Aerosol Technology

My research consists of understanding particle synthesis methods (e.g. flame reactor), particle properties (e.g. size, morphology, chemical composition, and electric charge) as well as the mechanisms explaining their formation in the gas phase (nucleation, surface growth, oxidation, and coagulation), and processing (e.g. sampling, filtration) to develop technological applications. Such technological applications involve catalyzers, plasmonic materials for health treatment, sensors, coatings, drug-delivery carriers, among others.

1. About me and my research contributions

My name is José Morán. Since April 2022 I have been a Post-doc Associate at Prof. Chris Hogan’s Laboratory belonging to the Particle Technology Laboratory (PTL) of the University of Minnesota, USA. Previously, I was a visiting researcher at the Energy and Particle Technology Laboratory (EPTL) working with Dr. Reza Kholghy at Carleton University in Canada. I obtained my PhD in Physics in France, in November 2021 at INSA de Rouen and worked at CORIA laboratory, my advisor was Prof. Jérôme Yon. I conducted my undergraduate studies at Universidad Técnica Federico Santa María in Chile. I carried out a MSc thesis in the EC2G (energy conversion and combustion group) where my advisor was Prof. Andrés Fuentes. My research domain is aerosol technology with particular emphasis on multi-scale numerical simulations of particle formation in the gas phase, and experiments mainly involving nanoparticle characterization.

• My PhD thesis entitled “Improving the numerical simulation of soot aerosol formation in flames” (English and French) is available here. Its corresponding Supporting Material is available here.
• My MSc thesis entitled “Numerical study of the influence of primary particle polydispersity and overlapping on soot aggregate of nanoparticle morphology” (Spanish) is available here.

In addition, during my (early) career I have investigated the following subjects:

Aerosol particle size and electric charges (experimental)

• I measured the 2D distribution of size and electric charge of KCl particles aerosolized in a wind tunnel connected in-line to an electrostatic precipitator (ESP). In this context, particles get electrically charged by a corona discharge in the ESP and sampled downstream of this device. I formulated the mathematical problem to be inverted for particle measurements based on two different experimental setups: (1) using a tandem-DMA (differential mobility analyzer) method for particles within the 50 to 250 nm diameter, and (2) using a DMA-APS (aerodynamic particle sizer) for particles in the 0.3 to 20 µm diameter. I developed numerical codes for conducting the inversion of these measurements. Also, I modeled the particle size-charge distribution based on a population balance model taking the field- and diffusion-charging into account. In this work, I observed that the measured unipolar charge distribution was accurately fitted by a Gaussian distribution, consistent with numerical simulations. In addition, I developed a theoretical explanation for this observation. See this paper for further details.

van der Waals interactions between nanoparticles (theoretical & numerical)

• Van der Waals (vdW) interactions may play a prominent role in a wide range of phenomena involving aerosol particles including coagulation, resuspension, restructuring, fragmentation, filtration, and impaction, among others. However, their understanding is currently limited especially when dealing with particles with complex morphology such as fractal-like agglomerates. In this work, we have studied the vdW interactions between molecule-agglomerate and agglomerate-agglomerate involving nanoparticles. We have introduced coarse-grained analytical and semi-analytical models for such interactions. See this paper for further details.

Bioaerosol filtration (experimental)

• Bioaerosols are aerosol particles of biological origin including viruses and bacteria. In particular, we have conducted experiments in collaboration with the Veterinary School of the U. of Minnesota (Prof. Montserrat Torremorell’s group) with viruses relevant to the swine industry notably PRRS (Porcine reproductive and respiratory syndrome) virus and influenza. We have also done experiments with bovine coronaviruses. We have recently published a review of existing and emerging control technologies for bioaerosol filtration in livestock housing. See this paper for further details. The image below shows a PRRS virus (From this website).

Aerosol particle coagulation (theoretical & numerical)

• I modeled the time between collisions of aerosol particles experiencing Brownian motion as a first-time passage (FTP) phenomenon. This FTP is shown to follow an exponential distribution parametrized by a collision frequency k consisting of the product between the particle collision kernel and number concentration. This explains why coagulation can be modeled as a Markov chain (as done in Monte Carlo Population Balance approaches), and why previous works were able to numerically obtain collision kernels based on FTP Langevin Dynamics simulations. This new theoretical development leads also to an analytical enhancement factor for aerosol particle coagulation experiencing diffusive movement at high concentrations (i.e. when the particle volume fraction is roughly >1000 ppm which is relevant in Aerosol Technology). In this high concentration limit, the classical Smoluckowski collision kernel fails to predict the actual particle collision kernel as the latter becomes a non-linear function of the particle concentration which is analytically shown in our work. See this paper for further details.

• I derived an equation for approximating the coagulation kernel of suspended particles valid in the transition of particle-particle interaction regime from a theoretical approach based on the Langevin equation. The main novelty of the work is the analytical link between the Langevin equation and particle coagulation kernels in the transition regime. In this work I also conducted population balance simulations of aerosol particle coagulation and the new method reveals a likely universal asymptotic limit (long times) for the kinetics of coagulation. See this paper or see this presentation for further details.

