Contact Time And Mechanism Of A Droplet Off-center Impact Upon Superhydrophobic Macro-ridges

Inhibiting the icing of impacting droplets holds significant implications for practical engineering applications.Traditional anti-icing methods,characterized by their short duration and incomplete ice removal,have proven insufficient.In light of this,a novel approach has emerged,leveraging the rebound characteristics of impacting droplets on superhydrophobic surfaces to fundamentally solve the problem of droplet icing,which has garnered considerable attention from researchers.Reducing the contact time between droplets and surfaces represents a pivotal aspect of this innovative approach,which promotes droplets rebound and improves anti-icing efficiency.To this end,researchers have mainly focused on reducing contact time by arranging macroridges on superhydrophobic surfaces.Notably,a significant reduction in contact time is observed when droplets centered impact the ridge.However,in practical applications,off-center impact is far more common,leading to a longer contact time between droplets and surfaces.Unfortunately,research on the mechanism of off-center impact on macroridges is currently limited.A deeper understanding of this mechanism and f a scale relationship for its contact time could serve as a critical theoretical foundation for the development of superhydrophobic surfaces for anti-icing purposes.This study investigates the mechanism of a droplet off-center impact on a macroscopic rectangular ridge via the lattice Boltzmann method(LBM),with a particular focus on the impact of off-center distance on contact time.The results indicate that off-center impact significantly prolongs the contact time compared to centered impact.This increase is mainly due to the asymmetric distribution of the droplet volume caused by the off-center distance,resulting in the asymmetric spreading and retraction of the liquid film.We derive a relationship between liquid volume and off-center distance based on the mechanism of the redistribution of droplet volume,and thus establish a qualitative scaling relationship for contact time.Based on the asymmetric spreading and retraction of the liquid film,we also derive and prove in detail the correction relationship of contact time caused by the asymmetricity of the liquid film due to longest retraction length after the liquid film wets the surface.Combined the two mechanisms,i.e.,liquid volume distribution and liquid film asymmetry,our study establishes a quantitative scaling relationship between contact time and off-center distance when a droplet off-center impacts on a rectangular macroridge.Experimental and simulation results demonstrate the accuracy of our scaling relationship in predicting contact time when the droplet impacts a superhydrophobic surface at a relatively high Weber number.In addition,based on the asymmetry mechanism of liquid film in off-center impacts.this article proposes a new idea for manipulating and reducing the contact time of droplets in off-center collisions for the first time.It is achieved by tilting the macroridge to reduce the asymmetry of the liquid film and thus lower the contact time.This study also explores the influence of the inclined angle of marco-ridges θi and off-center distance Δx*on the rebound dynamics of droplets in off-center impact via LBM simulations.The results show that for off-center impacts,tilting the macro-ridge can significantly reduce the contact time.For positive off-center impacts(Δx*>0),both the asymmetry of the liquid film and the contact time decrease with increasing inclined angle of the macro-ridge.For negative off-center impacts(Δx*<0),tilting the macroridge can minimize the asymmetry of the liquid film,causing its influence on the contact time to be ignored,and the contact time is only related to the volume change of the liquid.Based on these findings,this article ultimately establishes the scaling relationship between the contact time of droplets in off-center impact a inclined macroridge.This study partially addresses the current research gaps in the field of off-center impacts of droplets and provides a reliable theoretical basis for quantitatively calculating the contact time in such impacts.These findings may have implications for the application of superhydrophobic surfaces in areas such as anti-icing and selfcleaning,providing valuable guidance for further research in this area.