Enabling Superhydrophobicity-Guided Superwicking in Metal Alloys via a Nanosecond Laser-Based Surface Treatment Method.

Enabling capillary wicking on bulk metal alloys is challenging due to processing complexity at different size scales. This work presents a laser-chemical surface treatment to fabricate superwicking patterns guided by a superhydrophobic region over a large-area metal alloy surface. The laser-chemical surface treatment generates surface micro/nanostructures and desirable surface chemistry simultaneously. The superhydrophobic surface was first fabricated over the whole surface by laser treatment under water confinement and fluorosilane treatment; subsequently, superwicking stripes were processed by a second laser treatment in air and cyanosilane treatment. The resultant surface shows superwicking regions surrounded by superhydrophobic regions. During the process, superwicking regions possess dual-scale structures and polar nitrile surface chemistry. In contrast, random nanoscale structures and fluorocarbon chemistry are generated on the superhydrophobic region of the aluminum alloy 6061 substrates. The resultant superwicking region demonstrates self-propelling anti-gravity liquid transport for methanol and water. The combination of the capillary effect of the dual-scale surface microgrooves and the water affinitive nitrile group contributes toward the self-propelling movement of water and methanol at the superwicking region. The initial phase of wicking followed Washburn dynamics, whereas it entered a non-linear regime in the later phase. The wicking height and rate are regulated by microgroove geometry and spacing.

Hysteresis branch crossing and the Stoner-Wohlfarth model.

The Stoner-Wohlfarth model predicts the crossing of the ascending and descending branches of the hysteretic magnetization curve. This crossing behavior has widely been dismissed, with the claim that it violates the laws of thermodynamics. Experimental verification of hysteresis branch crossing has not been acknowledged in the literature. Here we show, both theoretically and experimentally, that the crossing of the ascending and descending branches of the magnetization curve is a robust, reproducible phenomenon which does not violate any fundamental law.

VO2-based switchable radiator for spacecraft thermal control.

Direct calorimetric measurements of a solid state passive switchable radiator for spacecraft thermal control have been performed in a simulated space environment. Dynamic emissivity control is provided by the thermochromic phase change in a multilayer VO2 thin film based resonant absorber. The measured radiated power difference between 300 K and 373 K was 480 W/m2 corresponding to a 7× difference in radiative cooling power. We present theoretical and experimental radiator values for both normal and hemispherical as well the optical properties of VO2 as determined via infrared spectroscopic ellipsometry.

Nanosecond mid-infrared pulse generation via modulated thermal emissivity.

We demonstrate the generation of nanosecond mid-infrared pulses via fast modulation of thermal emissivity enabled by the absorption of visible pump pulses in unpatterned silicon and gallium arsenide. The free-carrier dynamics in these materials result in nanosecond-scale modulation of thermal emissivity, which leads to nanosecond pulsed thermal emission. To our knowledge, the nanosecond thermal-emissivity modulation in this work is three orders of magnitude faster than what has been previously demonstrated. We also indirectly observed subnanosecond thermal pulses from hot carriers in semiconductors. The experiments are well described by our multiphysics model. Our method of converting visible pulses into the mid infrared using modulated emissivity obeys different scaling laws and can have significant wavelength tunability compared to approaches based on conventional nonlinearities.

Detection of explosives and related compounds by low-temperature plasma ambient ionization mass spectrometry.

Detection of explosives is important for public safety. A recently developed low-temperature plasma (LTP) probe for desorption and ionization of samples in the ambient environment ( Anal. Chem. 2008 , 80 , 9097 ) is applied in a comprehensive evaluation of analytical performance for rapid detection of 13 explosives and explosives-related compounds. The selected chemicals [pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT), cyclo-1,3,5-trimethylenetrinitramine (RDX), tetryl, cyclo-1,3,5,7-tetramethylenetetranitrate (HMX), hexamethylene triperoxide diamine (HMTD), 2,4-dinitrotoluene, 1,3-dinitrobenzene, 1,3,5-trinitrobenzene, 2-amino-4,6-dinitrotoluene, 4-amino-2,6-dinitrotoluene, 2,6-dinitrotoluene, and 4-nitrotoluene) were tested at levels in the range 1 pg-10 ng. Most showed remarkable sensitivity in the negative-ion mode, yielding limits of detection in the low picogram range, particularly when analyzed from a glass substrate heated to 120 °C. Ions typically formed from these molecules (M) by LTP include [M + NO(2)](-), [M](-), and [M - NO(2)](-). The LTP-mass spectrometry methodology displayed a linear signal response over three orders of magnitude of analyte amount for the studied explosives. In addition, the effects of synthetic matrices and different types of surfaces were evaluated. The data obtained demonstrate that LTP-MS allows detection of ultratrace amounts of explosives and confirmation of their identity. Tandem mass spectrometry (MS/MS) was used to confirm the presence of selected explosives at low levels; for example, TNT was confirmed at absolute levels as low as 0.6 pg. Linearity and intra- and interday precision were also evaluated, yielding results that demonstrate the potential usefulness and ruggedness of LTP-MS for the detection of explosives of different classes. The use of ion/molecule reactions to form adducts with particular explosives such as RDX and HMX was shown to enhance the selectivity and specificity. This was accomplished by merging the discharge gas with an appropriate reagent headspace vapor (e.g., from a 0.2% trifluoracetic acid solution).