• I have introduced a unified self-preserving size distribution (generalized Gamma distribution) to describe particles formed during coagulation. I found the simultaneous transition in the particle-flow regime (from free molecular to continuum) and particle-particle regime (from ballistic to diffusive) to explain a systematic change in agglomerate morphology when increasing the size of primary particles (similar observations were only made experimentally before). See this paper for further details.

New tools to simulate agglomerates and agglomeration (numerical)

• FracVAL: It is a tunable code to generate fractal-like agglomerates with prescribed fractal dimension, fractal prefactor, number of primary particles, and primary particle polydispersity. It is the first code able to take the polydispersity of primary particles into account in a rigorous way for a wide range of fractal parameters. It has been highly appreciated by different research groups around the world and the code has been used for particle-light interaction, particle hydrodynamics, morphology, and particle technology applications. The new version of this code includes raspberry-like agglomerates and a self-avoiding random walk model for protein morphology. It will also include a module for numerical TEM image generation as I explain in this presentation and will be available through this GitLab repository. In the same article, coworkers and I introduced a volume-based pair correlation function to describe the morphology of agglomerates of nanoparticles. This method was adapted to more complex aggregates during my PhD thesis. This method has also been extended by researchers interested in the morphology of liquid atomization particles (see the citations of FracVAL paper). The original code (in Fortran) is publicly available here.

• MCAC: The Monte Carlo Aggregation Code is a discrete element modeling (DEM) approach to simulate nanoparticle coagulation. I discovered an optimized way to solve the Langevin equation, which has some similarities with the DSMC used to solve the Boltzmann equation in fluid mechanics. I derived a new particle persistent distance (also called stopping distance) and its corresponding time-step to have good agreement with Langevin Dynamics. I validated the method theoretically based on a description of particle dynamics as random walks derived from a binomial statistical approach. See this paper for further details. This code is available through CORIA-CFD GitLab.

Simulating flame-made particle formation (numerical)

• Thanks to a collaboration with researchers from Chile and Canada I was able to simulate the formation of soot particles under ethylene diffusion flame conditions. In this work, I coupled a DEM code (my main work) with a macroscopic or population balance code that also solves the conservation of mass, linear momentum, and energy of the flame. This is the first time such a coupling is done. We also consider most of the relevant mechanisms of soot particle formation in flames including nucleation, surface growth, coagulation, and oxidation. The morphological aspects predicted by the population balance code are remarkably different from those predicted by the DEM code. In addition, these particles are considerably different (in size, and morphology) depending on the zones in the flame where they are formed notably those formed along the centerline are generally smaller and less ramified than those agglomerates formed along the wings of the flame. See this paper for further details.

• I have conducted DEM simulations of particle formation in flames. I have explained the overlapping of primary particles formed in flames. I explained the effect of surface growth on coagulation. I introduced models to correct aggregates’ volume and surface area based on average primary particle overlapping and coordination numbers. In this work, I was also able to simulate realistic soot particles that highly resemble those observed in experimental TEM images and was able to obtain a detailed morphological characterization of the simulated aggregates under 2 different flame conditions. See this paper for further details.

• One fundamental question regarding flame-made particle formation is related to the unitary sticking probability of particles upon collisions which has been questioned in the literature. I have introduced a potential energy criterion to explain the transition from reaction-limited (sticking probability «1) to diffusion-limited (sticking probability =1) nanoparticle coagulation regime at high temperatures. Particularly, I focused on a new approach to determine the rebound probability for fractal-like agglomerate collisions. This is the only work in the literature taking the van der Waals and electrostatic interactions into account in DEM simulations of nanoparticle formation under flame conditions. This is also the only work considering the evolution of soot maturity (here mainly manifested through a change in chemical composition and bulk density) into the coagulation kinetics. See this paper for further details.

Characterization of flame-made particles (experimental)

• I have conducted line-of-sight attenuation (LOSA) measurements of soot volume fraction to decide on the particle sampling points in the flame. Once coworkers and I decided on these points, I conducted the thermophoretic sampling experiments and analyzed the data for comparison with the time-resolved light-induced incandescence (TiRe-LII) technique (mainly carried out by coworkers). I reduced uncertainties in the thermophoretic sampling technique based on the LOSA technique to monitor the sampling in-situ and resolved in space (mm scale) and time (ms scale). This is explained in detail in this presentation. See this paper for further details.

• I have analyzed a large database of flame-made particle transmission electron microscopy (TEM) images (sampled by coworkers) to obtain morphological parameters of soot particles including their gyration diameter, number of monomers, and projected area, among others. Here we studied three different fuels namely ethylene, propane, and butane under different oxygen index conditions. Soot particles observed in these images have generally a fractal-like structure, a primary particle diameter between 5 to 60 nm, a primary particle polydispersity commonly below 1.3 in geometric standard deviation but some cases show larger values. Aggregates gyration diameter polydispersity is much larger and can be in the order of 2.5 in geometric standard deviation. See this paper for further details.

