Yannick Hallez

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.

Interaction modeling at the mesoscale

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

In particular, I built a massively parallel Particle-Field Brownian dynamics simulator for the study of the structuration and thermodynamics of dispersions of charged nanoparticles. Rather than using effective interactions between particles (often of the DLVO type), electrostatic forces are determined through a calculation of the 3D electric field generated by the charges present on all the simulated surfaces at each time step. This field is computed as the solution of the coupled Laplace and non-linear Poisson-Boltzmann equations. This allows for the calculation of the many-body forces exerted on particles for arbitrary particle and bounding surface geometries, without any pairwise additivity assumption. The image on the right is a simulation snapshot showing the electrostatic potential on the surface of clay-like nanodisks carrying a positive rim charge and a negative face charge.

Hydrodynamic interactions are also of interest, and are computed with the Accelerated Stokesian Dynamics (ASD) method in collaboration with Jeff Morris (CCNY). I’m also working on a coupling between the ASD and the Particle-Field method described above for electrostatics.

 I also contribute to the extension of the Accelerated Stokesian Dynamics (ASD) method for the calculation of hydrodynamic interactions between non-spherical objects, in collaboration with Jeff Morris (CCNY). On the left, the formation of tactoids is observed in a suspension of platelets under strong shear (V. Labalette’s Ph.D.). The accurate calculation of hydrodynamic interactions allows a reliable prediction of the suspension rheology and of its structuration kinetics.

Macroscopic description of transport phenomena in complex fluids

Fluids used in chemical engineering often involve several constituents which can interact: wet granular media, nanoparticle suspensions, emulsions, polymers… Transport phenomena in these complex fluids may depart significantly from what is predicted by the classical Fick, Fourier, and Newton-Stokes laws with constant transport coefficients. I work on the determination of the closure relations linking fluxes to macroscopic fields based on particle-scale simulations and on their testing at the macroscopic scale with CFD-like simulations and experiments. The image above shows the colloid concentration field in an evaporating suspension calculated at the macroscopic scale. The collective diffusion coefficient is obtained from thermodynamic data computed with integral equation theories and effective potentials accounting for surface charge renormalization. The results of this kind of simulations is compared to experimental data obtained in microfluidic devices in collaboration with Jean-Baptiste Salmon (LOF).

Research applications: coating, filtration, functional materials, self- and driven assembly, understanding and optimization of transport phenomena.

Research supervision:

– 5 PhD students: Lisa Guilbaud (2021), Cécilia Gestraud (2019), Vincet Labalette (2019), Yiannis Gergianakis (2016), Joseph Diatta (2014)

– 2 post-docs : Alexis Praga (2017), Florent Girard (2017)

Stays as a visiting researcher:

– Two weeks at the Levich Institute at the City College of the City University of New York with Jeff Morris.

Teaching activities: My teaching activities are in the Chemistry department of Université Toulouse III – Paul Sabatier, at the BS and master levels, in the Process Engineering master and in the Erasmus Mundus Master in Membrane Engineering. I teach transport phenomena, colloid and interface engineering, numerical methods for fluid flows and heat and mass transfer, python programming, transport phenomena labs, chemical engineering labs (distillation, absorption, filtration, extraction). I developed several pedagogical simulation tools with graphical interfaces to help students understand some physical concepts such as dynamic similarity, the link between Brownian motion and diffusion, wave propagation and resonance…

Collaborators: Martine Meireles and Patrice Bacchin (LGC Toulouse), Paul Duru (IMFT Toulouse), Etienne Palleau and Laurence Ressier (LPCNO Toulouse), Jean-Baptiste Salmon, Jacques Leng and Emmanuel Mignard (LOF Bordeaux), Jeff Morris (City College of New York), Gregor Trefalt (University of Geneva).