| Droplet impact on solid surfaces is a ubiquitous phenomenon in nature and industry field.The droplet dynamics as well as heat and mass transfer characteristics involved durnng droplet impact have significant effect on specific applications.For inkjet printing or spray cooling technology,it is desirable to enhance droplet deposition or heat transfer effectiveness between impacting droplets and underlying surfaces.However,when design water-repellent and insulating or anti-icing surfaces,it is often necessary to limit the contact and heat exchange between droplets and surfaces.It was reported that an impacting droplet can bounce off a surface which has good hydrophobicity,after spreading and retraction stages.For droplet bouncing on hydrophobic surfaces,short contact time and small contact area between droplets and surfaces endow it important application prospects in controlling droplet contact and heat transfer.Since heat and mass transfer mainly occurs during the contact period,the suppression of heat transfer can be achieved by shortening the contact time or regulating interfacial heat transfer rate.However,the effects of surface structure,droplet size and impact velocity on the contact time and corresponding heat transfer process need to be revealed.Therefore,this study performs basic research on the contact time between droplets and hydrophobic surfaces as well as heat transfer mechanisms,including the following aspects.Based on the idea of reducing droplet contact time through asymmetric bouncing,the contact time variations and inherent mechanisms for droplet impact on different scale cylindrical superhydrophobic surfaces were studied.A series of cylindrical superhydrophobic surfaces from submillimeter to millimeter scales were fabricated by Wire Electrical Discharge Machining(WEDM)and chemical etching.Droplet impact dynamics and corresponding contact time on the surfaces were analyzed through high-speed imaging.It was found that the contact time of conventional bouncing droplets on a superhydrophobic flat surface with micro-nano structures was almost constant durnng the experiments.The contact time between impacting droplets and the cylindrical surfaces with different diameters was reduced compared to the conventional bouncing,spanning several bouncing regimes.The contact time was generally shortest for droplets impacting a cylindrical surface with a comparable diameter.A theoretical model based on the retraction speed of droplets was established.It is verified that the contact time generally decreases with the cylinder diameter and the Weber number when the cylinders are smaller than the droplets.Besides,the contact time is shorter if droplets impact cylinders with smaller diameters or at larger Weber numbers,when the cylinders are larger.The contact time reduction mechanism for droplet impact on different scale cylindrical superhydrophobic surfaces is revealed theoretically.That is reducing the liquid film thickness above the cylinders to accelerate droplet retraction and reduce the retraction time.Inspired by the anisotropic wettability of rice-leaves,sub-millimeter macrostructure grooved superhydrophobic surfaces were designed and fabricated to study droplet bouncing types and contact time variations.By changing the Weber number of droplets,a rapid detachment phenomenon named"petal bouncing" appeared on the grooved superhydrophobic surfaces in droplet impact experiments.Compared to the conventional bouncing on a superhydrophobic flat surface,the contact time was reduced by-70%.Theoretical model proves that it is attributed to the interfacial energy stored during the downward penetration of impacting droplets.In the subsequent capillary emptying process,the interfacial energy partially transforms into kinetic energy,lifting the droplets rapidly.On the superhydrophobic surface with a lower groove depth,the impacting droplets touch the bottom of the grooves.Due to the Laplace force,the droplets turn into distinct wing-like liquid branches with vertical velocity components along the grooves.It further promotes the upward motion of the droplets and shortens the contact time.Impacting droplets with higher velocities break into smaller and more uniform child droplets on the grooved superhydrophobic surfaces.This shortens the overall contact time,extending the range of the Weber number for contact time reduction.Heat transfer process for conventional symmetric droplet bouncing was studied theoretically and experimentally,by regulating the solid/gas components and hydrophobicity of micro-pillared surfaces.Hydrophobic surfaces with different cavity fractions were fabricated by using photolithography and coating techniques.Droplet impact heat transfer experiments were performed on the heated surfaces.Temperature distributions and variations of impacting droplets were analyzed by high-speed infrared imaging,to calculate the temperature increase of the droplets and further estimate the experimental values of the dimensionless cooling effectiveness.Similarity solution was used to theoretically model the instantaneous heat flux at the interface.The total heat absorbed by bouncing droplets was calculated to obtain the theoretical values of cooling effectiveness,and compared with the experimental ones.The convection of fluid inside impacting droplets is considered in the model,which enhances the interfacial heat transfer.Results show that a surface with a higher cavity fraction is more hydrophobic,the droplet contact time is shorter and the interfacial heat flux is also weaker on the surface.Theoretical and experimental cooling effectiveness of droplets increases with the Weber number,but decreases with droplet diameter and surface cavity fraction.To reveal the transient heat transfer to bouncing droplets on hydrophobic surfaces,the interfacial heat transfer process during droplet contact period on hydrophobic foil surfaces was experimentally studied.Thin aluminum foils were coated to make the surfaces hydrophobic,from which impacting droplets can completely detach.Smooth hydrophilic surfaces were also used for comparison.Droplet impact dynamics and the variations of substrate temperature were recorded and analyzed.The heat transfer principle of a thin foil was analyzed in the cylindrical coordinate system.Finite difference method was applied to calculate the interfacial heat flux and total heat transfer.The effects of droplet impact velocity and surface wettability were also studied.Experimental results show that the interfacial heat flux is the highest near the three-phase contact line,and it decreases with time.During the retraction stage of droplets on the hydrophobic surface,the positions of the maximum heat flux gradually return toward the central region,while the highest heat flux always appears at the maximum spreading diameter on the hydrophilic surface.The Weber number and surface wettability do not significantly change the local heat flux at the interface,but they change the spreading and retracting speed of the impacting droplets as well as the solid/liquid contact area,thereby affecting the heat transfer between droplets and the underlying surfaces.Through the above research,the effects of surface structure and wettability as well as droplet diameter and velocity on the contact time and heat transfer characteristics for droplet impact are revealed.This provides a theoretical and experimental basis for the design of hydrophobic surfaces to control droplet contact and heat transfer. |
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