a skimmer device for concentrating an aerosol from a flowing gas stream , having an inlet with inlet aperture and inlet raceway , an outlet with virtual impact void and collector channel , and bulk flow divertors symmetrically disposed on either side of the long axis of flow , further characterized in that the downstream walls of the bulk flow divertors are concavedly curved and reverse the direction of bulk flow . in section , the four channels or passages of the “ skimmer ” thus form a “ crossed tee ” with concavedly contoured lateral arms curving back around . the lateral flow channels are for diverting the bulk flow into exhaust chimney spaces , and the chimney spaces are positioned proximate to the inlet element and anterior to the collection channel . in operation , the bulk flow streamlines are thereby folded more than 90 degrees away from the long axis of flow on the laterally disposed concave walls of the bulk flow channels . while counterintuitive , this was found using computational fluid dynamics and experimentation to dramatically reduce wall separation and related instabilities and to improve particle recoveries . large two - dimensional arrays of closely stacked inlet and skimmer elements are thus achieved by fitting the chimneys into spaces between parallel inlet elements . the interlinked problems of flow instability , manufacturability of arrays , and scale - up of chimney cross - sectional area to equalize pressure differentials in the bulk flow diverter exhaust ducts , particularly in two - dimensional arrays at high throughput , are uniquely solved with this geometry .