Enhancing Mechanism Of Liquid Film Fast Spreading On Superhydrophilic Surfaces

Rapid spreading of liquid film,as the precondition for the formation of a thin liquid film,is the basis of the heat and mass transfer enhancement.The wetting and spreading characteristics of the thin liquid film on the superhydrophilic surface and its underlying mechanism are vital for achieving the heat and mass transfer enhancement,yet there are still challenges to control and accelerate the spreading process.The liquid film spreading process on the superhydrophilic surface can be divided into the spreading process led by inertial force and capillary force,according to its dominant force.In this thesis,experimental observation,theoretical analysis,and numerical simulation methods and technical routes are employed to investigate the spreading characteristics of liquid film on the superhydrophilic surfaces,and the enhancing strategy of the spreading distance and speed of liquid film is proposed.The main contents of this thesis are as follows:Aiming at the rapid spreading dominated by inertial force,the high-speed camera was employed to investigate the liquid film spreading characteristics on superhydrophilic surfaces.The liquid film spreading characteristics including spherical cap liquid film,high viscous force and gravitational potential energy were analyzed to modify the previous prediction model of maximum spreading diameter.A new prediction model for the maximum spreading diameter on superhydrophilic surfaces was established.Comparing the model prediction results with the experimental results,it was found that the previous abnormal trend of the maximum spreading diameter on superhydrophilic surfaces at low Weber number（We<25）was settled.The maximum spreading diameter on superhydrophilic surfaces in the full range of high and low Weber numbers（1.91<We<290.08）was better predicted,with the deviation between the model and the experiment within 4%.The analysis of the energy contribution revealed that the introduction of the gravitational potential energy and auxiliary dissipation plays an important role in accurately predicting the maximum inertial spreading diamter on superhydrophilic surfaces.On this basis,the preexisting liquid film was introduced regarding to the characteristics of high viscous force of superhydrophilic surfaces.With infrared thermal imaging technology and numerical simulation,the effect of the preexisting liquid film on the inertial spreading process was investigated.The maximum spreading diameter in the inertial spreading stage first increases and then decreases with the increase of the preexisting liquid film thickness.The spreading diameter reaches the maximum when a coherent thin liquid film is formed on the superhydrophilic surface.Compared with the superhydrophilic dry surface,the thin preexisting liquid film（100μm）significantly increases the inertial spreading diameter,increasing the spreading factor from 2.5 to 3.7.Flow field evolution inside the liquid film was simulated,and the microscopic mechanism of the increasing of the inertial spreading diameter by preexisting thin liquid film was revealed.Internal vortexs are formed during the impacted droplet spreading on the relatively thick preexisting liquid film,and further inhibits the lateral spreading,thereby reducing the maximum spreading diameter.The lateral spreading of the impacted droplet on a relatively thin preexisting liquid film is promoted,thus significantly increasing the maximum spreading diameter.Aiming at the trade-off between the capillary driving force and flow resistance on the single scale structure during capillary spreading,the hierarchical nanowired surface composed of hollow nanowire bundles and microscale V-groove was introduced,and the influence mechanism of the hollow nanowire bundles on the capillary spreading process was explored by Confocal Laser Scanning Fluorescence Microscopy.It was revealed that the preferential capillary pumping occurs in the hollow nanowire bundles.The preferential capillary pumping phenomenon demonstrated the physical mechanism of the hollow nanowire bundles providing capillary pressure for the liquid film spreading.Compared with the solid micropillar structure of hierarchical micropillared surface that hinders the spreading of the liquid film,the hollow nanowire bundles drive the liquid film flow in the V-groove.Therefore,compared with the conventional hierarchical micropillared surface,the hierarchical nanowired surface achieves twice the capillary spreading rate.Wicking coefficient of the hierarchical micropillared surface is 3.73 mm/s^0.5,while that of the hierarchical nanowired surface is up to 6.54 mm/s^0.5.On the basis of above mechanism understanding,the influence of nanowire diameter,nanowire height,and surface tension and viscosity ratio on the capillary spreading rate was further investigated.The results showed that the capillary spreading rate increases with the decrease of the nanowire diameter,and increases with the increase of the nanowire height and surface tension and viscosity ratio.Both the high-speed microscopy and the Micro-PIV experiment proved that the interconnected microscale V-groove provides a liquid film transport channel for the capillary spreading process and significantly reduces the flow resistance.Based on the balance between capillary pressure and flow resistance,a prediction model of capillary spreading rate was established.The experimental results were well predicted by the model,further proved that the microscale V-groove provides a liquid transport channel for capillary spreading.According to the enhancement of the capillary spreading rate by hierarchical nanowired surfaces,the respective roles of hollow nanowire bundles and microscale V-groove in capillary evaporation process were further investigated and a strategy of thin film evaporation enhancement was introduced.The hierarchical nanowired surfaces with hollow nanowire bundles and microscale V-groove significantly increase the thin liquid film area and enhance the evaporation rate.At wall temperature of 60℃,the evaporation rate on the hierarchical nanowired surface is 12 times of that on the hydrophilic smooth copper surface.The calculation results of the evaporation flux showed that at wall temperature of 90℃,the evaporation flux on the hollow nanowire bundles is 33.02 g/（m²·s）,while that on the microscale V-groove and the spherical cap liquid film is 3.37 g/（m²·s）and 0.35 g/（m²·s）,respectively.The microscopic observation of Environmental Scanning Electron Microscope further revealed the mechanism of the evaporation at the top of the hollow nanowire bundles and the liquid supply in the microscale V-groove on hierarchical nanowired surface.Capillary evaporation mainly occurs at the top of the hollow nanowire bundles,microscale V-grooves provide liquid supply channels,and the capillary pressure driven supply flow coupled with the thin film evaporation synergistically maintains the efficient capillary evaporation process on hierarchical nanowired surface.