Soot Surface Research Articles

Role of oxygenated surface functional groups on the reactivity of soot particles: An experimental study

The oxidative reactivity of soot particles depends on several factors such as the elemental composition, the ratio of aliphatic to aromatic carbons, the curvature of graphene layers in soot, and the graphitic or amorphous character of soot. While several studies have highlighted the high reactivity of soot due to the presence of oxygenated groups on it, some studies conclude that the presence of oxygenated groups on soot may not affect its reactivity. In this work, to be able to exclusively study the effect of oxygenated functional groups on soot and its reactivity, a model compound, graphite, is first partially oxidized through a simplified Torr method using KMnO4 as an oxidizing agent to introduce oxygenated groups on it. The concentration of oxidizing agent is controlled to ensure the formation of partially-oxidized-graphite-oxides (POGO) without any significant changes in the structural characteristics so as to determine only the effect of oxygenated groups on oxidation. The POGO materials are then characterized using various techniques such as XRD for crystallite size, FTIR and EDX for oxygenated functional groups, SEM for surface morphological changes, and TGA for their weight-loss profiles during combustion in air. The results indicate a reduction in the activation energy for oxidation with the addition of oxygenated groups on the model soots (POGO), though, after the loss of such groups during oxidation, the activation energies of all the models were close to each other. This indicates that oxygenated groups on soot surface may enhance soot reactivity at low conversion level, but may not have a long-lasting effect on soot oxidation to affect its overall reactivity.

Transported JPDF modelling and measurements of soot at elevated pressures

Accurate measurements and modelling of soot formation in turbulent flames at elevated pressures form a crucial step towards design methods that can support the development of practical combustion devices. A mass and number density preserving sectional model is here combined with a transported joint-scalar probability density function (JDPF) method that enables a fully coupled scalar space of soot, gas-phase species and enthalpy. The approach is extended to the KAUST turbulent non-premixed ethylene-nitrogen flames at pressures from 1 to 5 bar via an updated global bimolecular (second order) nucleation step from acetylene to pyrene. The latter accounts for pressure-induced density effects with the rate fitted using comparisons with full detailed chemistry up to 20 bar pressure and with experimental data from a WSR/PFR configuration and laminar premixed flames. Soot surface growth is treated via a PAH analogy and soot oxidation is considered via O, OH and O2 using a Hertz-Knudsen approach. The impact of differential diffusion between soot and gas-phase particles is included by a gradual decline of diffusivity among soot sections. Comparisons with normalised experimental OH-PLIF and PAH-PLIF signals suggest good predictions of the evolution of the flame structure. Good agreement was also found for predicted soot volume statistics at all pressures. The importance of differential diffusion between soot and gas-phase species intensifies with pressure with the impact on PSDs more evident for larger particles which tend to be transported towards the fuel rich centreline leading to reduced soot oxidation.

Soot volume fraction and size measurements over laminar pool flames and pre-vaporised non-premixed flames of biofuels, methyl esters and blends with diesel

Previous studies have demonstrated that methyl ester-based biodiesels produce significantly less soot compared to petroleum diesel in standard diffusion flames. However, the roles of the oxygen content and the degrees of unsaturation (DoU) of the biodiesels on their sooting propensity have not been independently analysed. In the present study, the relationship between soot yield of biodiesels and the DoU of the fuel is systematically analysed in laminar pool flames and pre-vaporised diffusion flames. The spatial distribution of soot volume fraction of the flames fuelled with four biodiesel fuels, two methyl esters and their blends with diesel are quantitatively measured using laser induced incandescence. All six biofuels have different DoU but similar oxygen mass fractions, so that the effect of the DoU on soot formation could be separately considered. The soot yield using biodiesels was measured to be lower than that of diesel fuel by a factor up to 7.5 in pool flames and by a factor of 4.0 in vapour flames. In both flame setups, the soot reduction effect of biodiesels ranks in order of DoU. The more saturated the fuel molecule is, the lower the soot yield. The soot reduction effect of the blending ratio with biodiesel was found to be non-linear, and different in pool and vapour flames. The morphology, size, and number density of soot particles were analysed using scanning electronic microscopy. The results suggest that the shape of primary soot particles is close to spherical and the structure of the point-contact aggregates is similar in all cases. Biofuels not only produce smaller particle sizes, but also fewer particles compared to diesel. Chemical kinetic simulations suggest that blending of biodiesels reduces the soot yield primarily by slowing the soot surface growth process, and secondarily by prohibiting the soot nucleation and inception. Both the nucleation (C3H3 and benzene) and growth species (C2H2) for soot formation take place more quickly and intensely for the combustion of more unsaturated fuels, which results in the correspondingly larger soot yield.

