The likelihood of a particle in a fluid stream striking a solid obstacle depends on the Stokes Number, Stk, which is the ratio of the characteristic time needed for the particle to change its velocity to that needed for it to pass the obstacle. If Stk>>1, then particles will strike the obstacles, while Stk<<1 means that most particles are expected to pass around them within the fluid stream. Derivation of the expression for the Stokes number is provided here.
It can be seen from the expression for Stk that particle size is important, with finer particles more likely to be carried around an obstacle without striking it, while coarser (and more dense) particles cannot change their velocity so quickly and are likely to carry on in a straight line, so that impact occurs. Fluid velocity and viscosity are also relevant, with high velocity and low viscosity also favouring impact. These effects can be quantitatively explored in the simulation shown below - click on "start" to inject the particles, after setting the parameter values. (Note that the substrate (obstacle) size in this simulation is fixed at the rather low value of 0.3 mm, and the velocity range is only up to 1 m/s: in the next page, this formulation is used in a simulation of thermal spraying, in which a high velocity air stream carries particles towards a substrate.)