Mechanism And Regulation Of Coalescence-induced Droplet Jumping On Superhydrophobic Surfaces

The surface free energy of superhydrophobic surfaces is rather low.Droplet shows a perfect spherical "pearl" and can roll easily on superhydrophobic surfaces.When two small droplets coalesce on superhydrophobic surfaces,they will jump away from the surface due to the release of excess surface free energy,which is called the coalescence-induced droplet jumping.An outstanding feature of droplet jumping is that the droplets can be removed without external force or field.The coalescence-induced droplet jumping has showed its great potential in surface self-cleaning,anti-icing,process intensification,etc.In this dissertation,the coalescence-induced droplet jumping and its effect factors were investigated by experimental test,theoretical analysis and numerical simulation.The effects of initial droplet size(droplet radius and droplet radius ratio)and droplet arrangements on droplet jumping were revealed.And the interfacial structure effects were used to regulate droplet jumping process.The inherent relations between the impact position of liquid bridge on the substrate and droplet jumping velocity were understood.A set of experiment apparatus to trigger droplet jumping was designed and time-lapse images of coalescence-induced droplet jumping on transparent superhydrophobic surfaces were took by high-speed cameras.Generally,the overall jumping process can be divided into four stages,i.e.,the growth of liquid bridge,the impact of liquid bridge,the retraction of three-phase contact line,and the jumping of coalesced droplet.The liquid bridge radius was proportional to the square root of time,whereas the dimensionless prefactor was decreased from 1.18 to 0.83 due to the transition of liquid interface curvature.As the droplet coalescence process on superhydrophobic surfaces was inertia-dominated in the tested droplet radius range,the retraction dynamics of the three-phase contact line was self-similar,i.e.,the retraction velocity of the contact line showed limited dependence on initial droplet radii.The coalesced droplet finally jumped away from the substrate with a dimensionless jumping velocity of around 0.2,which followed an inertial-capillary scaling.The evolution of droplet jumping velocities at different initial droplet radii were investigated by lattice Boltzmann method,the critical droplet radius for the transition of coalescence regime on superhydrophobic surfaces was found to be 1.3?m.If the initial droplet radius was larger than 1.3?m,the droplet jumping velocity followed an inertial-capillary scaling with a dimensionless jumping velocity of around 0.2.And conversely,the droplet jumping velocity decreased steeply due to the increased viscous effects in droplet coalescence process,the aforementioned scaling collapsed.The effects of droplet radius ratio(the radius of large droplet to that of the small one)on droplet jumping velocity were explored by experimental observation and lattice Boltzmann method.The critical droplet radius ratio for droplet jumping on superhydrophobic surfaces was found and this value was about 0.56 when the droplet coalescence process was inertia-dominated.The critical droplet radius ratio would increase if the effect of viscosity played a key role in the coalescence process.Taking the coalescence of three equal-sized droplets as an example,the effects of initial droplet arrangements on droplet jumping were studied by lattice Boltzmann method.Depending on the initial position of droplets on the surface,the droplet coalescence behaviors were classified into two types:concentrated configurations and spaced configurations.When the droplet coalescence process on superhydrophobic surfaces was dominated by inertia,the jumping velocity decreased with increasing distribution angle firstly,and then it increased with distribution angle.As the droplet coalescence process was dominated by viscosity,the droplet jumping velocities no longer depend strongly on initial droplet arrangements.It was observed that spaced multidroplet coalescence results in drastically reduced jumping velocities compared with that for concentrated multidroplet coalescence,which showed the negative effect on jumping velocity for spaced configurations.Starting from the interfacial structure effect,the effects of liquid bridge impact on droplet jumping velocity were investigated by model analysis,lattice Boltzmann method and experimental observation.For droplets coalesce on superhydrophobic surfaces with raised structures,the liquid mass was redistributed and the retraction of droplet base areas was accelerated with the assistance of raised structures.The initial kinetic energy conversion efficiency was increased leading to a larger jumping velocity.For droplets coalesce on superhydrophobic surfaces with grooves,the impact of liquid bridge on the substrate was delayed and the growth of liquid bridge was extended due to large gaps between structures.The initial kinetic energy conversion efficiency was decreased leading to a smaller jumping velocity.Taking triangular prism as an example,the effect of structural parameters of triangular prism on droplet jumping velocity was studied.The results indicated that droplet jumping can be dramatically enhanced by exploiting structure effects,which was a promising reinforcement method.The initial kinetic energy conversion efficiency was determined by the apex angle and the height of triangular prism,which increased with the height of triangular prism for a fixed apex angle.Also,the initial kinetic energy conversion efficiency decreased with increasing apex angle for a fixed height.