The formation of liquid films on solid wall surfaces is a common phenomenon in engineering applications, directly affecting spectral radiation characteristics. When liquid adheres to a surface, a film or a cluster of droplets forms due to factors like surface tension and gravity, resulting in complex morphologies that are not accurately represented by simplified shapes such as circular arcs. The presence of these films significantly influences infrared target detection, optical windows, stealth coatings, solar photovoltaic panels, and porous structures used in solar interface evaporation by altering spectral radiation transmission and properties, thus affecting infrared signal transmission and energy conversion processes. Current studies focus primarily on the effects of liquid droplets, with analyses often limited to factors like droplet size, distribution, and surface properties. These studies typically employ large-scale parameters and geometric optics approximations. However, for thin liquid films at micro- and nanoscale thicknesses, there is limited in-depth research on their spectral radiation transmission and properties. In this paper, we aim to establish accurate models of surfaces covered with liquid films and investigate the spectral reflection characteristics of such surfaces at micro- and nanoscale levels.

We employ a theoretical liquid film profile derived from the Young-Laplace equation to construct microgroove structures with liquid films of varying thicknesses. The finite-difference time-domain (FDTD) method is used to simulate radiation transmission and assess the effects of liquid film thickness, groove width, angle of incidence, and wetting angle on the spectral reflection properties. The analysis covers the reflectance and electric field distribution across different wavelengths, providing insights into the spectral reflection patterns of surfaces with liquid films.

Simulations indicate that theoretical profiles of liquid films differ significantly from circular approximations, with reflectance differences reaching up to 7% in certain wavelength ranges. Compared to surfaces without liquid films, the presence of a liquid film on groove walls notably reduces the overall reflectance, with prominent local absorption peaks. At a wavelength of 3 μm, the reflectance difference reaches a maximum of 0.85 (Fig. 6). The influences of liquid film thickness, groove width, and wetting angle are especially pronounced for wavelengths greater than 4 μm, where reflectance curves exhibit multiple peaks and valleys. As the liquid film thickness, groove width, or wetting angle increases, these peaks shift toward longer wavelengths (Fig. 9). For instance, when the liquid film thickness increases from 1.5 μm to 2.0 μm (with a wetting angle of 60°), the average reflectance decreases from 0.895 to 0.815, representing an 8.94% decrease. In addition, reflectance generally decreases as the wetting angle increases, with no significant change observed beyond a wetting angle of 90° (Fig. 15). Increasing the angle of incidence further amplifies the variations in reflectance across different wavelengths (Fig. 11). Electromagnetic field intensity distributions at a wavelength of 3 μm demonstrate a distinct boundary at the liquid film curve, with lower intensity within the film and a notable enhancement near the sharp edges of the metal groove walls, suggesting potential plasmon resonance effects. Under inclined incidence, the electromagnetic field distribution becomes asymmetrical, though similar patterns are observed (Fig. 8 and Fig.13). The directional reflectance distribution indicates that, under normal incidence, reflected energy is mainly concentrated within a range of -40° to 40°. The width of this region and the peak reflectance within it are influenced by factors such as groove width. For example, at a wavelength of 2 μm, increasing the groove width by 2 μm results in a 0.002 increase in peak reflectance. Under inclined incidence, this central region shifts (Fig. 7 and Fig. 12).

Using the Young-Laplace equation to determine liquid film morphology, we establish a spectral radiation transmission model for copper microgrooves containing water films. The FDTD method is applied to simulate the effects of various parameters such as liquid film thickness, groove width, angle of incidence, and wetting angle on the spectral reflection properties of microgrooves covered with liquid films. The results show that liquid films significantly reduce the reflectance of microgroove structures, with a major absorption peak observed at a wavelength of 3 μm. For accurate spectral reflection analysis, the shape of the liquid film should not be simplified to an ideal circular arc. Compared to groove width and angle of incidence, liquid film thickness and wetting angle cause more pronounced variations in spectral reflectance. Reflectance generally decreases as the wetting angle increases. Under normal incidence, reflected energy consistently concentrates within the -40° to 40° range.