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Research On The Effect Of Droplet Size And Viscosity On The Dynamics Of Impacting On Superhydrophobic Surfaces

Posted on:2023-03-05Degree:MasterType:Thesis
Country:ChinaCandidate:Y L TaiFull Text:PDF
GTID:2530307154469294Subject:Engineering
Abstract/Summary:PDF Full Text Request
The phenomenon of droplet impacting on solid surfaces is ubiquitous in nature and in engineering science.In the past century,although the impact dynamics of droplets has been investigated by many studies,the internal physical mechanisms have not yet fully revealed.In this paper,the dynamics of droplets impinging on superhydrophobic surfaces is comprehensively investigated through a combination of numerical simulations,theory,and experiments.By modifying the existing lattice Boltzmann method model,an improved axisymmetric model is proposed for numerical simulation to study the bouncing dynamics of viscous micro-droplets with a diameter of 50 micron on superhydrophobic surfaces.It is shown that in the low-viscosity regime,the contact time of the micro-droplets remains constant within a certain range of We number(10<We<120).However,in the high-viscosity regime,the contact time increases with the impact velocity.A dimensionless contact number T≡WeRe-1/2=(ρD0ηU0321/2 is defined to describe the effect of viscosity through data fitting,meanwhile,a new time scaling law t~(D0/U0)T=(ρηD03U021/2 is deduced to characterize the contact time of the high-viscosity regime,and the simulated results agree well with the new scaling.In addition,to find out the internal physical mechanism and quantify the entire impact process,the kinetic energy,surface energy,viscous dissipation energy and velocity field during the impact process are analyzed.The results show that the viscous dissipation is not negligible even for relatively low-viscosity liquids.The bouncing dynamics of viscous macro-droplets with a diameter of about 2.2millimeter on superhydrophobic surfaces were investigated experimentally and numerically.It is shown that the contact number T can be used to characterize both the maximum spreading factor and the contact time of macroscopic droplets with different viscosities.Using energy conservation and contact number T,a new theoretical model for predicting the maximum spreading factor of macroscopic droplets with different viscosities is proposed.The predictions of the model agree with the experimental and simulated data.In addition,Liquid viscosity has a significant effect on the impact process,leading to an increase in the contact time,and it is found that the contact time scaling lawτof micro-droplets is equally applicable to macro-droplets.The experimental observations have shown that even on superhydrophobic surfaces,droplets only rebound under certain condition,thus,two critical transition lines are defined in the We-Oh plane that determine whether the droplets can rebound.Present simulations show that when the viscosity of the droplet is high(i.e.,Oh≥0.071),the viscous dissipation plays an important role in the droplet impact process;in the process of droplet impact,the spatial distribution of internal kinetic energy and viscous dissipation rate is not uniform,and the velocity field appears a symmetrical clockwise or counterclockwise vortex.
Keywords/Search Tags:Droplet impact, Superhydrophobic surface, Contact time, Spreading factor, Viscous dissipation, Lattice Boltzmann method
PDF Full Text Request
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