(60e) Non-Contact AFM Measurement of the Hamaker Constants for the Improvement of Trace Explosive Detection | AIChE

(60e) Non-Contact AFM Measurement of the Hamaker Constants for the Improvement of Trace Explosive Detection

Authors 

Fronczak, S. G. - Presenter, Purdue University
Browne, C., Purdue University
Krenek, E., Purdue University
Corti, D., Purdue University
Beaudoin, S. P., Purdue University
The detection of homemade explosive devices in public areas, such as airports, is a primary concern for public safety. Trace amounts of explosive particles can contaminate the clothing of individuals in contact with the explosive materials during manufacturing and/or transportation of the device. This particle-surface interaction is controlled primarily by interfacial intermolecular forces. Of these, the van der Waals (vdW) force is of great importance since it is omnipresent. The Hamaker constant, A, is a quantitative measure of the vdW interaction and captures each material’s compositional effects. Therefore, in order to improve the efficiency of detection checkpoints and better understand the adhesive nature of these materials, it is essential to determine the Hamaker constants for the explosives, the surfaces to which they adhere, and the swab materials used to collect the explosives from these surfaces during checkpoint screening.

Experimental attempts to determine A using an AFM are often plagued by deviations from expected behavior caused by issues inherent to the contact regime, such as surface roughness and deformation. Thus, we developed a method for estimating Hamaker constants from the non-contact approach regime of an AFM experiment (Fronczak et al., 2017, Langmuir 33, 714-725). This method relies on a quasi-dynamic understanding of the cantilever tip’s approach to contact, in which the inertial effects of the tip motion are accounted for when describing the trajectory of the tip as it approaches the substrate. The method was tested experimentally using silica, alumina and polystyrene substrates, and was demonstrated to yield estimates of A for these materials that were in very good agreement with previously published Lifshitz calculations.

This new method, as with other approaches, relies heavily on the accuracy of the geometric model governing the interaction between the AFM tip and the substrate. Therefore, original validation efforts were focused on remaining as true to the shape of cantilever tip as possible. A complex model of a truncated pyramid with a spherical cap was thus used in the first experimental interpretation. Although this geometry can be confirmed and the dimensions estimated via scanning electron microscopy (SEM), even high-resolution SEM analysis of the tip cannot provide sufficient detail to allow appropriately precise determination of the tip’s geometry. In addition, the numerical methods that are required to evaluate the Hamaker constants with high accuracy for complex tip geometry can quickly become prohibitively complex.

Therefore, we proposed an adaptation of the current method in which the geometric complexity of the cantilever tip was eliminated from the determination of the Hamaker constant by describing the tip as an ‘effective’ perfect sphere, thereby capturing all the geometric effects in the single dimension of this sphere. We began by measuring the interaction between the tip and well-defined surfaces for which the values for A are documented in the literature. After the forces against these well-known surfaces were measured, the interactions were modeled by assuming that our effective spherical tip had an effective radius of Reff. When the dimensions of this effective shape were properly captured, the Hamaker constants for the tip-surface interactions can be correctly calculated in all cases where this cantilever is applied. In this way, the dimensions of the effective tip shape were “calibrated” based on the attractive forces of the well-characterized ideal surfaces. Since an optimization procedure was already required to fit the geometric parameters for the pyramidal tip model, this amendment merely applies the same optimization principle to a simpler spherical tip model. Once we generated the tip’s Reffwe modeled the tip’s approach to contact using just this one geometric parameter. Then, this was used to determine the unknown A of a desired material.

We first demonstrated the practicality and accuracy of this updated method by comparing the results with both the original pyramid model and Lifshitz approximations (when available) for flat substrates composed of silica, polystyrene, highly ordered pyrolytic graphite (HOPG), sapphire (α-Al3O2), Plexiglas (PMMA), and acrylonitrile butadiene styrene (ABS). After calibrating the tip against these materials, we then measured the Hamaker constant of the previously undetermined trinitrotoluene (TNT) and several other commercial swabs used at airport security checkpoints. Since the dielectric response functions for an appropriate range of the electromagnetic spectrum is not available for these materials, the Hamaker constants could not be compared against predictions from the Lifshitz theory. The results of our measurements do agree, however, with those previously published in the literature, which were obtained from surface energy based experiments such as contact angle measurements and inverse gas chromatography. Finally, by using a geometric averaging function, the strength of the interaction between TNT and these swab materials are inferred, key information that is needed as input for generating improved detection protocols.