Article pubs.acs.org/ac
Adhesion of Explosives Michelle N. Chaffee-Cipich, Bryce D. Sturtevant, and Stephen P. Beaudoin* School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United States ABSTRACT: It is of increasing importance to understand how explosive particles adhere to surfaces in order to understand how to remove them for detection in airport or other security settings. In this study, adhesion forces between royal demolition explosive (cyclotrimethylenetrinitramine) (RDX), pentaerythritol tetranitrate (PETN), and trinitrotoluene (TNT) in their crystalline forms and aluminum coupons with three finishes, acrylic melamine (clear coating), polyester acrylic melamine (white coating) automotive finishes, and a green military-grade finish, were measured and modeled. The force measurements were performed using the atomic force microscopy (AFM)-based colloidal probe microscopy (CPM) method. Explosive particles were mounted on AFM cantilevers and repeatedly brought in and out of contact with the surfaces of interest while the required force needed to pull out of contact was recorded. An existing Matlab-based simulator was used to describe the observed adhesion force distributions, with excellent agreement. In these simulations, the measured topographies of the interacting surfaces were considered, although the geometries were approximated. The simulations were performed using a van der Waals force-based adhesion model and a composite effective Hamaker constant. It was determined that certain combinations of roughness on the interacting surfaces led to preferred particle-substrate orientations that produced extreme adhesion forces.
T
he ability to remove residual explosives from surfaces is crucial to detecting improvised explosive devices (IEDs) in a variety of settings. Trace amounts of explosives adhere to the hands and equipment of those handling explosives and are subsequently transferred to clothing, parcels, vehicles, or other surfaces.1 Improved methods to remove detectable amounts of these explosives from surfaces will allow more effective detection of explosives in a range of environments. Current sampling methods can be classified as either contact or noncontact.2−4 Contact sampling dislodges explosives particles through physical contact with a swab and collects particles that adhere more strongly to the swab than to the original surface. Noncontact sampling generally relies on momentum transfer between a moving fluid (generally air) and the residue to dislodge the particles from the surface, after which the entrained particles are directed into a collection device. Contact sampling is the more common means of explosives detection in airports, which is the application of interest here. As of 2004, explosive detectors utilizing the “swipe” sampling technique numbered in the 10 000s worldwide.5 In 2008, Verkouteren et al.5 evaluated the collection efficiency of the swipe sampling technique and concluded that to improve the efficiency, the adhesion between the particles and the swab must be improved; the application of greater swiping force was found to have little effect. As outlined in Burdick et al., the adhesion force between a particle and a surface is related to the force required to remove the particle from the surface through a moment balance. A schematic of the approach to describe this removal is shown in Figure 1. In general, the lowest energy mode of particle removal © XXXX American Chemical Society
Figure 1. Schematic showing features of a model spherical particle being removed from a rough surface by fluid motion. FL = lift force, FA = adhesion force, FD = drag force, MD = rolling moment, d = particle diameter, a = deformed particle contact radius, δ = asperity height, α = extent of particle deformation. Reproduced with kind permission from Kluwer Academic Publishers.
is via rolling.6 In this case, when the sum of the rolling moment, the sliding moment, and the lifting moment exceeds the adhesive moment, the particle is removed. A detailed description of this phenomenon is beyond the scope of this manuscript. Rather, this work is focused on describing the adhesion force, which sets the removal criterion. Received: September 22, 2012 Accepted: March 19, 2013
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dx.doi.org/10.1021/ac302758n | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
When micrometer-scale or smaller bodies are within ∼50 nm of contact with a solid surface, interfacial interaction forces exist between the two objects.7−13 One of the main attractive forces between a particle and a substrate in such systems is the van der Waals (vdW) interaction force. Electrostatic or capillary forces may also affect the attractive force between a particle and a substrate, depending on the conditions present when the particles were deposited and the subsequent treatment of the surface. This work will focus on vdW forces by experimentally mitigating capillary effects through humidity control and electrostatic effects by a radioactive element. vdW forces cannot be eliminated in the adhesion systems of interest, while capillary and electrostatic forces may or may not be important, depending on the environment at the sampling location. For this reason, vdW forces alone are characterized. The magnitude of the vdW force depends on the composition of the particle, substrate (the surface onto which the particle is adhered), and intervening media (taken to be air). The particle and substrate roughness, the particle and substrate geometries, the separation distance between the particle and substrate, and any deformation that occurs on either surface also affect vdW forces. We consider the adhesion of three explosive particle types, royal demolition explosive (cyclotrimethylenetrinitramine) (RDX), pentaerythritol tetranitrate (PETN), and trinitrotoluene (TNT), which have recently been considered in experiments by Zakon et al.14 against three coupons of Aluminum Alloy 3003, each of which is coated with a polyester primer and then one of three different surface topcoats: (1) acrylic melamine (clear coating); (2) polyester acrylic melamine (white coating); and (3) a proprietary green military grade finish (military finish). The coupons are macroscopically flat, with topography (micro- to nanoscale height variation) representative of typical automotive finishes. The results of this study show that the adhesion force between the explosives and these three coatings is affected most strongly by the coatings’ textures and to a lesser extent by their composition.
Figure 2. Schematic of two nominally spherical particles adhering to a nominally planar substrate: (a) smooth ideal surfaces and (b) rough “real” surfaces.
Materials and Methods. The particles studied include RDX, PETN, and TNT in their crystalline forms. Scanning electron microscope (SEM) micrographs of representative particles of each material are shown in Figure 3. Note that the magnification of the TNT particle is half that of the RDX and PETN particles because of the large particle size. It was necessary to use large TNT particles because smaller particles (