• Coworkers and I have introduced a horizontal planar multi-angle light-scattering (HPALS) approach for measuring flame-made particle properties in-situ in a diffusion flame. We used a horizontal laser plane (532 nm wavelength) to avoid any volume-of-measurement bias in the data when analyzed at different angles. I conducted the experimental measurements jointly with the first author and also developed the codes for post-processing the raw data including the deconvolution of measurements to obtain radially-resolved data. In this work, we have conducted a highly spatially-resolved (51 pix/mm) description of particle properties including their gyration diameter (the main property measured) as a function of the height above the burner and at different radial positions. See this paper for further details.

• Coworkers and I have measured in-situ the evolution of soot particle maturity characterized by the Amströng or dispersion exponent (linked to the power-law exponent of the absorption function E(m) as a function of the wavelength) by combining LOSA and emission measurements at four different wavelengths (500, 532, 660 and 810 nm). Thanks to the accurate measurements conducted at the EC2G group (mainly by coworkers) we were able to reveal a detailed map of soot maturity in the flame. In this paper, we also made an effort to review different methods for soot maturity characterization considering organic, graphitic, and amorphous soot particles as shown in the figure below. See this paper for further details.

Fractal-like agglomerate morphology and radiative properties (theoretical and numerical)

• I have contributed to the development of a generalized model for agglomerates of nanoparticle morphology (volume-based pair-correlation function) that explains the transition of the structure factor from a single primary particle to fractal-like agglomerates. Jointly with the first author, we developed the equations of this work, where I particularly contributed to characterizing the pair-correlation function of primary particles and I conducted the numerical simulations for comparison and using the theoretical equations of this work. This article was selected as cover in the Journal of Aerosol Science and is available here.

• We investigated the role played by primary particle polydispersity on the radiative properties of fractal-like agglomerates of nanoparticles. We compared Discrete Dipoles Approximation and the Generalized Multiparticle Mie methods. We thus obtained corrective factors to incorporate into the Rayleigh-Debye-Gans for fractal aggregates (RDG-FA). In addition, we observed an accurate and direct power-law between the radiative force and the aggregate’s volumetric effective-radius. See this paper for further details.

• I have investigated the effect of primary particle polydispersity on the morphological properties of fractal-like aggregates derived from numerical TEM images. I showed analytically this effect based on both Gaussian and Lognormal primary particle size distributions. I explored the Hough Transform algorithm for primary particle detection and the relative-optical-density method to infer the number of monomers from TEM images based on the gray intensity of the images. See this paper for further details.

• Coworkers have characterized soot particles emitted from different sources (e.g. industrial sites and power plants) and compared their measurements with numerically simulated aggregates. The latter was my work using FracVAL. See this paper for further details.

Others (numerical)

• Firstly, this topic is out of the domain of aerosol technology but is still part of my career so I decided to include it. Secondly, in this work, we intend to explore the economical performance of a small-scale wind farm for residential clients. The idea was to bring a quantitative assessment of the convenience of such projects to guide the public policy (taking increasing and big popularity) in renewable energy for Chile. See this paper for further details.

2. Numerical tools

Numerical codes for measurements, post-processing experimental data, and different numerical simulations are provided. The latter include population balance simulations of aerosol coagulation, aerosol particle charging, fractal-like agglomerates, Monte Carlo DEM simulations, and Langevin Dynamics are provided in the following page.

3. Teaching and tutorials

I would like to share teaching material related to aerosol science. For the moment I am sharing a few video tutorials related to aerosol metrology through the following page.

4. Scientific presentations

Rather than only providing a list of presentations I want here to share some PDF versions and videos for some of these presentations for the people interested in my work. Personally, I like to see presentations before reading papers because they give a big picture of a subject and summarize the content. Therefore, my presentations are available on the following page.

5. Outreach

I have contributed to the organization of a few research conferences with particular emphasis on Aerosol Technology and flame-made particles:

• 2023: Collaborated on the Aerosol Physics topic as chairman in one session of the 41st AAAR conference in Portland OR.
• 2021-Present: Collaboration in the Aerosol Technology working group for the European Aerosol Conference (EAC) and International Aerosol Conference (IAC). This includes EAC 2021, IAC 2022, and EAC 2023.
• 2020: Led the organization of the 1st French PhD students conference on soot particle research.

6. Photo and video gallery

We cannot make a research website without a gallery of pictures and videos! This is available on the following page.

7. Acknowledgment

I would like to thank all the people I have worked with throughout my career so far, including the professors and other people who may not be present in the pictures below. Thank you all!

8. Contact

If you are interested in my work or have a question using the material provided on this website or if you have feedback to improve codes or experiments, do not hesitate to contact me:

• Linkedin
• Researchgate
• Google Scholar