Ion-Induced Soot Nucleation Using a New Potential for Curved Aromatics

A potential able to capture the properties and interactions of curved polycyclic aromatic hydrocarbons (cPAHs) was developed and used to investigate the nucleation behaviour and structure of nascent soot particles. The flexoelectric charge polarisation of cPAHs caused by pentagon integration was included through the introduction of off-site virtual atoms, and enhanced dispersion interaction parameters were fitted. The electric polarisation and intermolecular interactions of cPAHs were accurately reproduced compared to ab initio calculations. This potential was used within molecular dynamics simulations to examine the homogeneous and heterogeneous nucleation behaviour of the cPAH corannulene and planar PAH coronene across a range of temperatures relevant to combustion. The enhanced interactions between cPAHs and potassium ions resulted in significant and rapid nucleation of stable clusters compared to all other systems, highlighting their importance in soot nucleation. In addition, the resulting cPAH clusters present morphologies distinct from the stacked planar PAH clusters.This work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk). K.B. is grateful to the Cambridge Trust and the Stanley Studentship at King’s College, Cambridge for their financial support. This project is also supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme

Day to Day Variability of Dynamic Knee Joint Stability in Healthy Individuals

The cause of osteoarthritis remains unknown; however abnormal joint mechanics are speculated to be an initiating factor [1]. Relating the Finite Helical Axis (FHA) to joint health may provide a means of predicting risk of joint degeneration [2]. To study dynamic knee joint stability using FHA and electromyography (EMG), it is valuable to quantify the day to day variance of these measures in a healthy population. It was hypothesized that there would be no statistically significant differences in FHA parameters or muscle patterns between days for healthy individuals. Three healthy females with intact anterior cruciate ligaments were recruited and tested 3 times during one week. Three-dimensional data for FHA determination was collected from reflective skin markers placed on each lower extremity (3 markers/segment) using an 8-camera (120 Hz) video motion capture system (Motion Analysis Corp.). A 16-channel EMG system recorded muscle activation patterns from 6 major muscles of the leg. Each subject performed two dynamic tasks: unconstrained knee flexion and extension while seated, and a single leg squat and rise. Data was analyzed using in house programs written in Matlab 7.1 (Mathworks Inc.). Four FHA parameters were described: location, translation, orientation and dispersion [2]. Muscle activity patterns were quantified using a wavelet analysis approach [3]. Due to the small sample size, a non-parametric Friedman’s test was used to detect differences in dynamic knee joint stability between days (p=0.05). Significant differences (p=0.028) were found for the extension phase of the squat in the contra lateral legs for location y, which describes the anterior/posterior location of the FHA in the knee. No significant differences were detected for any other FHA or EMG parameters. This finding suggests that the y location of the FHA during the extension phase of the squat task changes across days, and must be carefully interpreted in future studies

Exploring the internal structure of soot particles using nanoindentation: A reactive molecular dynamics study

The mechanical properties and internal structure of soot nanoparticles is investigated using reactive molecular dynamics simulations of nanoindentation of model soot particles. The particles that are provided as inputs to the simulations are generated using reactive molecular dynamics to create 3D networks of crosslinked coronene, circumanthracene and core-shell mixtures of coronene and circumanthracene. The results of the simulated nanoindentation experiments are analysed as a function of the degree of crosslinking (defined as the number of crosslinks per monomer in the particles), the size and the core-shell structure of the particles. In the case of homogeneous particles (i.e. those without a core-shell structure), the simulations show a unique relationship between the degree of crosslinking (CL) and the simulated hardness, Young’s modulus and deformation ratio. In the case of particles with a core-shell structure, a unique relationship was only found by considering the core-shell ratio and the degree of crosslinking in both the core and the shell. Our results allow for interpretation of the nanoindentation experiments as suggesting crosslinks are present in mature soot particles and preliminary evidence that crosslinks also are present within the interior of soot particles

