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In situ visualization of particle motions during wipe sampling of explosives and other trace particulate materials.

Surface texture tailoring has the potential to increase the effectiveness of dry particle collection wipes, as a wipe's topographical features control the intimate surface contact made with particles on the substrate (critical for van der Waals governed adhesion). However, texture-tailoring approaches have not yet been widely explored, in part because of a lack of understanding of the specific wipe topographies and wipe/particle interactions that maximize particle collection. Here we describe an in situ optical microscopy technique that enables direct observation of micrometer-scale particle-wipe interactions occurring at the wipe-substrate interface during contact sampling. The technique is demonstrated for nonwoven meta-aramid (Nomex®) collection wipes with particles ranging from 1 to 90 μm in diameter and substrates of different topographies (glass and a nylon coil zipper). Experiments with hemispherically coated Janus particles allow rolling motions to be distinguished from sliding motions, providing detailed information about how particles move prior to capture or release by the wipe. Particle-fiber and particle-particle interactions are seen to play important roles in particle capture, suggesting that conventional sphere-on-plane models are inadequate to describe adhesion behavior in these systems. Micrographs show how loose, flexible fibers in roughened textile wipes interrogate the valleys of uneven substrate topographies, allowing capture of particles that might otherwise be trapped within the substrate's grooves and depressions. The materials used in this work are specifically relevant to explosives detection, but the in situ visualization technique is transferable for the study of any application involving dry particle collection, such as toxic substance sampling and dust removal.