Description du sujet de thèse

Sticky Bubbles and Novel Liquid Foams

Contexte de travail

Liquid foams play a crucial role in the ecological and energy transition. With their excellent thermal insulation properties, they enhance the energy efficiency of buildings and infrastructure. They have various applications, such as cleaning and decontaminating surfaces and confined spaces, capturing and controlling dust on construction sites or industrial areas, wastewater remediation, and separating finely divided materials through flotation. Furthermore, new processes are emerging that utilize the properties of foams for extracting precious metals from recycled electronic devices (urban mining), promoting a more sustainable management of resources and urban waste. Other potential applications include soil improvement and rehabilitation, as well as underground CO2 storage. Liquid foams are multifunctional materials referred to as "complex" because they consist of an assembly of bubbles in varying concentrations within a liquid. To fully exploit their advantages, it is crucial to master their production methods, control their aging process over time, and predict their physical properties, both in their liquid and solid (hardened) states. Despite significant research advances over the past twenty years, many aspects still need to be mastered.
We propose to explore a new type of liquid foam in which bubbles, instead of repelling each other as in conventional foams, adhere upon contact until a sufficient force separates them. These adhesive bubble foams represent an unexplored domain, opening up a novel research field rich in potential discoveries and application prospects. Bubble adhesion is expected to profoundly alter the internal organization of the foam, leading, for example, to dense structures where bubbles form a compact network interspersed with unoccupied spaces (see illustration). These microstructural changes inevitably impact the foam's mechanical and dynamic properties. The objective of this thesis is to study these properties and identify their key characteristics.
The effect of bubble adhesion on the microstructure of foam will be studied from two perspectives: first, in a static foam, by analyzing the arrangement of bubbles as a function of their volume fraction; and second, during flow in confined geometries (microfluidics), designed to induce topological rearrangements and examine the resulting bubble reorganization. This approach will make it possible to manipulate these adhesive bubbles and place them in a novel configuration, where small groups of tightly interlocked bubbles form distinct 'clusters.' This original microstructure could lead to foams with innovative properties, paving the way for new applications.
The stress required to initiate the flow of a liquid foam and maintain a certain deformation rate (shear) is expected to be highly sensitive to adhesion forces that may arise at bubble contacts. The relationship between stress and shear rate will be measured using a rheometer to quantify this effect as a function of the adhesion force intensity and the volume fraction occupied by the bubbles. This study will help determine the critical fraction below which adhesion forces dominate bubble interactions.
Coarsening is a key process in the evolution of foams, as it leads to an increase in the average bubble size over time. This phenomenon results from gas exchange between bubbles through the diffusion of dissolved gas in the liquid. Here again, the presence of bubble adhesion is expected to have a significant impact, especially when the volume fraction of bubbles is low. Coarsening will be studied in a rotating cell, which compensates for gravity-induced flows caused by the large density difference between the bubbles and the suspending liquid. Time-lapse image acquisition, combined with AI-assisted image processing tools, will enable the analysis of bubble size distribution and average bubble size evolution, highlighting the role of adhesion. Coarsening will also be studied under applied shear, which controls the duration of transient bubble contacts. This duration is expected to compete with the timescale of contact formation due to adhesion forces, leading to significant modifications in the evolution of the average bubble size. Additionally, the time required for an adhesive contact to form will be investigated using a dedicated setup that allows for controlled 'collisions' between two bubbles.

Informations complémentaires

https://navier-lab.fr/les-emplois/these-bulles-collantes-et-nouvelles-mousses-liquides/