Towards a physical understanding of bubble–particle collisions in turbulence

The energy transition to mitigate global warming implies that the demand for minerals is expected to rise significantly. Many common minerals are separated from their ores through froth flotation, in which bubble–particle collisions in turbulence play a central role. This work systematically investigates bubble–particle collisions in statistically stationary homogeneous isotropic turbulence to reveal the underlying collision mechanisms and improve our predictive capabilities of the collision rate.

When gravity is negligible, point-bubble and point-particle simulations show that stronger turbulence leads to a net increase in the overall collision rate. The underlying mechanisms, namely spatial segregation and the ‘turnstile mechanisms’, are identified and explored. For the simulated parameters, the collision rate is usually overpredicted by the existing models.

Including gravity generally increases the collision rate by reducing segregation and increasing the collision velocity. Surprisingly, the addition of turbulence does not always increase the collision rate. This is due to bubble–particle segregation and nonlinear drag effects on the bubble. This peculiarity, and more generally the collision rate, are not captured well by the existing models. An existing particle–particle collision model is extended to the bubble–particle case. This extended model captures the simulated collision velocity excellently when particle inertia is weak.

Finite-size bubbles are subsequently considered, and a model that predicts the collision rate for a wide range of particle inertia is developed. The model predictions agree well with results from simulations of finite-size bubbles and point-particles in turbulence when the flow field near the bubble can be assumed as steady during a collision.

Finally, the first step towards the ultimate test of experimentally studying bubble–particle collisions in turbulence is taken. To this end, a versatile 3D-printed droplet generator that produces curable droplets is designed. These cured droplets can then be injected into turbulence facilities to study bubble–particle collisions.