Critical inertia for particle capture is determined by surface geometry at forward stagnation point

Aerosols are ubiquitous, and particle capture from particle-laden air as it flows past an obstacle is of widespread practical importance. Neglecting diffusion, previous work has shown that for a smooth curved surface in both Stokes flow and inviscid flow, only particles with inertia above a threshold value (quantified by the nondimensional Stokes number) collide with the surface. Here we show that the critical Stokes number decreases with increasing Reynolds number of the air flow, and the mechanism behind this threshold is the same at all finite Reynolds numbers but becomes qualitatively different in the limit of infinite Reynolds number (inviscid flow). In addition we show that in the latter case (inviscid flow) the threshold is set solely by the flow near the stagnation point, whereas at finite Reynolds numbers the threshold also depends on the flow far from the stagnation point. The threshold also depends on obstacle geometry and we show that fibers whose cross section is flattened along the flow direction have greater size selectivity than fibers with a circular cross section.