| Droplet impact phenomena appear in many industrial technologies,such as molten droplet deposition manufacture,binder-powder based 3D printing,and thermal spraying.Solid-liquid-air triple-phase flow and solid-liquid phase change usually occur in droplet impacting process,which make the droplet impacting process a complex physics process involving multiple factor effect.Affected by surface wettability and heat transfer,some local phenomena may occur during droplet impacting,which could have a great effect on related applications.For example,particles adhering to the droplet's surface could change the droplet surface properties,and air entrapment at the droplet's bottom surface would lead to defects in 3D printing.Therefore,understanding the droplet impact phenomena is significant for improving related industry applications.In this paper,the droplet impact dynamics was investigated numerically and experimentally.The lattice Boltzmann method(LBM),a mesoscopic method,was adopted to simulate the droplet dynamics after impacting.Effects of substrate wettability,particle wettability,particle size distribution,surrounding air and substrate roughness on the droplet impact dynamics were studied.The detailed contents are given as follows:1.Calculation stability and verification of multi-phase lattice Boltzmann model.By analyzing the time evolution of the maximum density in the computation domain during LBM calculation,a novel piecewise relaxation time was proposed to overcome the numerical instability at low liquid viscosities during the initial period of LBM calculation.Using the scheme of piecewise relaxation time,the phenomenon of a water droplet with a low viscosity(i.e.,at a low Oh number)impacting on hydrophilic hydrophobic surfaces were simulated.Simulation results match well with Scheller and Bousfield's empirical model and Pasandideh-fared et al.'s theoretical model.Simulation results also show droplet breakup behaviors at large We numbers.A map in terms of We number versus Oh number is obtained by LBM simulation for predicting the breakup occurrence.2.Lattice Boltzmann simulation of a droplet impacting on a layer of solid particles above a horizontal substrate.An isothermal solid-liquid-air triple-phase lattice Boltzmann method was developed combining a multi-component model and a particle motion model.Using this model,a droplet impacting on a layer of particles placed on the top of a horizontal substrate was first simulated.Numerical results show that hydrophilic and hydrophobic particles are transferred from the substrate to the droplet surface after impacting,which is driven by its adhesive force.Hydrophilic particles are more possible to adhere on droplet's surface than hydrophobic particle owning to its stronger adhesive force.Adhering particles which are hydrophobic hinder the droplet spreading motion.Hydrophilic particles attached to the droplet surface can weaken droplet oscillation motion,while hydrophobic particles attached to the droplet can enlarge droplet oscillation motion on a hydrophilic substrate or induce droplet's bouncing motion on a hydrophobic substrate.Droplet's maximum spread factors after impacting on hydrophilic particles are larger than those after impacting on hydrophobic particles on the same substrate at low We numbers(i.e.,We < 10),and differences of maximum spread factors between hydrophilic and hydrophobic particles become negligible at high We numbers(i.e.,We > 10).3.Experimental investigation of droplet dynamics after impacting on hydrophilic powder beds composing of uniform size and non-uniform size particles.Previous experiments of droplet impacting on hydrophilic powder beds ignored the effect of particle size distribution.In this paper an experimental investigation was carried out to study the dynamics of a water droplet after impacting on two types of hydrophilic Cu powder beds,composing of uniform size and non-uniform size particles,respectively.The contact angle of Cu powders is ranging from 54.4° to 75.1°.Experimental results show that a droplet can infiltrate into a hydrophilic Cu powder bed composing non-uniform size particles and its penetration time is independent of the We number.However,the droplet cannot infiltrate into a hydrophilic Cu powder bed composing of uniform size particles.Instead,the droplet bounces from the uniform powder bed owing to the non-stick particle-coated surface and then forms a liquid marble.The maximum spread factor on Cu powder beds are smaller than which of a Cu substrate.Relationships of maximum spread factors with We numbers are obtained.It is also found that a powder bed,composing of uniform size small Cu particles(with an average diameter of 2.3 ?m),behaves almost like a “super-hydrophobic” substrate,where the droplet can repeatedly jump after impact.The droplet contact time after impacting on this powder bed is equal to the first-order oscillation period of the droplet.4.Numerical investigation of air entrapment in a molten droplet impacting and solidifying on a cold smooth substrate.A novel multi-component solidification lattice Boltzmann method was proposed for simulating a molten droplet impacting and solidifying on a cold smooth substrate surrounded by air.A small pocket of air entrapped in the molten droplet adjacent to the cold surface after impacting was first simulated.Simulation results were in good agreement with Bhola and Chandra's theoretical model and Bouwhuis et al.'s empirical model.It is demonstrated that the no-slip velocity of the air on the solid surface prevents the air being squeezed out completely,and the air gap between the droplet and the substrate is compressed by the falling droplet.The compressed air makes the first contact away from the impact center and an air film is trapped within the droplet.This trapped air pocket eventually forms multiple air bubbles or a single air bubble depending on the surface wettability.5.Numerical investigation of droplet wetting behaviors and a molten droplet impacting and solidifying on a rough substrate.The roughness structure is considered as an array of micro-pillars on the top of the substrate.It is confirmed that droplets on a rough surface can exist in three states: hemi-wicking state,Wenzel state and Cassie-Baxter state.Simulated static contact angles of droplets on rough surfaces match well with theoretical predictions.A molten droplet impacting and solidifying on a cold rough substrate was studied numerically.Simulation results show that the air between the droplet and micro-pillars is compressed by the falling droplet,and the compressed air is pushed into grooves of the surface microstructure after impact.As a result,no air bubble is formed under the droplet bottom surface.Roughness has a small effect on the droplet spreading motion.The oscillation motion of the droplet on a rough surface is stronger than that on a smooth surface at the same wettability.Two rough substrates with wetting and non-wetting interlaced wettabilities are presented for different applications,which could artificially control morphologies of solidified droplet's bottom surfaces. |
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