Effects of prenol and isoprenol addition on soot and PAH formation in ethylene/methane counterflow diffusion flames

Prenol has attracted much attention due to its high heating value and novel octane hyperboosting phenomenon; while its soot formation behavior is not studied yet. In this study, experimental and numerical analysis of the effects of prenol and isoprenol addition on polycyclic aromatic hydrocarbon (PAH) and soot formation in non-premixed ethylene/methane flames is performed. Distribution of soot volume fraction and PAHs in the flames is measured via laser induced incandescence and laser induced fluorescence, respectively. The results show that both prenol and isoprenol addition enhances soot and PAH formation in ethylene/methane flames, and the prenol shows a stronger enhancing effect. Modeling of the sooting behavior is performed with a constructed gas-phase chemistry and a sectional method. The numerical results well capture the experimental observations and reveal that the increased concentrations of C2H2, C3H3 or C4H6 after prenol or isoprenol blending chemically promote benzene ring formation, subsequent PAH growth and soot formation processes. ROP (rate of production) analysis is performed to investigate the effect of prenol and isoprenol addition on rate-limiting ring formation process. In methane and ethylene flame, prenol or isoprenol blending promotes benzene formation in different ways, due to the discrepancies on the radical pool in two flames. Moreover, prenol also shows stronger effect on both of soot inception rate and surface growth rate than isoprenol, which is due to more PAHs and C2H2 formed in the prenol-doping flames. Reaction pathway analysis shows that decomposition of prenol generates C2H2 and C3H3 while isoprenol mainly generate C3H3 and almost no C2H2, the distinctive decomposition pathways of prenol results in its higher formation propensity of PAHs and soot.

Effects of ammonia addition on soot formation in ethylene laminar diffusion flames. Part 2. Further insights into soot inception, growth and oxidation

Ammonia, as an alternative fuel, is attracting unprecedented attention due to the absence of CO2 emission during its combustion. Effects of ammonia co-firing on soot formation in ethylene laminar diffusion flames have been studied in the first part of this series of papers, addressing the soot volume fraction change and reaction kinetics of gaseous soot precursors. This paper further investigates the effect of ammonia on evolution behaviors of soot particle, namely soot inception, growth and oxidation in the ethylene co-flow diffusion flame. The soot volume fraction (SVF) and spatial distribution in flames without/with ammonia addition (5 and 20 vol%) were measured using laser-induced incandescence (LII) method. A newly constructed C2H4/NH3/PAHs kinetic model consisting of 163 components and 1018 reactions and the CoFlame codes dedicated to simulating laminar co-flow diffusion flame coupled with fixed sectional soot particle model were used to model the flames and soot inception, surface growth, and oxidation processes in them. Numerical results well captured the experimental observation on both peak SVF and flame height, and confirmed that ammonia inhibited the soot formation in the flame, showing a peak SVF decrease of 2–4 % per 1 % ammonia addition. The numerical results revealed that the ammonia addition inhibited all soot inception, surface growth via HACA (hydrogen-abstraction-acetylene-addition) and PAH condensation, and oxidation via OH/O2 processes. Most importantly, inhibition of HACA reactions induced by ammonia was identified as the main reason for reducing soot formation. Kinetic analysis revealed that adding ammonia created a new pathway to consume H radical while decreasing the rate of H radical-generating reactions, leading to a decrease in H radical and ultimately a reduction in the HACA reaction rate.

