Jacob K. Miller

Bifurcation-based mass sensing using piezoelectrically-actuated microcantilevers

In conventional implementations, resonant chemical and biological sensors exploit chemomechanically-induced frequency shifts, which occur in linear systems, for analyte detection. In this letter, an alternative sensing approach, based upon dynamic transitions across saddle-node bifurcations is investigated. This technique not only has the potential to render improved sensor metrics but also to eliminate frequency tracking components from final device implementations. The present work details proof-of-concept experiments on bifurcation-based sensing, which were conducted using selectively functionalized, piezoelectrically-actuated microcantilevers. Preliminary results reveal the proposed sensing technique to be a viable alternative to existing resonant sensing methods.

Thermal and mechanical response of PBX 9501 under contact excitation

The thermal and mechanical responses of a cyclotetramethylene-tetranitramine-based explosive (PBX 9501) and two non-energetic mock materials (900-21 and PBS 9501) under high-frequency mechanical excitation are presented. Direct contact ultrasound transducers were used to excite samples through a frequency range of 50 kHz to 40 MHz. The mechanical response of each sample was approximated from a contact receiving transducer and trends were confirmed via laser Doppler vibrometry. The steady-state thermal response of the samples was measured at discrete excitation frequencies via infrared thermography. A maximum temperature rise of approximately 15 K was observed in PBX 9501, and the mock materials exhibited similar thermal characteristics. Temperature gradients were calculated to estimate the total heat generated within the samples due to the mechanical excitation. The active heating mechanisms were found to be highly dependent on the frequency of excitation. Possible mechanisms of heating at frequencies bel...

Thermal and mechanical response of particulate composite plates under inertial excitation

The thermal and mechanical, near-resonant responses of particulate composite plates formed from hydroxyl-terminated polybutadiene (HTPB) binder and varying volume ratios of ammonium chloride (NH4Cl) particles (50, 65, 75%) are investigated. Each test specimen is clamped and forced with three levels of band-limited, white noise inertial excitation (10–1000 Hz at 1.00, 1.86 and 2.44 g RMS). The mechanical response of each plate is recorded via scanning laser Doppler vibrometry. The plates are then excited at a single resonant frequency and the thermal response is recorded via infrared thermography. Comparisons are made between the mechanical operational deflection shapes of each plate and spatial temperature distributions, with correlation seen between the observed level of strain, as visualized by strain energy density, and heat generation. The effect of particle/binder ratio on both the thermal and mechanical responses is discussed. Acquired results are also compared to an analytical model of the system. ...

Heat generation in an elastic binder system with embedded discrete energetic particles due to high-frequency, periodic mechanical excitation

High-frequency mechanical excitation can induce heating within energetic materials and may lead to advances in explosives detection and defeat. In order to examine the nature of this mechanically induced heating, samples of an elastic binder (Sylgard 184) were embedded with inert and energetic particles placed in a fixed spatial pattern and were subsequently excited with an ultrasonic transducer at discrete frequencies from 100 kHz to 20 MHz. The temperature and velocity responses of the sample surfaces suggest that heating due to frictional effects occurred near the particles at excitation frequencies near the transducer resonance of 215 kHz. An analytical solution involving a heat point source was used to estimate heating rates and temperatures at the particle locations in this frequency region. Heating located near the sample surface at frequencies near and above 1 MHz was attributed to viscoelastic effects related to the surface motion of the samples. At elevated excitation parameters near the transdu...

On the Thermomechanical Response of HTPB-Based Composite Beams Under Near-Resonant Excitation

Currently, there is a pressing need to detect and identify explosive materials in both military and civilian settings. While these energetic materials vary widely in both form and composition, many traditional explosives consist of a polymeric binder material with embedded energetic crystals. Interestingly, many polymers exhibit considerable self-heating when subjected to harmonic loading, and the vapor pressures of many explosives exhibit a strong dependence on temperature. In light of these facts, thermomechanics represent an intriguing pathway for the stand-off detection of explosives, as the thermal signatures attributable to motion-induced heating may allow target energetic materials to be distinguished from their more innocuous counterparts. In the present work, the thermomechanical response of a sample from this class of materials is studied in depth. Despite the nature of the material as a polymer-based particulate composite, classical Euler–Bernoulli beam theory, along with the complex modulus representation for linear viscoelastic materials, was observed to yield predictions of the thermal and mechanical responses in agreement with experimental investigations. The results of the experiments conducted using a hydroxyl-terminated polybutadiene (HTPB) beam with embedded ammonium chloride (NH4Cl) crystals are presented. Multiple excitation levels are employed and the results are subsequently compared to the work’s analytical findings. [DOI: 10.1115/1.4029996]

Hydroxybenzylidene-indolinones, c-di-AMP synthase inhibitors, have antibacterial and anti-biofilm activities and also re-sensitize resistant bacteria to methicillin and vancomycin

c-di-AMP signaling regulates a myriad of physiological processes in Gram-positive bacteria and mycobacteria. c-di-AMP synthase (DAC) is essential in many human pathogens including Staphylococcus aureus, Listeria monocytogenes and Streptococcus pneumoniae and could become an important antibacterial drug target. In our continuing efforts to identify diverse DAC inhibitors, we uncovered hydroxybenzylidene-indolinones as new DAC inhibitors. Interestingly, these compounds also possess antibacterial activities and inhibit biofilm formation. Importantly, methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecalis could be re-sensitized to methicillin and vancomycin, respectively, by hydroxybenzylidene-indolinones.

