My research takes advantage of the tremendous opportunities offered by microfluidics to isolate and investigate the challenging multi-scale and multi-physics processes that occur in porous media. My microfluidic experiments are coupled with high-resolution imaging, cutting-edge metrology, and numerical simulations. This allows precise control, direct visualization, and accurate quantification of flow, transport, and reaction mechanisms at the microscale.

Because of the complexity and heterogeneity of large-scale systems, a mechanistic understanding of the processes at small scales is often needed to further develop integrated predictive modeling approaches from the micro to the macroscale. Moreover, the equations of continuum (large) scale models should be rooted in a correct description of pore scale processes. My research cuts across different domains: to understand blood flows in the microcirculation, to measure ionic currents through carbon nanotubes for selective ion transport, and to improve the knowledge of the pore-scale processes associated with the sequestration of CO2 into geological reservoirs, and the remediation of contaminated aquifers.

At the moment, my goals are to improve the estimation of storage capacity in geological reservoirs, reduce the footprint required to sequester CO2, assess the long-term stored CO2 behavior, and shorten the time to clean up contaminated aquifers.

Multiphase and reactive transport in porous media: microfluidics experiments

My interest in geological porous media started in 2013 when I joined the Department of Energy Resources Engineering at Stanford University. I used microfluidics to bring new insights into the debates that animate the subsurface scientific community for decades including the controversial use of Darcy’s law for modelling multiphase flow and the understanding of the complex feedback between hydro-bio-chemical systems. I developed advanced micro-Particle Image Velocimetry (micro-PIV) during micromodel experiments to measure velocity profiles with an unprecedented high resolution. My experiments allowed the identification and characterization of dissipative mechanisms that open new lines of research and explain long-standing discrepancies in the literature associated with geological CO2 sequestration and Enhanced Oil Recovery (Roman et al. 2017, 2020). My high-fidelity datasets are solicited by computational scientists who need to benchmark their models. That led me to start a fruitful collaboration with numerical modelers that resulted in the development of computational microfluidics for geosciences (Soulaine, Maes, and Roman, 2021). This new discipline intends to complement and augment microfluidic experiments. Using this combined experimental-numerical approach, we proposed the first validated two-phase reactive transport simulator at the pore-scale, and we derived scaling laws for the dissolution rate at the reservoir-scale.

snap-off.png

Single phase flow in sandstone micromodel

fluid: water seeded with microparticles

 

Micro-PIV measurements (Particle Image Velocimetry): exact velocity distribution at the pore-scale

Two-phase flows in sandstone micromodel
Movies here

We investigate flow instabilities during two-phase flows in porous media. Micro-PIV measurements allow us to get new insights onto fluid flow velocity fields at the pore-scale. 

Flow of Red Blood Cells in microchannels

My PhD research dealt with the study of blood flows in the cerebral microcirculation, which have implication in various pathologies impacting the microvascular architecture: hypertension, diabetes, Alzheimer, cancer... In the microcirculation, vessel sizes are similar to the size of a red blood cell. Thus, the dynamics of blood flows is particular at this scale. At microvascular bifurcations, a non homogeneous distribution of red blood cells and plasma is observed. This research attempts to clarify these distribution mechanisms. A microfluidic device has been developed in order to investigate flows of concentrated suspension of red blood cells. I first studied the metrological aspects specific to concentrated suspensions. Thus, various techniques have been developed and validated for the measurement of velocity fields, concentration and flow rates. In particular the dual-slit technique allows the measurement of red blood cells velocity profiles in microchannels with high resolution and accuracy (Roman et al, Microvasc. Res., 2012). With the dual-slit technique we are now able to measure the slip velocity of red blood cells at channel walls, which is not possible with micro-PIV techniques. The techniques developed now allow to explore different regimes depending on the size of the microchannels and to perform a parametric study of the phase separation effect at microvascular bifurcations.

In particular, this work has brought methodological protocols for the study of biological objects in microfluidics and quantitative results on the dynamics of blood flows. Movies here.

Flow of Red Blood Cells at a microbifurcation

Microdevices integrating individual carbon nanotube

Nanofluidics is the study of fluids confined into structures for which the dimensions are in the order of the size of large biomolecules such as proteins or DNA. This is also the size of the electrical double layer present on each surface immersed in an aqueous solution. There is an interest in developing a wide range of devices that will benefit from the unique behavior of liquids under confinement. We are especially interested in the study of the confinement of ionic bio-channels inside a carbon nanotube for selective ion transport. For that purpose, we develop a microfluidic device incorporating carbon nanotubes. At the same time, the confinement of ionic bio-channels inside carbon nanotube is investigated. This microdevice allows the characterization of the ion transport through carbon nanotubes with and without incorporated ionic bio-channels. This work has been done at the Laboratoire Charles Coulomb (Montpellier).