  Bubble-particle collisions in turbulence are central to a variety of
processes such as froth flotation. Despite their importance, details of the
collision process have not received much attention yet. This is compounded by
the sometimes counter-intuitive behaviour of bubbles and particles in
turbulence, as exemplified by the fact that they segregate in space. Although
bubble-particle relative behaviour is fundamentally different from that of
identical particles, the existing theoretical models are nearly all extensions
of theories for particle-particle collisions in turbulence. The adequacy of
these theories has yet to be assessed as appropriate data remain scarce to
date. In this investigation, we study the geometric collision rate by means of
direct numerical simulations of bubble-particle collisions in homogeneous
isotropic turbulence using the point-particle approach over a range of the
relevant parameters, including the Stokes and Reynolds numbers. We analyse the
spatial distribution of bubble and particles, and quantify to what extent their
segregation reduces the collision rate. This effect is countered by increased
approach velocities for bubble-particle compared to monodisperse pairs, which
we relate to the difference in how bubbles and particles respond to fluid
accelerations. We found that in the investigated parameter range, these
collision statistics are not altered significantly by the inclusion of a lift
force or different drag parametrisations, or when assuming infinite particle
density. Furthermore, we critically examine existing models and discuss
inconsistencies therein that contribute to the discrepancy.
