Effect of acoustic radiation on DMA resolution

Acoustic waves produced by pressure fluctuations in the turbulent exhaust region of differential mobility analyzers (DMAs) are radiated upstream, making the flow in the working region slightly unsteady and reducing DMA resolution. This claim is demonstrated in a variety of settings. First, a resonance has been observed in several DMAs where the resolution decreases drastically in a narrow range of flow rates. This resonance can only be acoustic, and must be excited by flow unsteadiness, which is only present downstream. Second, non-idealities observed in two very different DMAs are suppressed when they are operated under transonic conditions. This is consistent with the inability of acoustic waves to propagate upstream against a supersonic flow. Third, these non-idealities decrease when the turbulent exhaust region from which acoustic radiation would originate is not in direct sight with the DMA's working section. These findings suggest new strategies to achieve DMA resolving powers approaching 100 under subsonic conditions.