Timothy A. Sipkens

• PhD
• Research Officer at National Research Council Canada

Laser-induced incandescence (LII) is a widely used combustion diagnostic for in situ measurements of soot primary particle sizes and volume fractions in flames, exhaust gases, and the atmosphere. Increasingly, however, it is applied to characterize engineered nanomaterials, driven by the increasing industrial relevance of these materials and the fu...

The particle filtration efficiency (PFE) of a respirator or face mask is one of its key properties. While the physics of particle filtration results in the PFE being size-dependent, measurement standards are specified using a single, integrated PFE, for simplicity. This integrated PFE is commonly defined with respect to either the number (NPFE) or...

Axisymmetric tomography is used to extract quantitative information from line-of-sight measurements of gas flow and combustion fields. For instance, background oriented schlieren (BOS) measurements are typically inverted by tomographic reconstruction to estimate the density field of a high-speed or high-temperature flow. Conventional reconstruction...

Soot is an important material with impacts that depend on particle morphology. Transmission electron microscopy (TEM) represents one of the most direct routes to qualitatively assess particle characteristics. However, producing quantitative information requires robust image processing tools, which is complicated by the low image contrast and comple...

The optical properties of soot are crucial in estimating its climate impact through direct radiative forcing. Soot light absorption is typically quantified by the mass absorption cross-section (MACλ) or the absorption function E(mλ), which are wavelength dependent. Light absorbed by soot can be predicted from its MACλ using mass-concentration measu...

Black carbon (BC) is a strongly absorbing component of atmospheric aerosols that has a significant warming effect. BC particles are emitted from combustion sources as open-structured fractal aggregates. After emission, BC is often compacted due to capillary condensation of semivolatile vapors to form coatings. The addition of coatings influences th...

Non-contact methods are useful to improve the quality control of particle filtration media. The purpose of this paper is to investigate the correlation between the filtration efficiency of a porous sheet and its ultrasonic properties obtained using a non-contact technique. An air-coupled ultrasonic technique is used to obtain rapid measurements wit...

Carbonaceous particles, such as soot, make up a notable fraction of atmospheric particulate matter and contribute substantially to anthropogenic climate forcing, air pollution, and human health impacts. Thermal–optical analysis (TOA) is one of the most widespread methods used to speciate carbonaceous particles and divides total carbon (TC) into the...

Carbonaceous particles, such as soot, make up a notable fraction of atmospheric particulate matter and contribute substantially to anthropogenic climate forcing, air pollution, and human health. Thermal-optical analysis (TOA) is one of the most widespread methods used to speciate carbonaceous particles and divides total carbon (TC) into the operati...

Aerosol classifiers are instruments which can select suspended particles based upon a particular physical characteristic over a narrow range of that characteristic. Examples include the Differential Mobility Analyzer (DMA), the Centrifugal Particle Mass Analyzer (CPMA), and the Aerodynamic Aerosol Classifier (AAC). Often two of these classifiers wi...

We report a new workflow for background-oriented schlieren (BOS), termed “physics-informed BOS,” to extract density, velocity, and pressure fields from a pair of reference and distorted images. Our method uses a physics-informed neural network (PINN) to produce flow fields that simultaneously satisfy the measurement data and governing equations. Fo...

Forward error propagation is an established technique for uncertainty quantification (UQ). This article covers practical applications of forward error propagation in the context of particle counting measurements. We begin by presenting pertinent error models, including the Poisson noise model, and assess their role in UQ. Next, we describe several...

Standards governing face masks differ in the test methods used to determine sub-micron particle filtration efficiency (PFE), such that the meaning of PFE is not universal. Unifying the meaning of PFE requires data using these different test methods to drive improvements in standards. This simple data set provides the equivalence between two major t...

We report a new workflow for background-oriented schlieren (BOS), termed "physics-informed BOS," to extract density, velocity, and pressure fields from a pair of reference and distorted images. Our method uses a physics-informed neural network (PINN) to produce flow fields that simultaneously satisfy the measurement data and governing equations. Fo...