MRI-Based Knee Joint Laxity Measure in Healthy Individuals

A functional MRI based knee joint laxity device was developed by the current research group to enable three-dimensional (3D) evaluation of change in ligament length as a function of loading [1]. Previous studies have used the knee loading apparatus (KLA) to quantify knee joint laxity in the dominant leg of healthy individuals [1]; however anterior laxity of the knee is reported clinically as a left-right difference, and not absolute values [2]. Therefore, the purpose of this study was to quantify side-to-side differences in knee joint laxity using the KLA in a healthy population. It was hypothesized that there would be no statistically significant differences in knee joint laxity between legs for healthy individuals. One healthy female with an intact anterior cruciate ligament was recruited and tested 3 times during the span of 10 days. Magnetic Resonance (MR) imaging was used in conjunction with the KLA to obtain images of the knee joint geometry during an unloaded state and at an anterior load of 133 N. Sagittal images of the knee were manually digitized using SliceOmatic (Tomovision) to obtain 3D volumes of the femur and tibia. The displacement of the tibia at 133N was obtained from the 3D joint position of the femur relative to the tibia, specifically anterior displacement of the tibia [1]. Due to complications with data collection, results are only available for day 2 for the right leg, and days 1 and 3 for the left leg. The anterior displacement of the tibia under an anterior load of 133N was 1.29 mm for the right leg, and 0.62 ± 0.42 mm for the left leg. Due to the sample size of 1, statistical analysis was not performed. This is preliminary data; future studies will increase the number of subjects and collect data at multiple load levels

Flexoelectricity and the Formation of Carbon Nanoparticles in Flames

The formation of carbon nanoparticles in flames involves a nucleation step that remains poorly understood. Experimentally, carbon nuclei formation is known to depend strongly on the electrical aspects of combustion but modes of interaction between charged species in the flame and carbon precursors have yet to be found. We present evidence for flexoelectrically polarised aromatics contributing to carbon nanoparticulate nucleation. We imaged the nascent nanoparticles using high resolution transmission electron microscopy, which revealed that the majority of aromatics in the early carbon nanoparticles are fullerene-like and curved. The curvature induces a significant molecular flexoelectric dipole moment in the polyaromatic hydrocarbons. This electric polarization allows these molecules to strongly interact with chemi-ions produced during combustion, which we demonstrate using electronic structure calculations. The results indicate that the physical interaction between fullerene-like polar aromatics and chemi-ions is critically assisting the nucleation, and opens a new route to reduce pollution and improve flame-produced nanomaterials

Polar curved polycyclic aromatic hydrocarbons in soot formation

© 2018 The Combustion Institute. In this paper, we consider the impact of polar curved polycyclic aromatic hydrocarbons (cPAH) on the process of soot formation by employing electronic structure calculations to determine the earliest onset of curvature integration and the binding energy of curved homodimers. The earliest (smallest size) onset of curvature integration was found to be a six ring PAH with at least one pentagonal ring. The σ bonding in the presence of pentagons led to curvature, however, the π bonding strongly favored a planar geometry delaying the onset of curvature and therefore the induction of a flexoelectric dipole moment. The binding energies of cPAH dimers were found to be of similar magnitude to flat PAH containing one or two pentagons, with an alignment of the dipole moments vectors. For the more curved structures, steric effects reduced the dispersion interactions to significantly reduce the interaction energy compared with flat PAH. Homogeneous nucleation of cPAH at flame temperatures then appears unlikely, however, significant interactions are expected between chemi-ions and polar cPAH molecules suggesting heterogeneous nucleation should be explored.This project is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme

Self-assembly of curved aromatic molecules in nanoparticles

The self-assembly and structure of nanoparticles containing curved polycyclic aromatic hydrocarbon molecules (cPAHs) are investigated using molecular modelling. These polar fullerene-like molecules are receiving increased attention recently due to the steric and electronic properties caused by the inclusion of five-membered ring(s) within their hexagonal lattice. In this work, the curPAHIP potential is extended to describe the interactions between large cPAHs. It is then used within molecular dynamics simulations to produce nanoparticles containing cPAHs. Structural and energetic metrics, including diameter, density, intermolecular spacing, coordination number, alignment angle, radial distance, and energy value, are used to analyse systems containing cPAHs of different sizes and ratios, and containing flat PAHs or ions. Homogeneous cPAH particles are more tightly packed than their flat PAH counterparts, with large cPAHs displaying stacked columnar configurations absent in nanoparticles containing small cPAHs. Mixing cPAHs of different sizes disrupts the ordered mesophase and forms a core-shell structure in which the larger molecules make up the core and the smaller molecules comprise the shell, although this partitioning is less distinct compared to flat PAHs. In addition, the presence of flat PAHs and ions within cPAH nanoparticles promotes distinct arrangements dominated by weak dispersive interactions and strong electrostatic interactions, respectively.The ARCHER UK National Supercomputing Service (http: //www.archer.ac.uk), the Cambridge Trust and King's College, Cambridge, the National Research Foundation (NRF), Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme