In this paper, a superhydrophobic surface was prepared by chemical etching to study the evaporation crystallization characteristics of calcium sulfate droplets on the superhydrophobic surface and the influencing factors of calcium sulfate evaporation rate. In this study, calcium sulfate droplets were placed in a temperature- and humidity-controlled chamber for evaporation crystallization experiments according to the sessile droplet method, and the evaporation process of the droplets was visualized and recorded with a CCD camera until they were completely evaporated. Through image data processing, the contact angle, solid-liquid contact diameter and other parameters were determined, and the evaporation law of the droplet was further obtained. The evaporation experiment was performed using deionized water droplets and saturated calcium sulfate droplets with an initial volume of (3±0.1) μL at three different substrate temperatures (40±0.5), (50±0.5), (60±0.5) ℃ and three different humidity (30±5)%, (45±5)%, (60±5)%, respectively. At the same time, based on temperature and relative humidity, a multivariate fitting quadratic regression model was developed to describe its effect on the evaporation rate of droplets. The results showed that when the substrate temperature was 40, 50 and 60 ℃, the contact angle of calcium sulfate droplets and pure water droplets on the surface of hydrophilic aluminum sheet decreased linearly with the change of evaporation time. The first 90% of the evaporation of pure water droplets on the surface of the hydrophilic aluminum sheet maintained the CCR (constant contact radius) mode, and the last 10% showed the mixed mode; The entire evaporation process of calcium sulfate droplets on the surface of hydrophilic aluminum sheets showed the CCR mode. On the surface of the superhydrophobic aluminum sheet, pure water droplets and calcium sulfate droplets maintained the CCA (constant contact angle) mode in the first 60% of evaporation. In the late evaporation stage, evaporation modes of calcium sulfate droplets and pure water droplets were slightly different: pure water droplets in the evaporation processed sticky slip mode; while calcium sulfate droplets in the late evaporation stage edge salt increased. Under the action of gravity and Marangoni effect, the outside gradually formed a salt shell, the contact radius showed an upward trend, and the generated salt shell was likely to remove, indicating that the superhydrophobic surface was not conducive to the nailing of salt droplets. When the evaporation rate was low, it was more likely to form a salt shell on the outside. Once the salt shell was formed outside, the evaporation mechanism changed, and the water molecules inside the droplet needed to overcome the pressure difference between the inside and outside of the salt shell and further evaporate through the diffusion of the salt shell. In this paper, it is verified that the evaporation mode of calcium sulfate droplets is not related to the substrate temperature, but to the wettability of the substrate, and the evaporation rate of the droplets increases with the decrease of relative humidity and the increase of temperature. By fitting the quadratic regression model with R2=0.993 7 and analyzing the variance of the factors affecting droplet evaporation, the results show that temperature and relative humidity on the superhydrophobic surface have significant effects on the evaporation rate of calcium sulfate droplets. The research results of this paper provide effective theoretical support for the resource utilization of mine water. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.