Ultra-high strength steel (UHSS) alloys such as aluminized 22MnB5 are used to produce automotive parts through hot stamping. A thermometallurgical model that predicts the blank-heating profile and austenitization inside a roller hearth furnace is needed to maximize process efficiency while ensuring that the blanks are completely austenitized. Three...

The mass absorption cross-section (MAC) of combustion-generated soot is used in pollution and emissions measurements to quantify the mass concentration of soot and in atmospheric modelling to predict the radiative effects of soot on climate. Previous work has suggested that the MAC of soot particles may change with their size, due to (1) internal s...

The mass absorption cross-section (MAC) of combustion-generated soot is used in pollution and emissions measurements to quantify the mass concentration of soot and in atmospheric modelling to predict the radiative effects of soot on climate. Previous work has suggested that the MAC of soot particles may change with their size, due to (1) internal s...

This study explores nonwoven and woven fabrics to improve upon the performance of the widespread all-cotton mask, and examines the effect of layering, machine washing and drying on their filtration and breathability for submicron and supermicron particles. Individual materials were evaluated for their quality factor, Q, which combines filtration ef...

The particle filtration efficiency (PFE) of a respirator or face mask is one of its key properties. While the physics of particle filtration results in the PFE being size-dependent, measurement standards are specified using a single, integrated PFE, for simplicity. This integrated PFE is commonly defined with respect to either the number (NBFE) or...

Respirators, medical masks, and barrier face coverings all filter airborne particles using similar physical principles. However, they are tested for certification using a variety of standardized test methods, creating challenges for the comparison of differently certified products. We have performed systematic experiments to quantify and understand...

Tandem centrifugal particle mass analyzer (CPMA)-single-particle soot photometer (SP2) arrangements provide a measure of the non-refractory components of soot. In such a system, the CPMA is used to classify particles by mass-to-charge ratio (i.e., total particle mass mp), and the SP2 measures the mass of rBC (i.e., mrBC) within each particle. Data...

Respirators, medical masks, and barrier face coverings all filter airborne particles using similar physical principles yet are tested using a variety of variety of standardized test methods. To quantify and understand the effects of differences between the standardized test methods for N95 respirators (NIOSH TEB-APR-STP-0059 under US 42 CFR 84), me...

Gas flaring is a common practice in the oil and gas industry, where droplets of flowback water with varying levels of dissolved salts (mainly composed of sodium and chloride) often become entrained in the flared gas. In this study, we examine the mixing state of the aerosol produced by a laboratory flare with and without entrained droplets of sodiu...

Soot is an important material with impacts that depend on particle morphology. Transmission electron microscopy (TEM) represents one of the most direct routes to qualitatively assess particle characteristics. However, producing quantitative information requires robust image processing tools, which is complicated by the low image contrast and comple...

During a pandemic in which aerosol and droplet transmission is possible, such as the COVID-19 pandemic of 2020, the demand for face masks that meet medical or workplace standards can prevent most individuals from obtaining suitable protection. Cloth masks are widely believed to impede droplet and aerosol transmission, but most are constructed from...

A large-scale, laboratory turbulent diffusion flame was used to study the effects of fuel composition on soot size and morphology. The burner and fuels are typical of those used in the upstream oil and gas industry for gas flaring, a practice commonly used to dispose of excess gaseous hydrocarbons. Fuels were characterized by their carbon-to-hydrog...

The present study describes an improved inversion method for determining the two-dimensional mass distribution of non-refractory materials on refractory black carbon using a centrifugal particle mass analyzer (CPMA) and single-particle soot photometer (SP2) system. The novel approach is tested with several well-established regularization methods to...

Soot is a group of carbonaceous nanoparticles that contribute to climate change and can impact human health. The role soot plays in these scenarios depends significantly on their optical and transport properties, which are, in turn, largely determined by their size and shape. Transmission electron microscopy (TEM) remains one of the best ways of ac...

This note describes a new approach to identifying soot aggregates in transmission electron microscopy images using k-means clustering. As k-means generally performs poorly for images with noise or gradients, segmenting the image first requires image pre- and post-processing methods, including background removal, denoising, a measure of image textur...

The new approach here uses the k-means clustering algorithm, a kind of unsupervised machine learning. In this technique, we consider the Euclidean distance between image pixel values (whether those values correspond to grayscale images, colour images, of multiple layers that form a synthetic image) and a set of k mean values, where each mean has th...

During a pandemic in which aerosol and droplet transmission is possible, the demand for masks that meet medical or workplace standards can prevent most individuals or organizations from obtaining suitable protection. Cloth masks are widely believed to impede droplet and aerosol transmission but most are constructed from materials with unknown filtr...

Moving towards two-dimensional distributions of particle properties is important to the study of aerosol formation, aerosol climate impacts, and aerosols in material science. This paper builds on existing work to examine Bayesian or statistical approaches to inverting tandem particle mass analyzer (PMA) and differential mobility analyzer (DMA) data...

Incomplete combustion is the main source of airborne soot, which has negative impacts on public health and the environment. Understanding the morphological and chemical evolution of soot is important for assessing and mitigating the impact of soot emissions. Morphological and chemical structures of soot are commonly studied using microscopy or spec...

A pulsed gas Jet is imaged using the background-oriented schlieren technique to probe sensitivity to operating conditions, such as pressure ratio. Multiple inversion schemes will be considered to improve flow property estimates.

This paper provides a critical review of methods used to invert tandem measurements to determine the two-dimensional distribution of particle mass and mobility. We consider the performance of weighted least-squares analysis, Twomey-type approaches, a maximum entropy method, Tikhonov regularization (over a range of regularization parameters), and st...

Time-resolved laser-induced incandescence (TiRe-LII) is increasingly being used to characterize non-carbonaceous nanoparticles. However, there exist several measured phenomena, particularly on metal nanoparticles, that cannot be explained using traditional models. This paper shows that some of these phenomena may be due to errors caused by the Rayl...

This paper provides an overview of methods to evaluate transfer functions for the Couette centrifugal particle mass analyzer (CPMA) and aerosol particle mass analyzer (APM). The work first considers finite difference approaches to solving the partial differential equation governing particle motion, which represents an accurate but computationally-d...

The aerosol particle mass analyser (APM) and centrifugal particle mass analyser (CPMA) are devices used to characterize the ratio of a particle’s mass to its electromobility. This is accomplished by placing an electrical field across a gap between two concentric rotating electrodes. Centrifugal forces push the larger particles towards the outer wal...

In many time-resolved laser-induced incandescence (TiRe-LII) experiments, it is common practice to relate the intensity emitted by laser-heated nanoparticles to the detected LII signal through a factor (here called the intensity scaling factor, ISF) that includes the particle volume fraction and other parameters that may not be the focus of the ana...

Time-resolved laser-induced incandescence is used to infer the size distribution of gas-borne nanoparticles from time-resolved pyrometric measurements of the particle temperature after pulsed laser heating. The method is highly sensitive to aspects of the measurement strategy that are often not considered by practitioners, which often lead to discr...

Time-resolved laser-induced incandescence (TiRe-LII) experiments conducted on aerosolized Ag, commercial soot and Au nanoparticles energized with a 1064 nm laser pulse. Detected TiRe-LII signal from aerosolized Ag nanoparticles suggest that the observed signal could originate from electron neutral bremsstrahlung and not of incandescence origin. A n...

Aerosolized nanoparticles represent both great potential for the development of emerging technologies and one of the biggest challenges currently facing our planet. In the former case, aerosol-based synthesis techniques represent one of the most cost-effective approaches to generating engineered nanoparticles having applications that range from med...