Temperature dependence of ethanol and dimethyl ether chemical mixing effects on soot and gas products in ethylene pyrolysis

The present work aims to understand the temperature dependence of the fuel chemical mixing effects on soot and gas product formation in ethylene pyrolysis in a laminar flow reactor. The chemical and dilution effects of ethanol and dimethyl ether addition were studied experimentally by gas chromatography measurements and computationally using a detailed soot model coupled with PAHs chemistry. The chemical effects of ethanol and dimethyl ether mixing increased the production of CO, H2, and CH4, but the dilution effects decreased the production of H2, CH4, and C2H2. The chemical mixing effects inhibited the formation of C2H2 except for the case in which 50% ethanol was mixed with 50% ethylene at 1273 K. Ethanol and dimethyl ether mixing inhibited soot production, but the fuel mixing effects on soot reduction decreased with increasing temperature. The temperature dependence of the fuel mixing effects on soot production was dominated by the chemical effects rather than the dilution effects. Kinetic analysis of the soot formation process indicated that the methyl radical (CH3) from ethanol and dimethyl ether decomposition combined with C2 species (C2H2, C2H4) from ethylene pyrolysis to generate plenty of C3 species (C3H4–P, C3H6, C3H5-A, C3H4-A, C3H3), which further converted into benzene (C6H6) and polycyclic aromatic hydrocarbons (C10H8, C14H10, and C16H10). The reaction rate of the above C1+C2→C3→C6→PAHs pathway increased noticeably with increasing pyrolysis temperature, which strengthened the chemical effects of ethanol and dimethyl ether mixing on soot surface growth rate contributed by PAHs and therefore soot yield under high-temperature conditions.

Continuous soot surface growth over carbene active site through spin density migration and spontaneous dehydrogenation: A DFT study

Surface growth process contributes the majority of soot mass, but the detailed mechanism remains vague, especially at post-flame zone where reactive radicals like H· are insufficient to continuously generate active sites. In this work, the reactivity of carbene active center on soot surface towards different categories of hydrocarbons was evaluated through DFT calculation, and the self-sustaining mechanisms for continuous further growth were proposed. Acetylene, aromatic molecules (benzene and naphthalene) and resonance stabilized radicals (RSRs, cyclopentadienyl and indenyl) were selected as representative reactants. Potential energy surfaces were obtained at the M06-2X/cc-pVTZ level of theory, and spin density distributions were presented to analyze the evolution of active sites. The surface growth process was divided into two stages, i.e., initial capture and further growth. During initial capture process, carbene active site exhibits high reactivity towards all kinds of representative reactants, and the energy barriers are lower than 10 kJ/mol except for benzene (26.17 kJ/mol). Meanwhile, the initial capture processes are exothermic and capturing RSRs shows the largest exothermicity. After the carbene site was occupied through initial capture process, the graphene flake still tends to retain the radical-like behavior and shows open-shell character, providing possibilities for further surface growth. During further growth process, spin density migration mechanism combined with spontaneous dehydrogenation step could achieve continuous surface capture without involving exogenous radicals. For acetylene, the unpaired electrons will preferentially locate at the terminal carbon of the growing chain to continuously refresh the active site. Extra dehydrogenation steps are required for the further growth involving aromatics and RSRs, otherwise the surface growth steps would be less exothermic and more reversible. Covalent bonding of these sp2-hybridized hydrocarbon species and the carbene site will create a vulnerable tertiary CH bond. Spontaneous dehydrogenation through the breakage of the tertiary CH bond can reproduce active site with localized spin density, after which the further growth process will be energetically and kinetically more feasible.

