Yannick Hallez

Associate Professor
Colloids - Interactions - Electrostatics - Simulation

Associate Professor at the University of Toulouse (UT) in the Chemistry Department and at the Chemical Engineering Laboratory (LGC) since 2010, with an expertise in colloidal interactions, transport phenomena in colloidal dispersions, electrostatics, simulations, and high-performance computing.

I work on particle-particle and fluid-particle interactions in colloidal suspensions and their consequences at larger scale: self-assembly or driven assembly, structuration as a dispersed phase, thermodynamics and rheology.

Research

Meso-scale simulations

I develop and work with several simulation tools for the modeling of electrostatic, van der Waals, and hydrodynamic interactions in colloidal dispersions.

Particle-Field Brownian Dynamics
I develop a code allowing to study the structuration and thermodynamics of dispersions of charged nanoparticles. This is a meso-scale approach in which ion distributions are treated at the mean-field level but colloids are followed explicitly with Brownian Dynamics. No effective interaction potentials between colloids are used. Instead, at each time step of the Brownian Dynamics solver, the (modified) non-linear Poisson-Boltzmann equation is solved in the complex fluid domain delimited by the surfaces of colloids and the Laplace equation is solved inside the colloids. Many-body electrostatic forces are then determined for arbitrary particle geometries by numerical integration of the excess osmotic stress tensor. With this method, effective interaction potentials can be measured, possibly including charge regulation, and equations of state or meso-structures can be computed.

Accelerated Stokesian Dynamics (ASD)

I contributed to implementing ASD for clusters in collaboration with Jeff Morris (CCNY).

Atomic-scale simulations and machine learning

The mean-field simulation tool described above cannot account for ion correlations. Thereore, I also use Molecular Dynamics / Monte Carlo methods from LAMMPS for the fine description of ion distributions near charged interfaces. I am now working on strategies to couple these atomic-scale simulations to the meso-scale solver with machine learning tools. It will allow calculating structures at the meso-scale, with ion correlation effetcs included, while avoiding the numerical cost of performing primitive model simulations at the meso-scale.

Colloidal assembly driven by electrostatics

Structured assemblies of colloidal particles can be formed on a substrate once the latter has been imprinted with electric charges. This is possible with AFM nanoxerography or electric micro-contact printing. Charged particles can be assembled onto structured patches carrying an opposite charge exploiting electrophoresis, generally leading to monolayer deposits. Neutral particles can be assembled in multi-layer structures using dielectrophoretic forces. I performed several calculations of such assembly processes during collaborations with L. Ressier and E. Palleau from LPCNO who conducted these nice experiments.

Microfluidic osmotic compression

First chips in PDMS have been designed by Jean-Baptiste Salmon and coworkers to perform microfluidic compression experiments at the nano-liter scale, allowing the measurement of colloidal equations of state in a few hours instead of weeks. We have then adapted the design, using OSTEMER chips, to perform operando SAXS meso-structure measurements.

Teaching

I teach mainly in the Chemistry Department of the Faculty of Sciences and Engineering (FSI) at University of Toulouse (UT), at the BS and master levels, in the Process Engineering track and in the Erasmus Mundus Master in Membrane Engineering for Sustainable Development (MESD). I am also local coordinator of the MESD. I teach mainly Transport Phenomena, Colloid and Interface Engineering, Numerical Tools for Science in Python, and Computational Fluid Dynamics. I developed some labs and projects in which machine learning is coupled to a physical device with real-time measurements to illusrate the concept of numerical twins.

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