The evaporation of a hexane lens on a distilled water surface was experimentally and theoretically studied. The formation of the hexane lens was recorded by a high-speed camera from the side to observe the variations of the contact diameters and contact angles. The experimental results showed that the shape variation of the hexane lens experienced the spreading stage and the evaporation stage. The spreading stage lasted for about 6% of the lens lifetime. For most time of the evaporation stage, the square of the lens contact radius decreased linearly with time, while the contact angle remained almost unchanged. During the final rapid evaporation stage (about 2% of the lens lifetime), the shape of the hexane lens changed and the lens shrank rapidly until it disappeared. A theoretical model based on diffusion-controlled evaporation under the constant contact angle mode was developed to describe the evaporation of the hexane lens on the water surface. In terms of geometry, the model assumes that a lens is composed of upper and lower spherical caps, and the apparent contact angle is defined based on the intersection of the two caps. The results calculated using the model were found to be in good agreement with the experimental data. Finally, the effects of initial lens volume, water temperature, and water surface deformation on lens evaporation were discussed through calculations. The results showed that increase in the water temperature and deformation of the water surface accelerated the evaporation process.

The evaporation of hexane lenses on an ionic liquid (IL) (1-butyl-3-methylimidazolium hexafluorophosphate) surface is studied. The difference between the evaporation processes of the lens on the IL surface and on a distilled water (DW) surface with the same substrate liquid depth (2.6 mm) is primarily analyzed. The variation of the lens contact diameter D C and the deformation of the IL surface were experimentally observed. The results indicated that the spreading stage of a hexane lens was notably shorter in duration on the IL surface than on the DW surface. A hexane lens was pseudopartially wetted on the DW surface, and the plane position of the lens contact diameter remained level with the water surface throughout the evaporation process. In comparison, a hexane lens was partially wetted on the IL surface, and the plane position of the lens contact diameter was lower than the horizontal surface until the lens evaporated completely. The hexane lens evaporation on the IL surface was calculated by using the diffusion-controlled evaporation model under the constant contact angle mode. The calculated results agreed well with the experimental measurements. Finally, the evaporation of hexane lenses on the DW and the IL surfaces was compared through calculations. Although the maximum lens contact diameter on the DW surface was greater, it took a longer time for the lens to evaporate on the DW surface. This is because the more significant bending of the substrate liquid surface accelerated the lens evaporation. The results of this study offer a new approach for controlling droplet evaporation.

A theoretical model was established to predict the morphology evolution of a volatile liquid lens evaporation on another immiscible liquid substrate surface. The theoretical model considered the dynamic process of contact line motion. On the basis of the boundary conditions established at the contact line, the morphology change of the liquid lens was calculated by numerically solving the Young–Laplace differential equations for the three interfaces. The mass evaporation rate was calculated by the diffusion-controlled evaporation model. Then, an experimental system was established to record the process of a hexane lens evaporation on the surface of an ionic liquid with a depth of 4 mm. The calculated hexane lens radius variation matches well with the experimental measurements, which shows the rationality of the present model. The calculated results show that the evaporation pattern of the liquid lens follows the constant contact-angle evaporation mode for ∼70% of the lifetime. During the later stage of evaporation, the contact angle decreases, accompanied by contraction of the contact line, which is similar to the mixed evaporation mode in the later stage of sessile droplet evaporation on a solid substrate surface. Furthermore, the influences of the initial hexane lens volume and the ionic liquid temperature on the dynamic contact angle were theoretically summarized. This study helps to provide in-depth insights into regulating the lens evaporation process on another immiscible liquid substrate surface to control the particle deposition mode.

No abstract

No abstract