A new thermodeactivation model for soot surface reactivity toward O2 in turbulent hydrocarbon diffusion flames

ABSTRACT Accurate soot modeling in flames is one important tool contributing to an understanding of soot behavior in combustion. Soot models reflect the four principal processes that govern soot dynamics: nucleation, coagulation, surface growth, and oxidation. Existing oxidation models reflect both the temperature-dependent surface oxidation chemical kinetics and also the fact that the surfaces become less reactive as they age. This surface deactivation process has been modeled primarily by using in-flame soot measurements as a basis for developing functional forms and parameters to describe the decay of α, the ratio of active oxidation sites divided by total surface sites. These existing models, in general, overpredict oxidation rates in turbulent diffusion flames. This study presents an alternative soot aging factor model that is based on the analogous process of coal char oxidation deactivation, a well-studied problem. The proposed framework provides a basis for application to a wide range of turbulent diffusion flames. This new model is configured to provide the same parameter α that appears in existing approaches. As such it can be easily applied to the existing published models. The approach is implemented in the fixed-pivot sectional soot model, with the performance compared against literature data for ethylene/air turbulent diffusion flames. With only very minor adjustment from the baseline parameters developed for coal char, the model shows good agreement with the literature flame data.

Numerical Study of PAHs and Soot Emissions from Gasoline–Methanol, Gasoline–Ethanol, and Gasoline–n-Butanol Blend Surrogates

Soot formation is an intricate phenomenon, and soot propensity of a fuel is interwoven with the fuel composition, physical and chemical properties, and combustion environment. The present study examines the hypothesis that in addition to the chemical composition of the fuel, the sooting nature of the fuel is closely coupled with its chemical property known as octane sensitivity (S). With this motivation, the present study numerically investigates the effects of gasoline surrogate composition and its property, octane sensitivity (S), on polycyclic aromatic hydrocarbons (PAHs) and soot emissions. Four-component toluene primary reference fuel (TPRF)–alcohol blends, comprising iso-octane, n-heptane, toluene, and one of the three different alcohols- methanol, ethanol, and n-butanol, are used as gasoline surrogates. A total of 320 TPRF–alcohol mixtures, with S in the range of 1–10, are examined under laminar counterflow diffusion flame conditions. A detailed chemical mechanism coupled with a comprehensive soot model, which includes reactions for soot inception, surface growth, PAH condensation, and oxidation, is adopted. The analysis indicates that the toluene content in the fuel mixture has a prominent effect, while the alcohol content and octane sensitivity of the fuel have a weak correlation with the PAHs and soot. Thus, it is not clear if any of these three variables, namely, toluene content in the fuel, alcohol content in the fuel, and S, are individually sufficient to characterize the PAHs and soot across various blends. For this reason, a new variable (XCHO) based on the elemental composition of the fuel mixture is identified and it is shown that XCHO along with S of the fuel characterize soot emissions satisfactorily. Further, a reaction path analysis indicates that the efficacy of alcohols in reducing soot emissions follows the order: methanol > ethanol > n-butanol.

Investigation of conditional source-term estimation coupled with a semi-empirical model for soot predictions in two turbulent flames

The modelling of soot formation is investigated for two turbulent flames, at atmospheric and 3 atm pressure conditions. For the first time, a semi-empirical soot formulation that accounts for soot inception, coagulation, surface growth, and oxidation processes is coupled with the turbulent combustion model, Conditional Source-term Estimation (CSE) using Reynolds-Averaged Navier–Stokes equations. Detailed chemistry is included and an optically thin radiation model is considered. Non-adiabatic chemistry tabulations are created. Good agreement with the experiments is found for turbulent mixing and temperature fields in both flames, with some discrepancies believed to be due to the turbulence modelling approach. At 1 atm, the soot volume fractions are in reasonable agreement with the experiments, but typically smaller than the measurements with the centerline peak locating closer to the fuel exit. At 3 atm, good agreement between the numerical predictions and experimental data is achieved for the soot volume fraction within the experimental error. The centerline peak location is observed slightly farther downstream. Possible sources of discrepancies are examined and comparison with previously published numerical results is also undertaken. Differential diffusion and modified soot chemistry constants may bring further improvement. Without any particular tuning of soot chemistry, soot modelling within CSE is shown to be a promising approach.

