Simulation and experimental study on the dynamic characteristics of active droplets colliding with and wetting fine coal dust particles

To investigate the dynamic characteristics and evolution of active droplets colliding with fine coal dust particles, hydrophobic Changzhi anthracite was taken as the research object, and non-smooth spherical particle models with different fractal characteristics were constructed based on measured surface roughness. A combined research methodology incorporating active droplet wettability testing, scanning electron microscopy (SEM) analysis of coal samples, active droplet–coal dust particle collision experiments, and COMSOL simulation was employed to systematically investigate the dynamic wetting behavior and collision mechanism of active droplets on coal dust particle surfaces. The experimental results show that among sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), secondary alkyl sulfonate (SAS), and KuaiShen T solution, the KuaiShen T droplet with a mass concentration of 0.03% exhibits the optimal wettability, with both surface tension and contact angle reduced to the lowest values. Compared with raw coal samples, coal samples treated with active solutions show a significant decrease in surface height difference. In particular, the coal surface modified by KuaiShen T solution displays the most substantial variation, forming numerous vertical through-pore structures that increase the retention area of droplets. Collision experiments between active droplets and particles reveal that the droplet-particle size ratio and relative collision velocity are the dominant parameters affecting the collision morphology of droplets on coal dust surfaces. Effective wrapping of particles by droplets is achieved at a size ratio of 1.5 and collision velocities ranging from 2.5 to 3.0 m/s. Numerical simulation results indicate a strong correlation between coal dust surface roughness and droplet spreading behavior. High surface roughness tends to cause stress concentration and increase the risk of droplet breakup, whereas the reduced roughness after contact with active droplets facilitates stable droplet spreading. Increasing the collision velocity compensates for insufficient droplet kinetic energy and alleviates the formation of an air film at the top interface caused by the strong hydrophobicity of raw coal. At a collision velocity of 2.7 m/s, active droplets rapidly fill the surface microstructures via high-speed impact, forming a smooth liquid-solid transition layer and ultimately achieving complete spreading.