Modeling, Analysis, and Experimental Validation of a Bifurcation-Based Microsensor

The potential to detect very small amounts of added mass has driven research in chemical and biological sensors based on resonant micro- and nanoelectromechanical systems over the past two decades. While traditional resonant mass sensors utilize chemomechanically induced shifts in linear natural frequency for mass detection, alternate sensing approaches which exploit near-resonant nonlinear behaviors have garnered interest from the research community due to their potential to yield improved sensor metrics and to simplify final device implementations. This paper investigates the development of an amplitude-based mass sensing approach which utilizes the dynamic transitions that occur near a cyclic-fold/saddle-node bifurcation in the nonlinear frequency response of a piezoelectrically actuated microcantilever. Specifically, the work details the modeling, analysis, and experimental validation of this mass sensing technique. The experimental results presented here not only prove the feasibility of the proposed sensing approach but also allow for the direct evaluation of pertinent sensor metrics.

Phase Changes in Embedded HMX in Response to Periodic Mechanical Excitation

It is well known that energy can be spatially localized when explosives are mechanically deformed; however, the heat generation mechanisms associated with this localization process are not fully understood. In this work, mesoscale hot spot formation in ultrasonically-excited energetic materials has been imaged in real-time. More specifically, periodic, mechanical excitation has been applied to Dow Corning Sylgard® 184/octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) composite materials using contact piezoelectric transducers resulting in heating at various crystal locations. A thermally-induced phase transition from a β to δ non-centrosymmetric crystal structure for HMX results in the frequency doubling of incident laser radiation and can be used as a temperature proxy. In light of this, a high-repetition-rate 1064 nm Nd:YAG laser has been used to illuminate discrete HMX crystals, and a 532 nm filter has been applied to capture only the light emitted from δ-phase second harmonic generation (SHG). The visualization of δ-phase initiation and growth is useful for determining both heat generation mechanisms and heating rates at crystal/crystal and/or crystal/binder interfaces and contributes to the understanding and prediction of hot spots.

Polyimide-based magnetic microactuators for biofouling removal

Here we report on the development of novel polyimide-based flexible magnetic actuators for improving hydrocephalus shunts. The static and dynamic mechanical responses of the thin-film magnetic microdevices were quantitatively measured. The bacteria-removing capabilities of the microfabricated devices were also evaluated. Although additional evaluations are necessary, the preliminary results show promising potential for combatting bacteria-induced biofouling. Lastly, the thin-film microdevices are integrated into a single-pore silicone catheter to demonstrate a proof-of-concept, MEMS-enabled self-clearing, smart catheter.Here we report on the development of novel polyimide-based flexible magnetic actuators for improving hydrocephalus shunts. The static and dynamic mechanical responses of the thin-film magnetic microdevices were quantitatively measured. The bacteria-removing capabilities of the microfabricated devices were also evaluated. Although additional evaluations are necessary, the preliminary results show promising potential for combatting bacteria-induced biofouling. Lastly, the thin-film microdevices are integrated into a single-pore silicone catheter to demonstrate a proof-of-concept, MEMS-enabled self-clearing, smart catheter.

On the Thermomechanical Response of HTPB Composite Beams Under Near-Resonant Base Excitation

Currently, there is a pressing need to detect and identify explosive materials in both military and civilian settings. While these energetic materials vary widely in both form and composition, many traditional explosives consist of a polymeric binder material with embedded energetic crystals. Interestingly, many polymers exhibit considerable self-heating when subjected to harmonic loading, and the vapor pressures of many explosives exhibit a strong dependence on temperature. In light of these facts, thermomechanics represent an intriguing pathway for the stand-off detection of explosives, as the thermal signatures attributable to motion-induced heating may allow target energetic materials to be distinguished from their more innocuous counterparts. In the present work, the mechanical response of a polymeric particulate composite beam subjected to near-resonant base excitation is modeled using Euler-Bernoulli beam theory. Significant sources of heat generation are identified and used with distributed thermal models to characterize the system’s thermomechanical response. In addition, the results of experiments conducted using a hydroxyl-terminated polybutadiene (HTPB) beam with embedded ammonium chloride (NH4Cl) crystals are presented. The thermal and mechanical responses of the sample are recorded using infrared thermography and scanning laser Doppler vibrometry, and subsequently compared to the work’s analytical findings. By adopting the combined research approach utilized herein, the authors seek to build upon recent work and bridge the considerable gap that exists between theory and experiments in this specific field. To this end, the authors hope that this work will represent an integral step in enhancing the ability to successfully detect explosive materials.Copyright