena, including the Magnus effect, thermal convection phenomena in liquids, and the spatiotemporal evolution of butane plumes
In this study, the researchers used multifunctional metasurfaces to develop a non-invasive, real-time, full two-dimensional transparent flow field visualization system. This technology can directly observe turbulence, laminar flow, and vortices in transparent fluids in small-scale experimental environments, thereby deepening the understanding of these physical phenomena. In addition, this method is expected to show broad application potential in industrial design and visual inspection fields.
Paper Information
Fan, Q., Lin, P., Tan, L. et al. Non-invasive and fully two-dimensional quantitative visualization of transparent flow fields enabled by photonic spin-decoupled metasurfaces. Light Sci Appl 14, 113 (2025).
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