Flow kinematics in transparent replicas of geomaterial samples using innovative 3D printing/molding techniques

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INRAE presentation

The French National Research Institute for Agriculture, Food, and Environment (INRAE) is a major player in research and innovation. It is a community of 12,000 people with 272 research, experimental research, and support units located in 18 regional centres throughout France. Internationally, INRAE is among the top research organisations in the agricultural and food sciences, plant and animal sciences, as well as in ecology and environmental science. It is the world’s leading research organisation specialising in agriculture, food and the environment. INRAE’s goal is to be a key player in the transitions necessary to address major global challenges. Faced with a growing world population, climate change, resource scarcity, and declining biodiversity, the Institute has a major role to play in building solutions and supporting the necessary acceleration of agricultural, food and environmental transitions.

Work environment, missions and activities

Context and problematic

Geomaterial are complex porous material presenting a wide diversity of structures that set the flow kinematic of any fluids through it. When rainwater flows through rocks, or when expansive resin is injected to stabilize a soil and reinforce its mechanical properties to prevent crack, the way the injected solute is dispersed and homogenized in the surrounding media is directly driven by the local microstructure and the pore network. Understanding what drives and control the transport processes in porous media is therefore crucial for a broad range of applications, such as contaminant transport in soils and aquifers, drug delivery and nutrient transport in brain or plant tissues, heat exchangers, filters and catalytic processes in chemical or energy industry, biocalcification and soil reinforcement of hydraulic structures such as dam and dikes.
Transport processes are characterized by investigating the flow velocity fields, which is usually performed using direct flow visualization techniques such as particle image velocimetry (PIV) or particle tracking velocimetry (PTV). Such approach has been for instance successfully implemented to study model artificial porous media composed of randomly packed solid spheres, and to investigate how a blob of injected dye stretches and get mixed. Because of the opaque nature of porous media, direct flow visualization are usually prohibited within real soil microstructure, making flow kinematic characterization particularly challenging in 3D porous media. This ambitious experimental scientific project thus aim to tackle this experimental limitation by elaborating innovative 3D printing & molding techniques to replicate as closely as possible the microstructure of real porous material and geomaterial samples scanned via X-ray tomography. Lifting such technical barrier will pave the way for detailed investigations of porous media of increasingly complex microstructure, to fully characterize which microstructure features (pore size distribution, tortuosity, permeability), or heterogeneity (cracks, clogged pores) set the flow kinematic and controls the transport processes in 3D porous media.

Experimental methodology

The 3D printing & molding techniques will revolve around sacrifical molding : the 3D pore network of a geomaterial sample scanned via X-ray tomography will be printed and used as a sacrificial mold with an optically transparent material (PMMA, or PDMS) for the surrounding solid phase. The interstitial pore network will then be removed chemically to obtain a transparent replica of the scanned sample. Similar approaches have been recently developed for additive manufacturing to create complex microfluidic channels using sugar as a sacrificial mold [3], or to create transparent brain arteries models in PDMS using water soluble resin [2] (Figure 2). The latter study is extremely promising, as the authors managed to recreate a fullscale phantom model of brain arteries in PDMS from computed tomography angiography images. Adapting such approach to the field of geomechanics will not only unlock the challenging issue of opacity prohibiting direct visualizations in the porous media community, but also allow for a high level of control of the investigated microstructure, which will be useful to optimize transport processes through porous media by determining how local alteration of a microstructure may enhance transport processes. This promising approach yet requires rigorous tuning, as the technical feasibility strongly depends on the amount and quality of the interfaces through which the flow visualization is done. Homogeneity of the mold PMMA (or PDMS) is crucial, and will require precise tuning to allow for the use of refractive index matching techniques for flow direct visualization. The flow will then be characterized by reconstructing experimentally the 3D velocity field using successive scans of the flow velocity.

Training and skills

We are looking for a candidate with strong taste for experiments, a potent inclination for experimental curiosity and interest in 3D printing and molding techniques, and/or direct flow visualization. Past experience in laboratory work including flow visualization techniques (PIV/PTV), 3D printing, molding (PDMS/PMMA), or X-ray tomography will be profitable. The candidate is expected to have either a background in fluid mechanics, in additive manufacturing, or in geotechnical engineering. The postdoctorate will be involved at all stages of the project : elaboration of 3D printing technique, sacrificial molding, direct flow visualization, numerical image analysis for flow characterization (PIV, PTV, 3D flow reconstruction).

Expected skills : inclination for experiments, faculty for working independently and in a team, willingness to improve and explore new experimental techniques, constructive criticism, curiosity, perseverance and scientific rigour. A good level in English would be appreciated.

INRAE's life quality

By joining our teams, you benefit from (depending on the type of contract and its duration):

- up to 30 days of annual leave + 15 days "Reduction of Working Time" (for a full time);
- parenting support: CESU childcare, leisure services;
- skills development systems: training, career advise;
- social support: advice and listening, social assistance and loans;
- holiday and leisure services: holiday vouchers, accommodation at preferential rates;
- sports and cultural activities;
- collective catering.

How to apply

I send my CV and my motivation letter