We report a simple method that uses (i) emulsion shearing with oxidation to make core-shell particles, and (ii) emulsion shearing with surface-tension driven phase segregation to synthesize particles with complex surface compositions and morphologies. Subjecting eutectic gallium-indium, a liquid metal, to shear in an acidic carrier fluid we synthesized smooth liquid core-shell particles 6.4 nm to over 10 μm in diameter. Aggregates of these liquid particles can be reconfigured into larger structures using a focused ion beam. Using Field's metal melts we synthesized homogeneous nanoparticles and solid microparticles with different surface roughness and/or composition through shearing and phase separation. This extension of droplet emulsion technique, SLICE, applies fluidic shear to create micro- and nanoparticles in a tunable, green, and low-cost approach.

The origin of the odd-even effect in properties of self-assembled monolayers (SAMs) and/or technologies derived from them is poorly understood. We report that hydrophobicity and, hence, surface wetting of SAMs are dominated by the nature of the substrate (surface roughness and identity) and SAM tilt angle, which influences surface dipoles/orientation of the terminal moiety. We measured static contact angles (θs) made by water droplets on n-alkanethiolate SAMs with an odd (SAM(O)) or even (SAM(E)) number of carbons (average θs range of 105.8-112.1°). When SAMs were fabricated on smooth "template-stripped" metal (M(TS)) surfaces [root-mean-square (rms) roughness = 0.36 ± 0.01 nm for Au(TS) and 0.60 ± 0.04 nm for Ag(TS)], the odd-even effect, characterized by a zigzag oscillation in values of θs, was observed. We, however, did not observe the same effect with rougher "as-deposited" (M(AD)) surfaces (rms roughness = 2.27 ± 0.16 nm for Au(AD) and 5.13 ± 0.22 nm for Ag(AD)). The odd-even effect in hydrophobicity inverts when the substrate changes from Au(TS) (higher θs for SAM(E) than SAM(O), with average Δθs |n - (n + 1)| ≈ 3°) to Ag(TS) (higher θs for SAM(O) than SAM(E), with average Δθs |n - (n + 1)| ≈ 2°). A comparison of hydrophobicity across Ag(TS) and Au(TS) showed a statistically significant difference (Student's t test) between SAM(E) (Δθs |Ag evens - Au evens| ≈ 5°; p < 0.01) but failed to show statistically significant differences on SAM(O) (Δθs |Ag odds - Au odds| ≈ 1°; p > 0.1). From these results, we deduce that the roughness of the metal substrate (from comparison of M(AD) versus M(TS)) and orientation of the terminal -CH2CH3 (by comparing SAM(E) and SAM(O) on Au(TS) versus Ag(TS)) play major roles in the hydrophobicity and, by extension, general wetting properties of n-alkanethiolate SAMs.

A thorough understanding of the kinetics and dynamics of combusting mixtures is of considerable interest, especially in regimes beyond the reach of current experimental validation. The ReaxFF reactive force field method has provided a way to simulate large-scale systems of hydrogen combustion via a parameterized potential that can simulate bond breaking. This modeling approach has been applied to hydrogen combustion, as well as myriad other reactive chemical systems. In this work, we benchmark the performance of several common parameterizations of this potential against higherlevel quantum mechanical (QM) approaches. We demonstrate instances where these parameterizations of the ReaxFF potential fail both quantitatively and qualitatively to describe reactive events relevant for hydrogen combustion systems.

Hydrogen is a potential substitute for fossil fuels that would reduce the combustive emission of carbon dioxide. However, the low ignition energy needed to initiate oxidation imposes constraints on the efficiency and safety of hydrogen-based technologies. Microscopic details of the combustion processes, ephemeral transient species, and complex reaction networks are necessary to control and optimize the use of hydrogen as a commercial fuel. Here, we report estimates of the ignition time of hydrogen-oxygen mixtures over a wide range of equivalence ratios from extensive reactive molecular dynamics simulations. These data show that the shortest ignition time corresponds to a fuel-lean mixture with an equivalence ratio of 0.5, where the number of hydrogen and oxygen molecules in the initial mixture are identical, in good agreement with a recent chemical kinetic model. We find two signatures in the simulation data precede ignition at pressures above 200 MPa. First, there is a peak in hydrogen peroxide that signals ignition is imminent in about 100 ps. Second, we find a strong anticorrelation between the ignition time and the rate of energy dissipation, suggesting the role of thermal feedback in stimulating ignition.

Interest in low-cost analytical devices (especially for diagnostics) has recently increased; however, concomitant translation to the field has been slow, in part due to personnel and supply-chain challenges in resource-limited settings. Overcoming some of these challenges require the development of a method that takes advantage of locally available resources and/or skills. We report a Melt-and-mold fabrication (MnM Fab) approach to low-cost and simple devices that has the potential to be adapted locally since it requires a single material that is recyclable and simple skills to access multiple devices. We demonstrated this potential by fabricating entry level bio-analytical devices using an affordable low-melting metal alloy, Field's metal, with molds produced from known materials such as plastic (acrylonitrile-butadiene-styrene (ABS)), glass, and paper. We fabricated optical gratings then 4×4 well plates using the same recycled piece of metal. We then reconfigured the well plates into rapid prototype microfluidic devices with which we demonstrated laminar flow, droplet generation, and bubble formation from T-shaped channels. We conclude that this MnM-Fab method is capable of addressing some challenges typically encountered with device translation, such as technical know-how or material supply, and that it can be applied to other devices, as needed in the field, using a single moldable material.

While hydrogen is a promising source of clean energy, the safety and optimization of hydrogen technologies rely on controlling ignition through explosion limits: pressure-temperature boundaries separating explosive behavior from comparatively slow burning. Here, we show that the emergent nonequilibrium chemistry of combustible mixtures can exhibit the quantitative features of a phase transition. With stochastic simulations of the chemical kinetics for a model mechanism of hydrogen combustion, we show that the boundaries marking explosive domains of kinetic behavior are nonequilibrium critical points. Near the pressure of the second explosion limit, these critical points terminate the transient coexistence of dynamical phases-one that autoignites and another that progresses slowly. Below the critical point temperature, the chemistry of these phases is indistinguishable. In the large system limit, the pseudo-critical temperature converges to the temperature of the second explosion limit derived from mass-action kinetics.

The explosion limits of hydrogen-oxygen mixtures are macroscopic, temperature-pressure boundaries that divide the overall chemistry of hydrogen oxidation into slow-burning and explosive regimes. Here, we demonstrate that it is possible to recover the three chemical explosion limits of H2/O2 mixtures from nonequilibrium stochastic trajectories. This demonstration relies on the finding that, in explosive regimes, these trajectories have the quantitative features of a dynamical phase transition. Through computer simulations for both a generic and a reduced model for hydrogen oxidation, we find only one dominant reactive phase at temperatures below the explosion limits. At temperatures above the limits, however, a second phase transiently emerges from the chemistry. By locating the pseudo-critical temperature where two reactive phases are distinguishable, we construct all three explosion-limit boundaries for model hydrogen-oxygen mixtures of finite size.