Collision behaviour of a smaller particle into a larger stationary droplet

•Collision of spherical particle on a supported stationary droplet was studied.•Force balance and CFD method were used to quantify forces during collision.•Partial to complete sinking of particle observed with increase in Weber number.•Interface deformation during collision was captured well by the CFD model.•Particle sinking times obtained from numerical models agreed well with experiments.

The present study investigates the collision behaviour of a smaller particle into a larger stationary droplet – a phenomenon related to many process engineering applications. Experimentally, the collision process was studied using high speed video imaging involving glass ballotini particles (diameter: 1.13 ± 0.02 mm) and a supported stationary water droplet (diameter 3.41 ± 0.01 mm) at different particle impact velocities (Weber number range: 0.2–13.5). A transition from partial to complete penetration was observed with decrease in sinking time and significant shape deformation of the droplet when Weber number was increased. Numerically, a one dimensional transient force balance approach was adopted which included contributions of six major forces during the penetration process, including: gravity, capillary, fluid drag, buoyancy, pressure and added mass. It was found that the capillary force controlled the interaction process. Recognizing the limitation of using the one dimensional model to capture the details of the collision physics especially the movement of three phase contact line (TPCL) on the particle surface, a 3D computational fluid dynamics (CFD) model was developed using the multiphase volume of fluid (VOF) method combined with the dynamic meshing technique. The CFD model was in good agreement with experimental measurements of the sinking time of the particle and overall collision dynamics including shape deformation of the droplet.

Graphical abstractComparison of transient interactions between a stationary (supported) droplet and an impacting particle at Weber number 5.3 – (a) high-speed imaging (b) 1D force balance model (c) 3D VOF-dynamic meshing CFD simulation.Figure optionsDownload full-size imageDownload as PowerPoint slide