Droplet Impact onto a Spherical Particle in Mid-Air

Collision between a droplet and a particle has a wide range of applications in chemical and petrochemical industries, polyethylene synthesis, and particle coating. Various studies in the literature indicate that the collision products are very different depending on the size and velocity of the particle and droplet, particle wettability and roughness, and physical properties of the liquid and the surrounding gas. The collision outcome is a liquid film (i.e. lamella) and the objective of this thesis is to identify various impact products in different conditions and to study how each category of the above mentioned parameters or a combination of them affect the lamella formation. Investigation of the droplet impact was divided into two parts: drop impact onto a still particle, and droplet impact onto a moving particle in mid-air. Contribution of this thesis to the field can be summarized as following. First, studying the impact phenomenon in a wider range of both Weber number (0.1<We<1146) and droplet-to-particle diameter ratio (1.4<Dr<5.0) compared to what already exists in the literature. Both experimental and numerical tools were developed and used to study the head-on impact between a droplet and a particle. Second, studying the effect of impact velocity, particle wettability, and the amount that each of these parameter contributes on collision outcomes. The required conditions for a lamella to be formed was also studied, and how the lamella geometry changes in case the impact velocity is changed, or hydrophilic/hydrophobic types of particles are used. Third, investigation of the effect of liquid viscosity on lamella formation; what the dynamics of the liquid is inside the film, and how the fluid field inside the lamella is affected by the viscosity changes. Fourth, identifying the role of ambient gas in lamella formation and how each of the drag and lift forces contribute in creating the liquid film. Fifth, developing a pneumatic droplet generator capable of producing single drops with various droplet sizes. The breakup phenomenon in the nozzle and droplet velocity upon pinch-off were also investigated in detail.