Impactor Apparatus for the Study of Particle Rebound: Relative Humidity and Capillary Forces

Abstract

The effect of relative humidity (RH) on the adhesion of particles colliding with a hard surface was studied for submicron particles of liquid oleic acid, solid ammonium sulfate, and solid polystyrene latex (PSL). For this purpose, a three-arm impactor was designed and constructed. The three arms consisted of one impactor having an uncoated impaction plate (i.e., a rebound arm), one impactor having a viscous-liquid-coated impaction plate (i.e., a capture arm), and one impactor having no impaction plate (i.e., a null arm). The particle number concentrations downstream of each arm were measured by condensation particle counters (CPCs). Data were analyzed to obtain the particle rebound fraction. Use of ambient upstage pressure allowed measurements from 5 to 95% RH at the impaction plate. Particle rebound depended strongly on RH, even for non-hygroscopic PSL particles. The rebound fraction for PSL particles dropped monotonically from nearly unity at 50% RH to 0.4 at 95% RH. For ammonium sulfate, the rebound fraction dropped from nearly unity at 25% RH to 0.5 at 70% RH. The decreased rebound at higher RH was explained by the formation of a water meniscus. The resulting capillary forces inhibited particle detachment. A model, taking into account the impact kinetic energy compared to the contact adhesion energy arising from van der Waals and capillary forces, captured the observations well. The reduced rebound arising from increased adhesion at high RH, independent of particle water content, potentially confounds a recent assumption that non-rebounding atmospheric particles are liquid.

Evaluation of Particle Bounce in Various Collection Substrates to be Used as Vaporizer in Aerosol Mass Spectrometer

“…The low vacuum pressure with long particle trajectory in the AMS could not be simulated in the current experimental setup. To account for the effects of particle wall losses or other losses in the particle bounce fraction, three types of impactor setups were used (Bateman et al 2014) as displayed in Figure 1 (i.e., impactors with no substrate, grease-coated substrate, and uncoated substrate). A Dow Corning 976V high vacuum grease (Dow Corning, USA) was used to coat various substrates tested here.…”

“…Physical properties of particles, such as hygroscopicity, size, density, phase, elasticity, and hardness, play an important role in particle bounce behavior (Wall et al 1990;Stein et al 1994;Matthew et al 2008;Virtanen et al 2010;Virtanen et al 2011;Bateman et al 2014). Hygroscopic particles, such as salts, are known to have much less bounce than non-hygroscopic particles (Stein et al 1994).…”

“…The slower impaction velocity cannot be achieved in the AMS due to high vacuum pressure system. Bateman et al (2014) examined the effect of RH on particle bounce, and found that particle bounce was strongly dependent on the surrounding RH, even for non-hygroscopic particles, due to increased adhesion energy by capillary force. However, the RH inside the AMS should be low.…”

“…Several laboratory and field tests reported that a CE of less than 100% was mainly caused by particle bounce from the vaporizer, especially for solid particles (Jayne et al 2000;Huffman et al 2005;Quinn et al 2006;Liu et al 2007;Salcedo et al 2007). In addition, particle bounce has been considered as an important issue for impactor techniques that use the inertial force of particles (i.e., high impaction velocity) for sampling or separation (Virtanen et al 2010;Bateman et al 2014).…”

“…The DMA-impactor method (Stein et al 1994;Bateman et al 2014) was applied to evaluate particle bounce behavior under a constant impact velocity (»104 m/s) and a dried condition (RH <20%), which is similar to the condition whereby particle bounce can occur in the AMS vaporizer. Collection substrates (i.e., vaporizers) of various materials, geometry, porosity, and mesh were constructed and tested by using solid and liquid particles.…”