[PhD defence] 20/02/2024, Florian CAJOT: "Modelling water transfer in soil in the presence of amphiphilic materials: Application to the rhizosphere".

Research news 23 February 2024

Title of thesis

"Modelling soil water transfer in the presence of amphiphilic matter: application to the rhizosphere".

Date and place

Oral defense scheduled on Friday 01 March 2024 at 11.00 am
Venue: Avignon University - Jean-Henri Fabre Campus 301 Rue Baruch de Spinoza, 84140 Avignon
Room: Agrosciences lecture theatre
By videoconference : https://v-au.univ-avignon.fr/live/bbb-soutenance-these-f-cajot-1er-mars-2024/ 




Laboratory: UMR 1144 - EMMAH - Mediterranean Environment and Modelling of Agro-Hydrosystems

Research team : SWIFT - Soil Water Interactions and transFer Team



Composition of the jury

MR PHILIPPE BELTRAME Avignon University Thesis supervisor
Ms ANDREA SCHNEPF Institute for Biological and Geographical Research (IBG) Rapporteur
MR UWE THIELE University of Münster - Institute of Theoretical Physics Rapporteur
Ms ANNETTE BERARD INRAE Thesis co-supervisor
MR CLAUDE DOUSSAN INRAE Thesis co-supervisor
MR LAURENT LIMAT  Université de Paris Cité Examiner
MR CYPRIEN SOULAINE Institute of Earth Sciences of Orléans (ISTO) Examiner

Summary of the thesis

Organic matter is an important component of soils and, in particular, in the rhizosphere (the volume of soil influenced by root activity), specific compounds are present in the form of polymers such as ExoPolySaccharides (EPS). These compounds, exuded by roots and micro-organisms, are amphiphilic. They influence water transfer in the root zone. Understanding how organic matter modifies water transfer in the soil is an open question to which modelling can provide some answers. To take account of the variable hydrophobic/hydrophilic nature of organic matter, we propose a paradigm shift in the representation of soil wettability. Instead of associating wettability with a simple contact angle, we propose an energetic conceptual framework, in which an energy functional represents the effective interactions between the soil matrix and the surface of free water. This interaction energy contains both attractive (hydrophilic) and repulsive (hydrophobic) interactions, in order to reproduce the amphiphilic nature of the molecules. By including the energies at work in the soil, i.e. the energy associated with the wettability, gravity and surface tension of the water, we can define the free energy of the system. We have derived the equation governing water saturation, which leads to a Darcy-Richards type equation coupled with a free energy equation. This system of equations ensures consistency with thermodynamics. The system of PDEs is solved numerically using the finite element method implemented in C++ with the Oomph-lib library based on an adaptive spatial and temporal scheme. The model is first tested for its behaviour with respect to hydrophobicity effects in the soil. In this respect, it is capable of reproducing the finite horizontal spread of water on a hydrophobic porous medium. Applied to a vertically stratified porous medium with alternating hydrophilic and hydrophobic layers, the model simulates, as a function of the average water content, the phenomena of water trapping in the hydrophilic layers or intermittent flow and front instability. In addition, the numerical analysis of bifurcation highlights hysteresis phenomena. This is the first time that a PDE system has captured all these phenomena in a single soil model. In the second part, we adapt and experimentally test our model to the specific characteristics of rhizospheric soil. We constructed a model medium consisting of sand with added EPS derived from chia mucilage. Experiments were carried out to characterise the parameters of the model. In particular, we studied experimentally and numerically the infiltration of a drop placed on the sand/EPS model medium. With regard to the model, we showed that the laws governing the flow of water infiltrating the matrix are modified in the presence of organic matter. Once calibrated, the model can reproduce various phenomena specific to the presence of amphiphilic matter observed in our experiments or in the literature. In particular, the 'buffer' effect during wetting and the various stages of droplet imbibition. Finally, an initial application was carried out to simulate the rewetting phase of the rhizosphere on the scale of a root segment.

Key words :
porous media, wettability, exopolysaccharides, free energy, numerical modelling

Mots clés associés
thesis defence