Fabrication of terahertz metamaterials by laser printing.

A laser printing technique was used to fabricate split-ring resonators (SRRs) on Si substrates for terahertz (THz) metamaterials and their resonance behavior evaluated by THz time-domain spectroscopy. The laser-printed Ag SRRs exhibited sharp edge definition and excellent thickness uniformity, which resulted in an electromagnetic response similar to that from identical Au SRR structures prepared by conventional photolithography. These results demonstrate that laser printing is a practical alternative to conventional photolithography for fabricating metamaterial structures at terahertz frequencies, since it allows their design to be easily modified and optimized.

Screening of agrochemicals in foodstuffs using low-temperature plasma (LTP) ambient ionization mass spectrometry.

Low-temperature plasma (LTP) permits direct ambient ionization and mass analysis of samples in their native environment with minimal or no prior preparation. LTP utilizes dielectric barrier discharges (DBDs) to create a low power plasma which is guided by gas flow onto the sample from which analytes are desorbed and ionized. In this study, the potential of LTP-MS for the detection of pesticide residues in food is demonstrated. Thirteen multi-class agricultural chemicals were studied (ametryn, amitraz, atrazine, buprofezin, DEET, diphenylamine, ethoxyquin, imazalil, isofenphos-methyl, isoproturon, malathion, parathion-ethyl and terbuthylazine). To evaluate the potential of the proposed approach, LTP-MS experiments were performed directly on fruit peels as well as on fruit/vegetable extracts. Most of the agrochemicals examined displayed remarkable sensitivity in the positive ion mode, giving limits of detection (LOD) for the direct measurement in the low picogram range. Tandem mass spectrometry (MS/MS) was used to confirm identification of selected pesticides by using for these experiments spiked fruit/vegetable extracts (QuEChERS, a standard sample treatment protocol) at levels as low as 1 pg, absolute, for some of the analytes. Comparisons of the data obtained by direct LTP-MS were made with the slower but more accurate conventional LC-MS/MS procedure. Herbicides spiked in aqueous solutions were detectable at LODs as low as 0.5 microg L(-1) without the need for any sample preparation. The results demonstrate that ambient LTP-MS can be applied for the detection and confirmation of traces of agrochemicals in actual market-purchased produce and in natural water samples. Quantitative analysis was also performed in a few selected cases and displayed a relatively high degree of linearity over four orders of magnitude.

Direct olive oil analysis by low-temperature plasma (LTP) ambient ionization mass spectrometry.

A fast, reagentless, and direct method is presented for the mass spectrometric analysis of olive oil without any sample pretreatment whatsoever. An ambient ionization technique, the low-temperature plasma (LTP) probe, based on dielectric barrier discharge, is used to detect both minor and trace components (free fatty acids, phenolics and volatiles) in raw untreated olive oil. The method allows the measurement of free fatty acids (the main quality control parameter used to grade olive oil according to quality classes), selected bioactive phenolic compounds, and volatiles. The advantages and limitations of the direct analysis of extremely complex mixtures by the ambient ionization/tandem mass spectrometry combination are discussed and illustrated. The data presage the possible large-scale application of direct mass spectrometric analysis methods in the characterization of olive oil and other foodstuffs.

Low-temperature plasma probe for ambient desorption ionization.

A low-temperature plasma (LTP) probe has been developed for ambient desorption ionization. An ac electric field is used to induce a dielectric barrier discharge through use of a specially designed electrode configuration. The low-temperature plasma is extracted from the probe where it interacts directly with the sample being analyzed, desorbing and ionizing surface molecules in the ambient environment. This allows experiments to be performed without damage to the sample or underlying substrate and, in the case of biological analysis on skin surfaces, without electrical shock or perceptible heating. Positive or negative ions are produced from a wide range of chemical compounds in the pure stateand as mixtures in the gaseous, solution, or condensed phases, using He, Ar, N2, or ambient air as the discharge gas. Limited fragmentation occurs, although it is greater in the cases of the molecular than the atomic discharge gases. The effectiveness of the LTP probe has been demonstrated by recording characteristic mass spectra and tandem mass spectra of samples containing hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT) from poly(tetrafluoroethylene) (PTFE) surfaces where limits of detection are as low as 5 pg. Other performance characteristics, when using a commercial ion trap mass spectrometer, include 3-4 orders of magnitude linear dynamic range in favorable cases. Demonstration applications include direct analysis of cocaine from human skin, determination of active ingredients directly in drug tablets, and analysis of toxic and therapeutic compounds in complex biological samples. Ionization of chemicals directly from bulk aqueous solution has been demonstrated, where limits of detection are as low as 1 ppb. Large surface area sampling and control of fragmentation by a simple adjustment of the electrode configuration during operation are other demonstrated characteristics of the method.