Evaporation And Deposition Dynamics Research Of Binary Aqueous Droplets On The Superhydrophobic Surfaces

In this paper,the experimental exploration,theoretical analysis and numerical simulation were used to investigate the evaporating dynamics of binary colloidal aqueous droplets on superhydrophobic surfaces at different ethanol concentrations.The temporal evolutions of different parameters,including the contact radii between the liquid droplets and the surface,the contact angles and the evaporation rates,are analyzed and compared with existing theoretical models.The effects of ethanol concentration and relative humidity on the evaporation dynamics are illustrated.It is shown that the evaporation modes and evaporation rates are greatly affected by ethanol concentration.Droplets with lower ethanol concentrations tend to evaporate at a low rate,staying in the constant contact angle mode nearly in the whole process,while those with higher ethanol concentrations evaporate faster and take the constant contact radius mode.The evaporation rate decreases significantly when the relative humidity is increased and vice versa.However,the relative humidity has little impact on the evaporation mode.Due to the significant difference between the volatility of ethanol and water,the analysis of evaporation rate is an important way to explore the evaporation of components from binary aqueous droplets at different stages.it was found that the main evaporation component is ethanol in the early stage,while the main evaporation component is water in late period by using the existing theoretical models to fit the experimental data.The dynamics behavior of colloidal microsphere deposition is the main object of this dissertation.The deposition of colloidal microspheres on superhydrophobic surface was characterized by three-dimensional confocal microscope system and scanning electron microscope,and the deposition process of colloidal microspheres was photographed by color camera.The results show that most of the colloidal microspheres will float on top of the water droplets at the end of evaporation and eventually deposit in the form of a two-dimensional cake-like structure.At low ethanol concentration,liquid marble will be formed and deposited into a three-dimensional hemispherical structure.An increase in the concentration of ethanol is deposited as a ring-like structure similar to coffee stains,and the higher the concentration of ethanol,the larger the ring size.In addition,the evaporation flux distribution between the gas and liquid interfaces of different droplets under different morphologies was investigated by numerical simulation: if the droplet contact angle is higher than 90°,the evaporation flux will be larger at the top of the droplet and smaller at the three phase contact line;if the droplet contact angle is equal to 90°,the evaporation flux will be evenly distributed;if the droplet contact angle is lower than 90°,the evaporation flux at the top of the droplet will be significantly smaller than that at the three phase contact line.