Photocatalytic Role of Atmospheric Soot Particles under Visible-Light Irradiation: Reactive Oxygen Species Generation, Self-Oxidation Process, and Induced Higher Oxidative Potential and Cytotoxicity

It is known that there are semiconductor oxides involved in mineral dust, which have photocatalytic properties. However, soot particles contained in carbonaceous aerosol and their photoactivity under sunlight are rarely realized. In this study, reactive oxygen species (ROS) such as superoxide anions and hydroxyl radicals were generated upon visible-light irradiation of soot particles, and the production activity was consistent with the carbonaceous core content, indicating that the atmospheric soot particles can serve as a potential photocatalyst. The increase of oxygen-containing functional groups, environmentally persistent free radicals, oxygenated polycyclic aromatic hydrocarbons, and the oxidative potential (OP) of soot after irradiation confirmed the occurrence of visible-light-triggered photocatalytic oxidation of the soot itself. The mechanism analyses suggested that the carbonaceous core caused the production of ROS, which subsequently oxidize the extractable organic species on the soot surface. It is oxidized organic extracts that are responsible for the enhancements of the OP, cell mortality, and intracellular ROS generation. These new findings shed light on both the photocatalytic role of the soot and the importance of ROS during the photochemical self-oxidation of soot triggered by visible light and will promote a more comprehensive understanding of both the atmospheric chemical behavior and health effects of soot particles.

Oxidation evolutions of soot surface and pore properties with different O2-NO2 mixtures and temperatures

Soot oxidation evolutions at various oxidation degrees under different O2-NO2 mixtures and temperatures were performed. Results indicated that the internal burning-out process represented by shell-like particles at 500 °C independent on NO2 concentration, while the development of hollow shells for the soot was suppressed with increasing the temperature. A tendency with soot toward a less ordered arrangement with short and defective graphene layers after oxidizing at 500 °C was also observed, which was suppressed with added NO2 and increasing temperature. Many carbon oxygen functionalities produced at 40% oxidation and the intensity of the peak gradually increased as the increase of the oxidation degree, while the increase of the peak intensity became less sensitive with added NO2 and increasing temperature. Abundant micropores and mesopores formed inside the particles during the soot oxidation process depending on oxidizer and temperature, micropores (smaller than 2 nm) were extensively generated at the early stages of the oxidation while mesopores increased sharply after 20% oxidation. The trends were correspondent to the oxidation modes with different oxidizer and temperature. The correlationship observation suggested that soot oxidation behavior was dependent on competing rates of reaction kinetics versus mass transport of oxidizer according to the oxidation evolution of surface and pore.

An experimental investigation of soot morphology and nanostructure in high-pressure co-flow laminar methane diffusion flames

Pressure is a significant factor affecting soot formation and oxidation. However, there are only scarce studies in the literature that probe soot samples in high-pressure laminar diffusion flames and then analyze the soot nanostructure quantitatively. In this study, a novel pneumatic probe sampling method was proposed to obtain soot samples in methane co-flow laminar diffusion flames with pressures ranging from 0.2 to 0.8 MPa. Compared with thermophoretic sampling, this method was more suitable for long-time sampling to obtain adequate soot samples in flames with low soot concentration. The soot morphology and nanostructure were analyzed quantitatively by high-resolution transmission electron microscopy (HRTEM). The results show that as the pressure increases, the soot samples at 10, 20, 30, and 40 mm heights above burner (HAB) exhibit larger soot particle size, more ordered nanostructure, longer crystallite fringe, the larger size of poly-aromatic hydrocarbons (PAHs), lower fringe tortuosity, and smaller fringe spacing. In particular, the soot samples at 10 mm HAB showed the largest differences in morphology and nanostructure as the pressure increased. The quantitative analysis results suggested that elevated pressure increased graphitization, maturity, and oxidation resistance of soot, which was most possibly attributed to enhanced soot nucleation and surface growth by pressure.

