A Novel Method for Evaluating the Adhesion of Explosives Residues
The adhesion between explosives residues and surfaces is of great interest to the Homeland Security community because this phenomenon defines the sampling challenge when trying to detect explosives using contact sampling/ion mobility spectrometry (IMS) methods at air transportation checkpoints. Specifically, the adhesion between explosives residues and baggage, persons, and the swipes used to collect the residues for detection is not adequately understood. As a result, the magnitude of the sampling challenge is not well defined. This limits the effectiveness of the methods employed to detect the residues.
The goal of this work is to measure directly the force with which an explosive powder or compounded formulation adheres to a range of surfaces of interest in air transportation security settings. In this context, “adhesion” refers to the force required to separate a residue from a surface. Such separation can occur when there is failure at the interface between the residue and the surface, or when the residue fails internally while under tensile load.
Because every explosive particle has a different shape and topography; compounded residues deform under their adhesive load against a surface; and the force holding residues onto the surface (as well as the internal force holding the residue together) are tightly related to the topography of interacting surfaces, it is necessary to study the behavior of many particles or residues against a surface in order to identify the limiting adhesion force.
This project has developed a method to measure the adhesion behavior of a population of explosive particles or residues against any surface of interest (swab or baggage, for example) using an advanced interpretation of classic experiments performed using a centrifuge. Specifically, residues of interest are placed onto a surface of interest, which is then mounted in a centrifuge so that the surface is parallel with the axis of rotation of the centrifuge and the residue faces outward. Next, the centrifuge is rotated and the particles or residue that remain adhered to the surface are counted as a function of the rotational speed of the centrifuge. This process allows the force required to remove any individual particle to be determined readily.
A modeling and simulation scheme has been developed that allows the adhesion forces of all of the particles studied to be described in terms of a simple closed form expression that can be evaluated with a handheld calculator. One adjustable constant is embedded in the expression, and this constant is “tuned” using the experimental adhesion results. The net result of this work will be a “look-up table” containing the fitted constants for each explosive-surface pair of interest (surfaces in this case including swabs, baggage, or other surfaces where residues may be detected), indexed as a function of the size of the particles in the residues. This is a new project; however, the method has been demonstrated and proven successful. Work is underway to transition the proven approach to systems of explosives and surfaces of interest to the Homeland Security community.
The purpose of this work is to develop essential understanding that does not currently exist on the way that residues deform and fail when under a swiping load. This knowledge will enable the fabrication of improved swabs, swabbing protocols and instrumentation.Year 4 Annual Report
Stephen P. Beaudoin
Students Currently Involved in Project
- Cara Coultas-McKenney