Aerodynamic Removal and Characterization of Particulate Trace Residues on Model Surfaces

Abstract

Trace residues of illicit substances and biological contamination of the surfaces remain accessible for long times because of their volatility. Although the non-contact surface sampling has been demonstrated using state-of-the-art sampling/detection platforms, the resuspension rates and the forces acting a nonspherical particle in the surface boundary flow are not well understood. The effectiveness and the sampling throughput of trace residues detection can be greatly enhanced by integrating the analytical instrument with a properly-designed sampling system. Aerodynamic particle resuspension is a function of many parameters, such as particle size, morphology, material, humidity, and turbulence. This work presents morphological analysis and analysis of the removal rates of two different trace explosives residues in an aerodynamic flow cell as a function of particle size and flow velocity. Samples of Trimethylenetrinitramine (RDX) and 2,4,6- Trinitrotoluene (TNT) are prepared by dry transfer method on the glass surface and interrogated under various flow conditions. The samples are examined optically and binned by their apparent area. They are also visualized by Scanning Electron Microscopy and Optical Profilometry to compute the height of the sample. This method is also applied to size-controlled reference particles in 5-50-micron range, the particle size range chosen based on the typical size distribution associated with trace explosives found in fingerprints. The resuspension efficiency is calculated as a function of the particle size and compared with the trace explosive particles in the same size range. Removal efficiency is proportional to the height (critical dimension) and inversely proportional to the dimension in the plane of the substrate. The particle removal data from the flow cell experiment is compared with the Computational Fluid Dynamics (CFD) modelling to correlate the removal with the wall shear stress showing a direct correlation with the particle removal rates.