Fresh water resources, on which humans depend, are becoming scarce. Solar desalination technology has advantages such as sustainability, low cost, and environmental friendliness, thus meeting the requirements of China sustainable development path. Solar desalination technology based on interfacial evaporators has become an active area of research. Aluminum-based metals are widely used in interfacial evaporators because of their low weight, corrosion resistance, good thermal conductivity, and excellent processability. However, the surface of aluminum has a solar reflectivity of 60%?90%, which is unfavorable for energy absorption, and traditional processing techniques have difficulty in efficiently preparing antireflective microstructures on the aluminum surface. Ultrafast lasers can be used to process aluminum-based interfacial evaporators. In view of this, this paper proposes picosecond laser processing of superwetting aluminum-based interfacial evaporators. First, targeting the processing of microstructures, the corresponding relationship between the laser process parameters and the characteristic parameters of the microstructures (points, lines, and surfaces) is established. Subsequently, based on simulated solar desalination experiments, the influence law of the characteristic size and distribution characteristics of microstructures on the desalination performance of the evaporators is revealed, thus providing a theoretical basis and technical preparation approach for the industrial application of interfacial evaporators.

The picosecond laser processing system used in the experiment (Fig. 1) consists of a picosecond laser, a 3D scanning galvanometer, and a four-axis motion platform. The wavelength of the picosecond laser is 1064 nm, the adjustment range of the repetition frequency is 10?1000 kHz, the laser pulse width is less than 15 ps, and the diameter of the focused spot is approximately 50 μm. The laser beam is generated by the picosecond laser, and after passing through the collimation system (to keep the light rays parallel), the light shutter (a switch to pass or block the laser pulse), and the attenuator (composed of a half-wave plate and a linear polarizer to control the laser energy), it enters the galvanometer scanning system and is focused on the sample surface through the flat field focusing lens (with a focal length of 326 mm). The sample has a thickness of 2 mm, and length and width of 20 mm. The sample is ultrasonically cleaned with absolute ethanol for 10 min before and after processing. After laser processing, geometric parameters such as the depth and width of the microstructures on the aluminum surface are observed using a confocal laser microscope, and the microscopic morphology of the laser-processed aluminum surface is observed using a scanning electron microscope. A xenon light source is used to simulate sunlight, and a solar desalination simulation experiment is conducted (Fig. 2). The mass change of the evaporating dish is measured every 10 min using a precision balance to determine the evaporation efficiency.

The diameter and depth of the point structure gradually increase with the increase in the single-pulse energy of the laser (Fig. 3); as the number of pulses increases, the diameter of the point structure first increases and then decreases, and the depth gradually increases (Fig. 5). With the increase in the single-pulse energy of the laser, the width of the line structure first increases and then decreases, and the depth gradually increases; with the increase in scanning speed, the width of the line structure first increases and then decreases, and the depth gradually decreases. The fewer the number of scans, the larger is the width of the line structure, and with the increase in the number of scans, the width of the line structure shows a gradually decreasing trend (Fig. 7). The ranges of the process parameters of the picosecond laser processing for the line structure are as follows: single-pulse energy of the laser, 322.64?465.65 μJ; scanning speed, 100?200 mm/s; and number of scans, 5?20. For the surface stepped structure, when the scanning interval is 80?120 μm, a stepped structure with obvious contour features can be obtained (Fig. 9). For the point array structured evaporator, when the scanning interval is 150 μm and the depth is 45 μm, the maximum evaporation rate of the evaporator reaches 6.63 kg·m^-2·h^-1 (Fig. 11 and Fig. 12). For the line array structured evaporator, when the scanning interval is 120 μm and the depth is 35 μm, the water production within 1 h of the evaporator can reach 6.84 kg·m^-2 (Fig. 13 and Fig. 14). For the stepped structured evaporator, the evaporation rate first increases and then decreases with the increase in step width. The stepped structure with a and b of 720 μm and c of 240 μm has a maximum evaporation rate of 7.56 kg·m^-2·h^-1, achieving the best performance (Fig. 15 and Fig. 16). The desalination rate of the stepped structured evaporator in a real environment can reach 2.325 kg·m^-2·h^-1, and the mass concentration of ions in the desalinated seawater is reduced by more than three orders of magnitude, meeting the requirements of the World Health Organization for drinking water (Fig. 18).

This paper proposes a picosecond-laser processing of superwetting aluminum-based interfacial evaporators. First, targeting the processing of microstructures, processing experiments of point, line, and surface structures are conducted, and a correlation model between the laser process parameters and the characteristic parameters of the microstructures is established. It is found that the depth of the point structure gradually increases with the increase in single-pulse energy and number of pulses; the diameter of the point structure gradually increases with the increase in single-pulse energy, and first increases and then decreases with the increase in number of pulses. A lower scanning speed, a higher single-pulse energy, and multiple scans can obtain a line structure with a larger depth-to-width ratio. When processing the surface structure (stepped structure), the scanning interval should be selected within the range of 80?120 μm. By comparing the desalination effects of different structural interfacial evaporators and combining the efficiency of interface processing, it is found that the optimal structure is the stepped structure, and its optimized water production can reach 7.56 kg·m^-2·h^-1 when the light intensity is 0.804 W/cm². The water production in a real environment can reach 2.325 kg·m^-2·h^-1, and the mass concentration of ions in the desalinated seawater is significantly reduced, meeting the standards of the World Health Organization for drinking water. This approach is expected to be used in small and medium-sized desalination plants and portable water purification devices.