In the literature, two different frameworks exist for describing the rheology of solid/liquid suspensions: (1) the “viscous” framework in terms of the relative suspension viscosity, η_r, as a function of the reduced solid volume fraction, f=f_m, with f_m the maximum flowable packing fraction, and (2) the “frictional” framework in terms of a macroscopic friction coefficient, μ, as a function of the viscous number, I_v, defined as the ratio of the viscous shear to the wall-normal particle stress. Our goal is to compare the two different frameworks, focusing on the effect of friction between particles. We have conducted a particle-resolved direct numerical simulation study of a dense non-Brownian suspension of neutrally buoyant spheres in slow plane Couette flow. We varied the bulk solid volume fraction from f_b ¼ 0:1 to 0.6 and considered three different Coulomb friction coefficients: μ_c ¼ 0, 0.2, and 0.39. We find that η_r scales well with f=f_m, with f_m obtained from fitting the Maron–Pierce correlation. We also find that μ scales well with I_v. Furthermore, we find a monotonic relation between f=f_m and I_v, which depends only weakly on μ_c. Since η_r ¼ μ=I_v, we thus find that the two frameworks are largely equivalent and that both account implicitly for Coulomb friction. However, we find that the normal particle stress differences, N₁ and N₂, when normalized with the total shear stress and plotted against either f=f_m or I_v, remain explicitly dependent on μ_c in a manner that is not yet fully understood.

Non-photochemical laser-induced nucleation (NPLIN) has emerged as a promising primary nucleation control technique offering spatiotemporal control over crystallization with potential for polymorph control. So far, NPLIN was mostly investigated in milliliter vials, through laborious manual counting of the crystallized vials by visual inspection. Microfluidics represents an alternative to acquiring automated and statistically reliable data. Thus we designed a droplet-based microfluidic platform capable of identifying the droplets with crystals emerging upon Nd:YAG laser irradiation using the deep learning method. In our experiments, we used supersaturated solutions of KCl in water, and the effect of laser intensity, wavelength (1064, 532, and 355 nm), solution supersaturation (S), solution filtration, and intentional doping with nanoparticles on the nucleation probability is quantified and compared to control cooling crystallization experiments. Ability of dielectric polarization and the nanoparticle heating mechanisms proposed for NPLIN to explain the acquired results is tested. Solutions with lower supersaturation (S = 1.05) exhibit significantly higher NPLIN probabilities than those in the control experiments for all laser wavelengths above a threshold intensity (50 MW/cm²). At higher supersaturation studied (S = 1.10), irradiation was already effective at lower laser intensities (10 MW/cm²). No significant wavelength effect was observed besides irradiation with 355 nm light at higher laser intensities (≥50 MW/cm²). Solution filtration and intentional doping experiments showed that nanoimpurities might play a significant role in explaining NPLIN phenomena.

A droplet-based microfluidic platform is presented to study the nucleation kinetics of calcium oxalate monohydrate (COM), the most common constituent of kidney stones, while carefully monitoring the pseudo-polymorphic transitions. The precipitation kinetics of COM is studied as a function of supersaturation and pH as well as in the presence of inhibitors of stone formation, magnesium ions (Mg2+), and osteopontin (OPN). We rationalize the trends observed in the measured nucleation rates leveraging a solution chemistry model validated using isothermal solubility measurements. In equimolar calcium and oxalate ion concentrations with different buffer solutions, dramatically slower kinetics is observed at pH 6.0 compared to pHs 3.6 and 8.6. The addition of both Mg2+ and OPN to the solution slows down kinetics appreciably. Interestingly, complete nucleation inhibition is observed at significantly lower OPN, namely, 3.2 × 10-8 M, than Mg2+ concentrations, 0.875 × 10-4 M. The observed inhibition effect of OPN emphasizes the often-overlooked role of macromolecules on COM nucleation due to their low concentration presence in urine. Moreover, analysis of growth rates calculated from observed lag times suggests that inhibition in the presence of Mg2+ cannot be explained solely on altered supersaturation. The presented study highlights the potential of microfluidics in overcoming a major challenge in nephrolithiasis research, the overwhelming physiochemical complexity of urine.

Nonphotochemical laser-induced nucleation (NPLIN) is a promising primary nucleation control method, yet its underlying mechanism remains elusive. To contribute to the discussion on whether the polarization of laser irradiation in NPLIN experiments influences the polymorphic outcome, we revisit NPLIN experiments with aqueous glycine solutions with supersaturations ranging between S = 1.5 and S = 1.7 irradiated by nanosecond pulses (∼7 ns) of near-infrared wavelength (1064 nm). Systematically altering laser light excitation properties, including the number of pulses and type of polarization, we quantified the nucleation kinetics and characterized the polymorphic form that crystallized upon laser irradiation. Due to the stochasticity of the nucleation process, a large number of samples (>100 per each experimental point) were studied under carefully controlled experimental conditions such as the ambient temperature, cooling rate, and aging period. We observed significant differences among laser-irradiated, spontaneously nucleated, and crash-cooled samples in terms of nucleation kinetics and polymorphic form. This result indicates that laser irradiation provides a different polymorph-forming pathway in comparison to crash-cooling and spontaneous nucleation. However, no clear dependence between the polymorphic form and the polarization of laser irradiation is observed. We discuss our results in the context of previous reports supported thorough quantification of sample heating in NPLIN experiments.

