Study On The Behavior And Mechanism Of Micron Particle Impact On Liquid Surfaces

Impacts of particles on liquid surfaces are observed frequently in nature and industrial processes and are also the fundamental problem of many subjects such as fluid mechanics and particle dynamics.In this study,the dynamics and energy transformation during the impacts of micron particles on liquid surfaces were investigated.Findings could provide fundamental insights to related natural phenomena and provide guidance for related industrial technologies.Firstly,the motion behaviors and flotation conditions of spherical particles after contacting the liquid surface with zero velocity were studied numerically.The particle's motion is determined by the Bond number,density ratio and contact angle.The limit density ratios for particles to float after impacting the liquid surface were obtained,which agree well with the experimental results,and the effects of Bond number and contact angle on the particle's dynamic floatability were analyzed.The expression of the limit density ratio of small spheres was proposed based on energy balance,which showed that small particles can float after impact only when the contact angle is larger than 63.3°.Secondly,a high-speed microphotography experimental system and a numerical simulation method were established to study the impacts of hydrophobic spherical particles.Two modes were observed after PMMA particles impacting on water surfaces,namely,submergence and oscillation.According to the motion characteristics of the particle and gas–liquid interface,the impact process was divided into different stages,and the evolution of the dominant forces(hydrodynamic force and surface tension)and energy conversion mechanism during each stage were revealed.The effects of Reynolds number,density ratio and contact angle on the motion of the particle,the three phase contact line and the evolution of the dominant forces during the critical sinking process after vertical impact were analyzed.Expressions of the work done by the two forces were given,and a critical criterion for the particle to submerge was proposed.The prediction accuracy of the criterion on the experimental results is much higher than that of the existing criteria.Different from the vertical impact,the wetted part of the particle surface after oblique impact is non-axisymmetric along the motion direction,which makes the particle's trajectory deviate under the forces deviating from the motion direction,and the particle rotate under the viscous torque.The decrease of impact angle weakens the impact force exerting on the particle,and enlarges the nonaxisymmetry of particle wetting,the rotation and trajectory deviation of the particle.The effects of the Weber number and Reynolds number on the global motion behaviors were analyzed.It was found that when the Reynolds number is greater than 500,its effect can be neglected.The critical sinking velocity increases with the decrease of impact angle.A critical criterion for the particle to submerge after oblique impact was proposed based on energy balance,which predicted the experimental results accurately.Finally,the vertical impacts of the hydrophobic ellipsoidal particles were studied numerically.The increase of axis ratio decreases the surface tension and hydrodynamic force acting on the particle,and thus changes the motion behaviors of the particle and the fluids.The proposed impact mode phase diagram shows that ellipsoidal particles(without contact angle hysteresis)with axis ratio greater than 0.8 exhibit three impact modes of submergence,oscillation and rebound,while the rebound mode disappears when the axis ratio is less than 0.8.The larger the axis ratio,the smaller the critical sinking and critical rebound velocities are,and the larger the velocity range of rebound mode will be.