Hypersonic impaction of ultrafine particles

The performance of a highly supersonic aerosol impactor as a size-discriminating instrument is explored experimentally with ultrafine particles having diameters as small as 51 Å. A hypersonic jet is formed by expansion of a gas-particle mixture through an orifice of diameter d_n, from a source region maintained at a pressure p_o into an evacuated region kept at a background pressure p₁ several hundred times smaller than p_o. Facing the jet perpendicularly, at a variable distance, L, from the nozzle exit, is a bounce-free flat target plate which collects a fraction, E, of the incoming particles. The collection efficiency, E(d_{p, L)}, is determined on-line by using monodisperse charged particles of diameter d_p and measuring the electrical current they transport to the conducting target plate, which is grounded through an electrometer. The impactor shows sharp separation efficiency curves E(d_{p, L)} only when L is smaller than some critical distance L^*. For L > L^*, the curves E(L) become non-monotonic as a result of some gas dynamic reasons not fully understood yet. Provisionally, over the limited range of pressure ratios explored, it appears that L^* d_n = 0.13 √( p_o p₁). The size discrimination behaviour deteriorates also when L d_n becomes smaller than one, though this lower limit seems to be somewhat more flexible and geometry dependent than the upper one. Within the region 0.8 < L d_n < 0.13 √( p_o p₁), E(d_{p, L)} depends in a step-like manner on both these variables. The relatively sharp transition between E = 0 and E = 1 occurs roughly when the Stokes number in the impact region, S ∼ 32S_o L d_n, is approximately one, where S_o is a standard Stokes number based on the nozzle exit diameter, d_n, and the gas sound speed, c_o, at source conditions. For particles sufficiently small to be in the free-molecule limit, S_o = 0.1983 ρ_pd_pc_o² (d_np_o), where ρ_p is their density. The fact that the fundamental variable S governing the impaction process is proportional to the product Ld_p, makes these two factors conjugates of each other, and allows determining the diameter of an unknown monodisperse aerosol by measuring the value of L at which the capture efficiency E undergoes a step. This feature makes it possible to operate this impactor as an aerodynamic size spectrometer for ultrafine particles. With a pumping capacity of 500 l/min and an electrical noise level of 10^-16 A, we have been able to 'measure' NaCl particles with a diameter as small as 51 Å, and to discriminate between particle diameters only 3-5 Å apart.