This work investigates how the interatomic surface potential influences MD-derived thermal accommodation coefficients (TACs). Iron, copper, and silicon surfaces are considered over a range of temperatures that include their melting points. Several classes of potentials are reviewed, including two-body, three-body, and bond-order force fields. MD-de...

We present the first use of background-oriented schlieren (BOS) tomography as a combustion diagnostic. BOS imaging captures deflections of light due to gradients in the optical density of a variable index medium, such as a flame. Multiple cameras are combined to tomographically-reconstruct the refractive index, which can be related to key combustio...

Over the last two decades, considerable focus has been placed on improving the fidelity of TiRe-LII heating and cooling models, with the objective of improving the robustness of inferred aerosol parameters. While simple models fail to capture all the salient features of experimental cooling curves, more elaborate models that account for such featur...

Candidate LII measurement models presented in the literature cannot explain several commonly-observed phenomena in experimental data. A new model is proposed based on electron neutral bremsstrahlung emission and inverse neutral bremsstrahlung absorption during laser-induced incandescence on silver nanoparticles.

The thermal accommodation coefficient (TAC) defines the efficiency of energy transfer during gas-surface collisions and is one the leading uncertainties in TiRe-LII analyses. For soot, the TAC is conflated with unknowns in surface composition and reactivity; while, for synthetic nanoparticles the TAC is often coupled with synthesis, making it chall...

Mid-wavelength infrared (MWIR) cameras are increasingly used to obtain quantitative estimates of single-species hydrocarbon concentrations. However, in principle, it is also possible to quantify each component of a mixture using multiple MWIR cameras equipped with different filters. This paper presents the first attempt to do this, via MWIR images...

Multiple models of varying complexity have been developed to analyze TiRe-LII data, but often with little regard as to how model complexity affects the robustness of inferred results. This study applies Bayesian model selection to evaluate three competing soot sublimation models for time-resolved laser-induced incandescence (TiRe-LII), and demonstr...

Competing theories have been proposed to account for how the latent heat of vaporization of liquid iron varies with temperature, but experimental confirmation remains elusive, particularly at high temperatures. We propose time-resolved laser-induced incandescence measurements on iron nanoparticles combined with Bayesian model plausibility, as a nov...

Time-resolved laser-induced incandescence (TiRe-LII) data can be used to infer spatially and temporally resolved volume fractions and primary particle size distributions of soot-laden aerosols, but these estimates are corrupted by measurement noise as well as uncertainties in the spectroscopic and heat transfer submodels used to interpret the data....

Laser-induced incandescence (LII) models fail to reproduce some aspects of experimental trends. Recent experiments for high-fluence suggest that a laser-induced plasma is generated during the laser pulse. The plasma temperature is higher than the nanoparticle temperature. A new model based on plasma bremsstrahlung emission is added to the LII model...

This constitutes a software package demonstrating the use of the fluence curve expressions association with work published in the Optics Express entitled "Defining regimes and analytical expressions for fluence curves in pulsed laser heating of aerosolized nanoparticles" by Sipkens and Daun.

This paper presents a novel error model for TiRe-LII signals and illustrates how the model can be used to diagnose a detection system, quantify uncertainties in TiRe-LII, and characterize fluctuations in the measured process. Noise in a single TiRe-LII measurement shot obeys a Poisson–Gaussian noise model. Variation in the aerosol results in shot-t...

Gas distributions imaged by chemical species tomography (CST) vary in quality due to the discretization scheme, arrangement of optical paths, errors in the measurement model, and prior information included in reconstruction. There is currently no mathematically-rigorous framework for comparing the finite bases available to discretize a CST domain....

This software is an implementation of the general error model described in the work entitled "General error model for analysis of laser-induced incandescence signals" by T. A. Sipkens and coworkers, which simulates signals according to the described error model (sufficient for generating Figure 1 in the associated work) and demonstrates a fitting p...