Inside Cover: Generation and Release of OH Radicals from the Reaction of H2O with O2 over Soot (Angew. Chem. Int. Ed. 21/2022)

A conventional view is that particulate matter (PM) in the atmosphere is a major sink of OH radicals. By contrast, Xiao Cheng Zeng, Joseph S. Francisco, Hong He, and co-workers demonstrate in their Communication (e202201638) that water and O2 react on carbonaceous soot surface and give rise to gaseous OH radicals under irradiation. This new chemical mechanism for the generation of OH radicals indicates that airborne soot particles possess important catalytic oxidation capacity.

Resonant charge transfer in the interaction of hyperthermal anions with a technical graphite-like conducting surface

It was found that when the hyperthermal anions Br-, S- and SH- interact with the cleaned surface of soot, the intensity of the ion yield curves (IYC) of fragmentary anions S- and SH- decreases, while Br- does not. Based on the DFT calculations and the use of the induced charge model, it was shown that HOMO of the S- and SH- anions is located in the energy range of the conduction band of graphite, which allows an additional electron to tunnel from these anions to the surface and this leads to their neutralization.

Effect of CO2/H2O addition on laminar diffusion flame structure and soot formation of oxygen-enriched ethylene

Oxygen-enriched combustion technology has an important application prospect, in which Oxy-steam combustion is a new generation of potential carbon capture technology. In this paper, the effects of O2/CO2 and O2/H2O atmosphere on soot formation in laminar diffusion flame were studied. The flame image was taken by a color (CCD) camera in the visible band, the directional emission power in R and G bands was calibrated, and the two-dimensional distribution of temperature and soot volume fraction in the flame was reconstructed. The experimental results are compared with the numerical results, and the numerical results well predict the temperature of oxygen-enriched flame in O2/N2/CO2 atmosphere, but overestimate the soot volume fraction. The laminar diffusion flame of ethylene in two kinds of oxygen-rich atmosphere was numerically studied by using a step-by-step decoupling method to separate the density, transport, thermal and chemical effects of the diluent by introducing four virtual substances. The calculation results show that combustion in O2/H2O and O2/CO2 atmosphere reduces the flame peak temperature, but the effect on the flame centerline temperature distribution is different. The addition of H2O enhanced the concentration of OH through the reverse reaction rate of OH + H2=H + H2O, while the addition of CO2 decreased the concentration of OH.O2/CO2 combustion leads to the increase of flame height and radius, while in O2/H2O atmosphere, the flame height is shortened. Both CO2 and H2O have a good inhibitory effect on soot formation. CO2 in the oxidant affects soot nucleation and surface growth rate mainly by reducing flame temperature and H mole fraction, but not by promoting soot oxidation process. Water vapor also inhibits soot formation by increasing OH concentration through chemical effect.

Molecular Dynamics Study on the Condensation of PAH Molecules on Quasi Soot Surfaces.

In this paper, the condensation efficiency of polycyclic aromatic hydrocarbon (PAH) molecules up to coronene, from 500 to 2000 K, is calculated based on hundreds of collisions between a PAH molecule and the quasi soot surface, which is composed of stacked coronene molecules with periodic boundary conditions, using molecular dynamics simulations. The results show that the condensation efficiency increases with the PAH molecular mass but decreases as the temperature increases, following a Gaussian function. Meanwhile, when the presence of aliphatic chains on soot particle surfaces is considered, the condensation efficiency can be lowered by up to 40%, being affected more significantly at higher temperatures. A condensation efficiency model is thus proposed from the molecular trajectories. Finally, when this newly proposed PAH condensation efficiency model is adopted, better agreement with the experiments is achieved in predicting soot volume fractions of an ethylene/oxygen/nitrogen mixture in a tandem jet-stirred reactor and a plug-flow reactor.

