Tracing the flow of colloidal particles in the subsurface Colloidal particles could be used to remove contaminants from underground aquifers, but getting them to the affected area is a complex task. Dr. Sophie Roman and her team in the TRACE-it project are investigating how colloidal particles flow through geological environments, which could in future lead to methods of effectively driving them to a particular region of interest. The aquifers beneath our feet hold large quantities of groundwater, which are often a primary source of drinking water, yet they are not insulated from the environment above ground, and contaminants can seep into the soil and filter downwards. In these circumstances colloidal particles, essentially a suspension of particles within a fluid, can be used to remediate groundwater supplies and maintain water quality. “Colloidal particles can be used in subsurface environments to remove contaminants,” explains Dr Sophie Roman, Associate Professor at the University of Orléans, working in the Porous Media Research Group of the Institut des Sciences de la Terre d’Orléans. This first requires a deep understanding of the flow of colloidal particles however, a topic at the heart of Dr. Roman’s work as Principal Investigator of the ERC-backed TRACE-it project. “We want to see how we can actually drive these particles, these colloids - between tens of nanometres and a micrometre in size - towards a particular region of interest. This might be contaminants in an aquifer, or a dissolving fracture in rock,” she outlines. Depending on the remediation strategy, these particles may be engineered metal nanoparticles, naturally occurring mineral grains, or even living bacteria. “Geological environments are made of grains, voids, and fractures, forming what are called porous media. In such complex structures, they are many different paths and directing particles to a specific region is not easy.”
From rivers and aquifers at the field scale to tiny interconnected pores between grains at the pore scale: a porous medium is a material (like sand or rock) whose connected voids store and transmit water underground. Image credit Noura Eddaoui
region of interest, now as part of her role in the project Dr. Roman is investigating how it works in geological porous media. With diffusiophoresis, particles move due to differences in concentration gradients of dissolved species. “If colloids are in a straight channel for example, with a high salt concentration on one side and a low salt concentration on the other, the colloids will then move to the side with a higher or lower concentration, depending on the properties of the salts and the properties of the particles,” explains Dr. Roman. The idea is to use concentration gradients of different species to essentially drive colloids in a particular
direction. “Diffusiophoresis is a specific mechanism to displace colloids,” continues Dr. Roman. “In the project we are conducting microfluidics experiments, considering models of spherical particles, of a fairly regular size. We aim to replicate - on a microfluidic chip the geometry of porous media, the network of channels that can be found in aquifers and reservoirs, which typically have a diameter around the size of a human hair. So we’re doing this research at a very small scale.”
Microfluidics Developments for Geosciences
a substrate and covered by a transparent material, then when the chips are placed under a microscope, researchers can directly visualise the flow processes within porous media. Dr. Roman and her colleagues are looking to gain deep insights into the nature of the flow and the behaviour of colloidal particles from these experiments. “We are developing techniques to measure flow velocities in these experiments, and to get access to certain types of chemical information. We are continuously developing new methods in our team, we want to probe the properties of chemical gradients,” she says. Image processing and spectroscopy techniques are also being used in the project to build a deeper picture. “The way light interacts with our materials gives us information about its chemical composition. This is quite effective in detecting certain minerals with different chemistries,” explains Dr. Roman. “We also want to adapt this technique to detect species in solution, that are dissolved in the fluid, thus probing chemical gradients in situ and in real time. When water is in contact with minerals some of them may dissolve in the solution. This gives you water that is particularly rich in calcium, magnesium or other components.” The project team has also developed the first geo-electrical monitoring method on microfluidic chips. Geophysicists relate electrical signals to physico-chemical processes in the soil and subsurface, now in the TRACE-it project Dr Roman plans to use them to probe concentration gradients at the pore-scale. “We’re working to improve different microfluidic methods, which can then be adapted for a variety of applications,” she continues.
Diffusiophoresis is the motion of charged particles (10 nm–10 µm in diameter) induced by solute concentration gradients. Depending on the properties of the solute (e.g., salts, organic compounds) and of the particle surface, the motion can occur toward either higher or lower concentrations.
The Wider Aim The wider aim in the project is to include diffusiophoresis in models to describe the transport of particles in porous media, which is typically neglected in current models. The importance of the mechanism is likely to
colloids, sometimes the velocity of water may be more important, there are still unanswered questions in this area,” says Dr. Roman. “There will also be some cases where we cannot ignore diffusiophoresis, where it will actually lead to the deviation of particles from a particular path.”
“We want to see how we can actually drive these particles, these colloids, towards a particular region of interest. This might be contaminants in an aquifer, or a dissolving fracture in rock.” vary in different situations. “We can see that diffusiophoresis will not have a big influence on particle transport in some types of system, as generally, the velocity of particles due to diffusiophoresis is quite slow. Water velocity in the subsurface is also quite slow, but greater in most cases than the velocity due to diffusiophoresis. In some cases diffusiophoresis might be an important mechanism to displace
Applications of colloids This research supports the longer-term goal of using colloidal particles for specific engineering applications, such as remediating groundwater, or limiting the dissolution of minerals. As part of the TRACE-it project researchers have published a paper on mineral dissolution, which Dr. Roman says holds important implications. “When a
This network of channels is etched onto Left: A microfluidic chip with integrated electrodes used to monitor, through geo-electrical measurements and direct visualisation, the dissolution of a calcite grain inside a microchannel.
Diffusiophoresis A mechanism called diffusiophoresis can be used to essentially drive particles to a Geo-electrical monitoring under the microscope. Images credit: Flore Rembert
The microfluidic chip is observed under a microscope to watch the chemical reaction in real time, while a Spectral Induced Polarization device measures the accompanying electrical signals.
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When acid is injected into a microchannel containing a calcite grain, the grain gradually dissolves. If particles are injected as well, they may form a passivation layer around the grain, slowing or inhibiting its dissolution.
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Diffusiophoresis: chemical gradients generated by dissolution drive colloids for CO2 storage integrity and groundwater remediation.” Image credit: Florian Cajot.
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