Meet the project - 'Mixing by Interfaces: How Does Water Infiltration Control Mixing and Reaction in Soils?'

Every time it rains, something remarkable happens beneath our feet. As water seeps into the soil, it sets off a cascade of chemical and physical processes that determine how nutrients, contaminants, and life itself move through the ground. Yet despite its ubiquity, scientists still know surprisingly little about what happens during these moments of infiltration — especially at the tiny scales where air, water, and soil meet.

Gaute Linga, who leads the Young CAS project “Mixing by Interfaces: How Does Water Infiltration Control Mixing and Reaction in Soils?”, has now begun his two-month research stay at CAS. Together with his team, he is uncovering how shifting water fronts influence the way substances mix and react underground — with the goal of redefining what we know about one of nature’s most essential elements: water.

“The main objective of the project is to describe and explain how dynamic water infiltration fronts control mixing and reactions in soils,” Linga explains. “In the process, we aim to answer related research questions, including: Do moving fluid interfaces enhance mixing and reaction? And what are the key statistical properties on small scales governing effective laws at larger scales?”
 

Water: Beautiful, Complex, and Essential

Linga’s fascination with water began long before this project. “Water is an essential compound for life on Earth, but apart from that I have always found it extremely fascinating, beautiful and multi-faceted,” he says. “From the fast motion of turbulent eddies in a river to the slow motion of droplets sliding down your window on a rainy day.”

What particularly captures his curiosity is how water and air interact in porous materials such as soil. This mix of elements behaves in ways that cannot be predicted from the microscopic laws of physics alone. “Although water infiltration fronts occur everywhere in the soil below us — for example, every time there is rainfall — we know very little about how they affect chemical processes,” he adds.
 

Peering into the Soil, One Pore at a Time

To tackle these questions, the project will combine theory, numerical modelling, and experiments in an unusually integrated way. The team plans to take what Linga calls “a physicist’s approach,” using minimal models that simplify the complexity of real soils while preserving the key processes.

“We will consider synthetically generated two-dimensional flow domains where cylindrical obstacles represent the solid grains of the soil,” Linga explains. “This allows for reproducible flow settings that are comparable between simulations and experiments.” To isolate the effect of mixing itself, the team will also consider simple, fast reaction kinetics — a controlled way of examining how chemical reactions respond to the movement of water through soil. By isolating specific mechanisms — such as how moving water interfaces stir up chemical mixtures — the researchers hope to identify universal features that will hold true even in more complex, realistic settings.

Their work unfolds on three fronts: improving simulation methods, modelling the merging of fluid clusters as a stochastic process, and characterising how mixing and reaction behave on larger scales where the fine details blur together. The outcome will be unique numerical data and theories revealing how air–water interfaces drive mixing and reaction — data that will be shared openly with the scientific community.
 

Collaboration at Its Core

For Linga, CAS provides the perfect environment for this type of close collaboration. “CAS is a wonderful arena for this type of research because it allows us, a group of scientists specifically tailored for the problem, to sit together and work directly on the same problem,” he says.

The project brings together a small but complementary group of researchers: theoreticians, computational scientists, and experimentalists. “Since the group is diverse, the core of the work will be to implement numerical models, analyse simulations and experimental data, and develop predictive theories together,” he says. “In particular, if possible, we will apply the same analysis and theoretical ideas to numerical and experimental data to investigate whether they agree or are complementary.”

Linga chose his team carefully. “The core idea for the project lies right in the middle of the research topics of Tomas Aquino, who runs an ERC Starting Grant related to chemical reactions in soils, Marcel Moura, an experimentalist and specialist on infiltration fronts, and myself,” he explains. “In addition, we have selected people with complementary skills within simulation methods for solute transport and multiphase flow, experimental data analysis, stochastic processes and modelling.”
 

Driven by Curiosity

The project exemplifies CAS’s mission to support curiosity-driven research — work that deepens our understanding of the world rather than chasing immediate applications. As Linga puts it:
 

Basic research allows us to ask questions and seek answers without focusing on direct payoffs. Most technological breakthroughs that shape today’s society ultimately resulted from curiosity, not from the need to solve immediate problems.

Looking Ahead

By the end of their stay at CAS, Linga and his colleagues aim to establish new theoretical frameworks for predicting how mixing and reactions occur during water infiltration. The work could inform fields ranging from hydrology and environmental science to engineering and climate modelling. The team also plans to make their unique simulation and experimental data publicly available, paving the way for further advances across disciplines where solutes and multiple phases interact.
 

The main outcome from this project will be new models to understand and predict mixing and reactions in soils, in particular in the context of dynamic, time-dependent flows,” Linga says. “It will open up possibilities to study more complicated versions of our problem with more realistic geometries, chemistry, and flow conditions.

As the team dives into this intricate world of moving interfaces, their findings could reshape how we understand the hidden dynamics beneath the Earth’s surface — and how water, the most familiar of substances, continues to surprise us.

It is a pleasure to have Gaute Linga and his team at CAS this term!