In the domain of energetic nanomaterials, more specifically nano-sized explosives and oxidizers, many small scale production methods have been explored up to now. So far only limited attempts have been made to scale up the production to tens or maximally a few hunderds of grams. This paper provides a review of these small scale production methods as well as characterization techniques for nanometric explosives and oxidizers. As a result of the limited scale-up, the application of energetic nanomaterials in typical propellant and explosive formulations is currently very limited. This might be caused by the fact that a clear and commonly shared view on which energetic nanomaterials and production processes it would be economically beneficial and feasible to invest in is lacking at the moment. Furthermore, a considerable number of technical challenges can be expected regarding the processing of energetic nanomaterials on a composition level. To manage these challenges, this review proposes several technical solutions which may contribute to a better understanding of the benefits, risks and costs involved in the use and scale-up of energetic nanomaterials and, if considered economically feasible, a more widespread application of these nanomaterials in the defense and space domains.

A tensile module system placed within a Scanning Electron Microscope (SEM) was utilized to conduct in-situ tensile testing of propellant samples. The tensile module system allows for real-time in-situ SEM analysis of the samples to determine the failure mechanism of the propellant material under tensile force. The focus of this study was to vary the experimental parameters of the tensile module system and analyze how they affect the failure mechanism of the samples. The experimental parameters varied included strain rate and sample temperature (-54, +25 and +40°C). Stress-strain diagrams were recorded during the in-situ tensile tests, and these results were coupled with the in-situ images and videos of the samples captured with SEM analysis. The experiments conducted at -54°C showed a different failure behavior of the propellant sample due to its rigidity at this low temperature, while experiments conducted at +25 and +40°C displayed a similar failure mechanism. For future testing using this tensile tester, special attention should be given to improved temperature control of the specimen, especially at low temperatures.

In an attempt to further contribute to the characterization of explosive compositions, small scale Floret tests were performed using four RDX grades, differing in product quality. A Floret test provides a measure - by indentation of a copper block - of detonation spreading or the initiability and shock wave divergence and is applied in particular to explosives used in initiation trains. Both as-received RDX and PBXs (based on the AFX-757 composition, a hard target penetrator explosive) containing these RDX grades were tested in the Floret test set-up. It was found that the Floret test method, when applied to granular, as-received RDX, was not able to discriminate between the overall RDX product qualities on the basis of the resulting volume of the indentation in the copper block. For the Floret test data of the PBX samples, a division into two parts, where one of the RDX lots shows a lower dent volume compared to the other RDX lots tested. Based on the results presented in this paper with granular RDX and a PBX composition and earlier results with a different type of PBX (based on PBXN-109, an insensitive high explosive used in a wide range of munitions), the Floret test could be developed into a screening test for shock sensitivity and product quality, without the need for complex and large volume casting of specific PBX compositions.

Studying links between triacetone triperoxide (TATP) samples from crime scenes and suspects can assist in criminal investigations. Isotope ratio mass spectrometry (IRMS) and gas chromatography (GC)-IRMS were used to measure the isotopic compositions of TATP and its precursors acetone and hydrogen peroxide. In total, 31 TATP samples were synthesized with different raw material combinations and reaction conditions. For carbon, a good differentiation and a linear relationship were observed for acetone–TATP combinations. The extent of negative (δ¹³C) fractionation depended on the reaction yield. Limited enrichment was observed for the hydrogen isotope (δ²H) values of the TATP samples probably due to a constant exchange of hydrogen atoms in aqueous solution. For oxygen (δ¹⁸O), the small isotopic range and excess of water in hydrogen peroxide resulted in poor differentiation. GC-IRMS and IRMS data were comparable except for one TATP sample prepared with high acid concentration demonstrating the potential of compound-specific isotope analysis. Carbon IRMS has practical use in forensic TATP investigations.

An explosive composition, derived from AFX-757, was systematically varied by using three different qualities of Class I RDX. The effect of internal defect structure of the RDX crystal on the shock sensitivity of a polymer bonded explosive is generally accepted (Doherty and Watt, 2008). Here the response to a mechanical non-shock stimulus is studied using an explosion-driven deformation test as well as the ballistic impact chamber. No correlation between RDX crystal quality and deformation sensitivity is observed. The DDT behavior (Deflagration to Detonation Transition) of the three plastic bonded explosives, although similar in composition, is distinct regarding the rate of diameter increase in the explosion-driven deformation test. Recovered polymer bonded explosive from the explosion-driven deformation test responds equally fast or slower in the ballistic impact chamber. Based on our experimental results the shear rate threshold as a single parameter describing mechanical sensitivity is challenged, and preference is given to the development of an ignition criterion based on inter-granular sliding friction under the action of a normal pressure.

The failure mechanism of a propellant consisting of hydroxyl terminated poly-butadiene filled with ammonium perchlorate and aluminum (HTPB/AP/Al) was determined by performing in-situ uniaxial tensile tests in a scanning electron microscope (SEM). The experimental test plan contained uniaxial tensile test experiments performed at room temperature (25 °C) at three different strain rates (30, 150 and 750 μm min⁻¹). The in-situ images and in-situ videos collected by the SEM were correlated with the stress-strain diagrams created with the tensile experiments, in order to relate the failure mechanism to the features found in the stress-strain diagram. No significant strain rate dependency of the failure mechanism was observed when working with strain rates up to 750 μm min⁻¹ and working at room temperature. The stress-strain diagram showed indications of existing cracks and voids opening up prior to the creation of new cracks and/or voids in the sample, debonding of binder with AP particles as well as nucleation and coalescence of voids. On the fracture surfaces of the samples, it was apparent that the binder cleanly separated from the large AP particles but had a better bond with the aluminum particles. However, a difference in the appearance of a short drawing phase in the stress-strain diagram of the propellant is observed at different strain rates. The presented results clearly demonstrate the major advantage of the combination of microscopic tensile tests with microscopic observations, linking the stress-strain behavior to the mechanical deformation processes taking place in these propellant samples at the microscopic level.