Knowledge of the thermal accommodation coefficient (TAC) at high surface temperatures is critical in many applications, including sizing of aerosolized nanoparticles through time-resolved laser-induced incandescence (TiRe-LII). Unfortunately, an understanding of the fundamental physics that underlies this parameter is elusive. To this end, we carry...

Measurements of time-resolved laser-induced incandescence (TiRe-LII) are used to infer the size and concentration of nanoparticles, among other quantities-of-interest. Reliable models of nanoparticle heating and cooling are required to obtain accurate estimates from TiRe-LII data. Fundamental research into nanoscale heat transfer has resulted in a...

This paper examines the method be calculating temperatures and scaling factor, which is related to the volume fraction, from time-resolved laser-induced incandescence data on in-flame soot. A Bayesian analysis is used to estimate the probability densities for these variables, and to introduce prior information into the procedure in order to narrow...

Auto-compensating laser-induced incandescence (AC-LII) can be used to infer the particle volume fraction of a soot-laden aerosol. This estimate requires detailed knowledge of multiple model parameters (nuisance parameters), such as the absorption function of soot, calibration coefficients, and laser sheet thickness, which are imperfectly known. Thi...

Fluence curves are a powerful tool for understanding the mechanisms underlying nanosecond pulse laser heating of aerosolized nanoparticles, which is relevant to laser-induced incandescence (LII). This paper presents analytical expressions encompassing the entirety of the fluence domain considered in LII and uses them to formally define fluence regi...

This paper presents a comparative analysis of time-resolved laser-induced incandescence measurements of iron, silver, and molybdenum aerosols. Both the variation of peak temperature with fluence and the temperature decay curves strongly depend on the melting point and latent heat of vaporization of the nanoparticles. Recovered nanoparticle sizes ar...

Nanoparticle characterization is important in determining the functionality and environmental impact of nanoparticles. Laser-induced incandescence (LII) is an in situ diagnostic that estimates nanoparticle characteristics, including their size, thermal and optical properties, and volume fraction. In LII, aerosolized nanoparticles are heated to inca...

Auto-correlated laser-induced incandescence (AC-LII) infers the soot volume fraction (SVF) of soot particles by comparing the spectral incandescence from laser-energized particles to the pyrometrically inferred peak soot temperature. This calculation requires detailed knowledge of model parameters such as the absorption function of soot, which may...

With nanotechnology becoming an increasingly important field in contemporary science, there is a growing demand for a better understanding of energy exchange on the nanoscale. Techniques, such as time-resolved laser-induced incandescence, for example, require accurate models of gas-surface interaction to correctly predict nanoparticle characteristi...

While time-resolved laser-induced incandescence (TiRe-LII) shows promise as a diagnostic for sizing aerosolized iron nanoparticles, the spectroscopic and heat transfer models needed to interpret TiRe-LII measurements on iron nanoparticles remain uncertain. This paper focuses on three key aspects of the models: the thermal accommodation coefficient;...

This paper summarizes the results of Time-Resolved Laser-Induced Incandescence (TiRe-LII) measurements of iron nanoparticles in He, Ne, Ar, N2, CO, CO2, and N2O. The iron nanoparticles are formed in solution and then aerosolized with a pneumatic atomizer using various carrier gases, so the nanoparticle size is the same for each aerosol and the TiRe...

Conditional Source-term Estimation (CSE) obtains the conditional species mass fractions by inverting a Fredholm integral equation of the first kind. In the present work, a Bayesian framework is used to compare two different regularisation methods: zeroth-order temporal Tikhonov regulatisation and first-order spatial Tikhonov regularisation. The obj...

This paper describes the application of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used mainly for measuring soot primary particles, to size silicon nanoparticles formed within a plasma reactor. Inferring nanoparticle sizes from TiRe-LII data requires knowledge of the heat transfer through which the laser-heated n...

The capabilities of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used almost exclusively to measure soot primary particles, could potentially be extended to size aerosolized metal nanoparticles. In order to do this, however, it is necessary to characterize the thermal accommodation coefficient, α, which specifies th...