If you look at a rain drop rolling down a car’s windshield you will see that it frequently leaves behind a thin water trail. As the drop moves down the glass, the area just behind it does not get immediately dry: a thin water film persists, sometimes even for a long time after the drop passes by. If the glass surface were not so smooth, such films would remain for much longer, and this is precisely what happens inside porous media like natural soils and rocks. When a wet portion of the soil gets dry, say after some hours of sunshine following a storm, thin liquid films remain on the surface of the soil grains. On a windshield, those liquid films are already quite interesting to look at, but inside a porous medium they can do so much more: they can interconnect different parts of the soil, like a whole set of water bridges forming a large network of water streets and avenues. This analogy may seem a bit far-fetched but it is quite descriptive: just like our streets, the water films act as an invisible transportation network in an invisible water city. Plant roots can use this network to obtain nutrients from far away, but pollutants can also take a high-speed road to spread quickly in the soil. In this project, we will study the network of interconnected liquid films inside a porous medium. Unlike the windshield, natural soils are not transparent, so we will work with a custom-built porous medium made of glass. We will study the formation and stability of the liquid films network and how to design the system to obtain the most efficient transportation network. Our experiments will be backed up by computer simulations and theoretical modeling based on network theory, the same branch of mathematics used to describe the traffic of cars in a city. This project will give us a better understanding of how liquids are transported inside natural soils and how this knowledge can be used to mitigate the effects of pollution spreading.
The flow of liquids and gases inside porous networks is a rather common process. It happens for example when rain falls on a dry soil: as the water moves in, it displaces air from the pores between the soil grains. It is also very important for many industrial and environmental applications related for example to the storage of CO2 inside depleted oil reservoirs and the remediation of contaminated soils.
In many of those fluid displacement processes, thin layers of liquid are left on the surface of the grains forming the porous network (for example, seemingly dry soils frequently have thin layers of water covering their grains). Those thin layers play a significant role: they can connect distant parts of the system. This effect brings some positive and negative consequences. The enhanced thin film connectivity is used by plants to obtain water and nutrients, but it also provides a pathway for the fast spreading of pollutants inside the soils. It is very important to understand these effects and this is the primary goal of this project: to produce a physics-grounded explanation for the stability and transport properties of the thin liquid film network. This will be done via experiments, theoretical analysis and numerical simulations.
Our experimental approach will be based on the use of custom-built transparent porous samples, where we can directly map the whole thin film network. This mapping is very useful and prior to our recent work it had never been experimentally obtained. The ability to map the film network will serve as an input for a new theoretical investigation of the problem, based on solid concepts from network theory (graph theory). This approach, coupled with network simulations, will allow us to have a full understanding of the physics of the problem. This new understanding will allow us for example to propose physics-based numerical routines to better describe the transport of liquids and the spreading of pollutants inside dry soils.