Variations in surface functional groups, carbon chemical state and graphitization degree during thermal deactivation of diesel soot particles

The thermal deactivation of diesel soot particles exerts a significant influence on the control strategy for the regeneration of diesel particulate filters (DPFs). This work focused on the changes in the surface functional groups, carbon chemical state, and graphitization degree during thermal treatment in an inert gas environment at intermediate temperatures of 600°C, 800°C, and 1000°C and explore the chemical species that were desorbed from the diesel soot surface during thermal treatment using a thermogravimetric analyser coupled with a gas-chromatograph mass spectrometer (TGA-GC/MS). The surface functional groups and carbon chemical state were characterized using Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The graphitization degree was evaluated by means of Raman spectroscopy (RS). The concentrations of aliphatic C–H, C–OH, C=O, and O–C=O groups are reduced for diesel soot and carbon black when increasing the thermal treatment temperature, while the sp2/sp3 hybridized ratio and graphitization degree enhance. These results provide comprehensive evidence of the decreased reactivity of soot samples. Among oxygenated functional groups, the percentage reduction during thermal treatment is the largest for the O–C=O groups owing to its worst thermodynamic stability. TGA-GC/MS results show that the aliphatic and aromatic chains and oxygenated species would be desorbed from the soot surface during 1000°C thermal treatment of diesel soot.

Effect of ion-exchangeable calcium on carbonaceous particulate matter formation during coal pyrolysis

This work investigated the effect of ion-exchangeable calcium on the mass yield, size distribution and structural properties of carbonaceous particulate matter from coal pyrolysis. The raw, acid washed and calcium-exchanged coal samples were pyrolyzed in a flat-flame burner at 1600 K. The particulate matter was collected with a three-stage dilution sampler. Then, the particle size distributions were measured by a low pressure impactor (DLPI+) and a scan mobility particle sizer (SMPS). The results show that, with the addition of ion-exchangeable calcium, the soot yield reduces, and the size growth of soot particles is inhibited. Transmission electron microscopy (TEM) analysis shows that ion-exchangeable calcium diminishes the sizes of disordered cores in soot primary particles, accelerates the graphitization of soot and makes the soot aggregates more compact. The chemical structures derived from Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) indicate that the graphitization degree of the soot is first enhanced and then inhibited with increasing calcium loading amount. Additionally, the inorganic species in the size-segregated soot particles were characterized. The fractions of calcium and sodium in soot show opposite trend with particle size. Unlike Na, which nucleates before soot inception, Ca tends to participate in both the soot nucleation and surface growth processes, and is enriched at larger particle size.

Interfacial dynamics of viscous droplets impacting a superhydrophobic candle soot surface: Overview and comparison

The impact of droplets on textured or rough surfaces has garnered remarkable appreciation due to its multifarious applications such as self-cleaning, anti-icing, and anti-fouling, leading to a plethora of engineered superhydrophobic surfaces (SHPs) exhibiting different interfacial dynamics during impact. However, the prime limiting factors in using these surfaces abundantly arise from their long fabrication time and concurrent high cost. Here, we propose using carbon soot nanoparticle (CSNPs) coated fractal superhydrophobic surfaces prepared from flame deposition as an alternative to overcome the limitations. We establish our claim by exploring the dynamic wetting behavior of the soot-coated surface in terms of key droplet impact parameters such as rebounding, contact time, impalement transition, and splashing dynamics. A systematic investigation is undertaken by considering a vast range of viscosity and impact conditions. One of the significant observations is the absence of the partial rebound regime during the impact of water droplets on the CSNPs surface, unlike most of the existing superhydrophobic surfaces under similar impact conditions. Furthermore, the surface promotes droplet splashing for moderately viscous solutions at high impact velocities, also characterized by unified scaling laws based on different non-dimensional numbers. Finally, a regime map is proposed to elucidate the complete dynamic wetting characteristics of these CSNPs' surfaces for viscous fluids, which further reflects competitive and equal, if not superior, wetting behavior compared to a series of existing non-wetting surfaces. The results are expected to promote CSNPs based surfaces in applications such as self-cleaning, oil-water separation, and thermal management.