Agglomeration occurs in environmental and industrial processes, especially at low temperatures where particle sintering or coalescence is rather slow. Here, the growth and structure of particles undergoing agglomeration (coagulation in the absence of coalescence, condensation, or surface growth) are investigated from the free molecular to the continuum regime by discrete element modeling (DEM). Particles coagulating in the free molecular regime follow ballistic trajectories described by an event-driven method, whereas in the near-continuum (gas-slip) and continuum regimes, Langevin dynamics describe their diffusive motion. Agglomerates containing about 10-30 primary particles, on the average, attain their asymptotic fractal dimension, D(f), of 1.91 or 1.78 by ballistic or diffusion-limited cluster-cluster agglomeration, corresponding to coagulation in the free molecular or continuum regimes, respectively. A correlation is proposed for the asymptotic evolution of agglomerate D(f) as a function of the average number of constituent primary particles, n̅(p). Agglomerates exhibit considerably broader self-preserving size distribution (SPSD) by coagulation than spherical particles: the number-based geometric standard deviations of the SPSD agglomerate radius of gyration in the free molecular and continuum regimes are 2.27 and 1.95, respectively, compared to ∼1.45 for spheres. In the transition regime, agglomerates exhibit a quasi-SPSD whose geometric standard deviation passes through a minimum at Knudsen number Kn ≈ 0.2. In contrast, the asymptotic D(f) shifts linearly from 1.91 in the free molecular regime to 1.78 in the continuum regime. Population balance models using the radius of gyration as collision radius underestimate (up to about 80%) the small tail of the SPSD and slightly overpredict the overall agglomerate coagulation rate, as they do not account for cluster interpenetration during coagulation. In the continuum regime, when a recently developed agglomeration rate is used in population balance equations, the resulting SPSD is in excellent agreement with that obtained by DEM.

Combustion is essential to the manufacture of carbon black, fumed oxides, optical fibers and, recently, new high-value products like carbon nanotubes, nanosilver and biomagnetic nanofluids that are driven to market predominantly by startups. This technology is attractive for material synthesis for its proven scalability as it does not involve the tedious steps of wet chemistry and can readily form stably metastable compositions and high purity products. Recent advances in aerosol and combustion sciences reveal that coagulation and sintering and/or surface growth control product particle size and morphology through the high temperature particle residence time, self-preserving size distribution and power laws for fractal-like particles. This motivates synthesis of an array of unique particle compositions and morphologies primarily by spray combustion leading to new catalysts, gas sensors and bio-materials and, most recently, to hand-held devices such as breath analysis sensors for monitoring chronic illnesses. In particular, multi-scale process design integrating mesoscale and molecular dynamics facilitates understanding of combustion product development. The latter contributes also to understanding of aggregation and surface growth of nascent soot, a bona fide nanostructured material! So here nascent soot dynamics, after nucleation or inception, are investigated through accounting of soot agglomeration and surface growth by acetylene pyrolysis. Neglecting the fractal-like nature of soot underestimates its mobility diameter and polydispersity up to 40%. The evolution of nascent soot structure from spheres to aggregates is quantified by the mass fractal dimension and mass-mobility exponent, in excellent agreement with microscopic and mass-mobility measurements in a standard burner-stabilized stagnation ethylene flame. Surface growth chemically bonds the constituent primary particles of these aggregates, while the effect of soot volume fraction on soot morphology is elucidated. Based on aggregate projected area, a scaling law is derived for determining the primary particle size of nascent soot aggregates from mass-mobility measurements rather than tedious image counting.

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The crystallinity of gold nanoparticles during coalescence or sintering is investigated by molecular dynamics. The method is validated by the attainment of the Au melting temperature that increases with increasing particle size approaching the Au melting point. The morphology and crystal dynamics of nanoparticles of (un)equal size during sintering are elucidated. The characteristic sintering time of particle pairs is determined by tracing their surface area evolution during coalescence. The crystallinity is quantified by the disorder variable indicating the system's degree of disorder. The atoms at the grain boundaries are amorphous, especially during particle adhesion and during sintering when grains of different orientation are formed. Initial grain orientation affects final particle morphology leading to exposure of different crystal surfaces that can affect the performance of Au nanoparticles (e.g., catalytic efficiency). Coalescence between crystalline and amorphous nanoparticles of different size results in polycrystalline particles of increasing crystallinity with time and temperature. Crystallinity affects the sintering rate and mechanism. Such simulations of free-standing Au nanoparticle coalescence are relevant also to Au nanoparticles on supports that do not exhibit strong affinity or strong metal support interactions.

Piezoelectric fluoropolymers convert mechanical energy to electricity and are ideal for sustainably providing power to electronic devices. To convert mechanical energy, a net polarization must be induced in the fluoropolymer, which is currently achieved via an energy-intensive electrical poling process. Eliminating this process will enable the low-energy production of efficient energy harvesters. Here, by combining molecular dynamics simulations, piezoresponse force microscopy, and electrodynamic measurements, we reveal a hitherto unseen polarization locking phenomena of poly(vinylidene fluoride–co–trifluoroethylene) (PVDF-TrFE) perpendicular to the basal plane of two-dimensional (2D) Ti3C2Tx MXene nanosheets. This polarization locking, driven by strong electrostatic interactions enabled exceptional energy harvesting performance, with a measured piezoelectric charge coefficient, d33, of −52.0 picocoulombs per newton, significantly higher than electrically poled PVDF-TrFE (approximately −38 picocoulombs per newton). This study provides a new fundamental and low-energy input mechanism of poling fluoropolymers, which enables new levels of performance in electromechanical technologies.