Droplet impact on a layer of solid particles placed above a substrate: A 3D lattice Boltzmann study

A 3D isothermal solid-liquid-air lattice Boltzmann method (LBM) is used to study numerically a droplet impacting on a layer of particles placed on the top of a horizontal substrate. Effects of wettabilities of the particle and the substrate on particles transfer from the substrate to droplet's surface after droplet's impact are simulated. It is shown that hydrophilic particles are more easily to adhere on droplet's surface than hydrophobic particles after droplet impact because of stronger adhesive force. For droplet's subsequent spreading motion, adhering particles which are hydrophilic move in the same direction as liquid flow of the droplet. On the other hand, adhering particles which are hydrophobic move in the direction opposite to liquid flow of the droplet, which hinders droplet's spreading motion. It is found that 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 Weber numbers (i.e., We < 10), and differences of maximum spread factors between hydrophilic and hydrophobic particles become negligible at high Weber numbers (i.e., We > 10). It is also found that hydrophilic particles attached on droplet's surface can weaken droplet's oscillation motion, while hydrophobic particles attached to the droplet can enlarge droplet's oscillation motion on a hydrophilic substrate or induce droplet's bouncing motion on a hydrophobic substrate. Maximum spread factors obtained from simulation are shown to match reasonably well with existing experimental data and correlation equations, validating the accuracy of the present numerical model. (C) 2019 Published by Elsevier Ltd.