Periodic Reporting for period 4 - METAFOAM (Novel assembly strategies in liquid dispersion via interface control – towards cellular metamaterials)
Periodo di rendicontazione: 2023-11-01 al 2025-04-30
Many modern materials derive their function from internal architecture. Solid foams, for example, consist of densely packed pores that yield lightweight materials with valuable mechanical, thermal, and acoustic properties. While research has identified the structural features needed to optimize such functions—including in so-called metamaterials—a major challenge remains: how to manufacture these complex architectures efficiently, affordably, and at scale.
METAFOAM set out to address this challenge by developing strategies to guide the spontaneous organization of bubbles during foam formation. By controlling and analysing this process, the project aimed to elucidate the scientific mechanisms which could help to create foams with novel, tunable architectures and properties, distinct from conventional materials.
To achieve this, we designed model systems that explored how viscoelastic bubble interfaces and embedded elastic objects influence foam structure. We developed new formulations using liquid templates, engineered polymeric skins to tune bubble surface properties, and controlled solidification kinetics. The development of several original custom instruments allowed us to characterize individual bubbles—behaving like miniature balloons—and to study their interactions within the foam.
Our findings demonstrate that these interactions can indeed drive the formation of new foam architectures, representing key progress toward self-assembled architected materials.
The project enabled the PI, after relocating from Paris-Orsay to Strasbourg, to build a dynamic, interdisciplinary research group. Collaborations were established across Europe and with academic and industrial partners such as BASF, URGO, and TECLIS. Additional PhD students joined with external funding, and all non-permanent team members have since transitioned successfully into academia or industry. METAFOAM has resulted in 30 peer-reviewed publications (with four more underway), and was widely disseminated through 9 invited lectures, 26 oral presentations, and 9 posters. The scientific momentum and visibility it generated continue to support new research directions and collaborations.
METAFOAM set out to address this challenge by developing strategies to guide the spontaneous organization of bubbles during foam formation. By controlling and analysing this process, the project aimed to elucidate the scientific mechanisms which could help to create foams with novel, tunable architectures and properties, distinct from conventional materials.
To achieve this, we designed model systems that explored how viscoelastic bubble interfaces and embedded elastic objects influence foam structure. We developed new formulations using liquid templates, engineered polymeric skins to tune bubble surface properties, and controlled solidification kinetics. The development of several original custom instruments allowed us to characterize individual bubbles—behaving like miniature balloons—and to study their interactions within the foam.
Our findings demonstrate that these interactions can indeed drive the formation of new foam architectures, representing key progress toward self-assembled architected materials.
The project enabled the PI, after relocating from Paris-Orsay to Strasbourg, to build a dynamic, interdisciplinary research group. Collaborations were established across Europe and with academic and industrial partners such as BASF, URGO, and TECLIS. Additional PhD students joined with external funding, and all non-permanent team members have since transitioned successfully into academia or industry. METAFOAM has resulted in 30 peer-reviewed publications (with four more underway), and was widely disseminated through 9 invited lectures, 26 oral presentations, and 9 posters. The scientific momentum and visibility it generated continue to support new research directions and collaborations.
Since the PI joined the host institution just prior to the launch of METAFOAM, the initial phase of the project was dedicated to equipping and setting up the laboratory. All commercial instrumentation was installed and operational within the first two years—despite delays caused by the COVID-19 pandemic—while the development of bespoke, home-built instruments continued throughout the project. The final device was completed one year before the project’s conclusion.
The project resulted in the development of three original technological platforms:
1. A multi-stage microfluidic chip featuring a novel flexible geometry for bubble/drop generation. This system harnesses gravity to sequentially transport bubbles/drops through distinct fluid layers, enabling the deposition of multi-layer coatings.
2. A microfluidic Thin Film Pressure Balance, allowing for the controlled creation, manipulation, and observation of freestanding thin films undergoing solidification. This tool enables the study of film stability and transformations in real time.
3. A LabView-controlled dual-droplet device designed to: (i) Generate and hold two drops or bubbles with complex interfacial properties under well-controlled conditions; (ii) Apply a novel pressure-based protocol to extract viscoelastic interfacial properties; (iii) Analyse interactions between the two droplets/bubbles.
Using these and other techniques, METAFOAM tackled several key scientific challenges in parallel:
A central goal was to establish model formulations for generating bubbles and drops with complex viscoelastic interfaces, capable of undergoing mechanical self-assembly in a liquid matrix that could then be solidified into architected materials. We achieved significant advances in four material systems: (i) Hydrogel foams, (ii) Polyurethane foams, (iii) Polystyrene foams, and (iv) Silicone emulsions. Across all systems, we attained unprecedented control over solidification kinetics and final foam/emulsion morphologies. Notably, we developed the first solidification route for hydrogels from the gas phase, and the first UV-curable polyurethane foam system, enabling on-demand solidification.
A key side discovery emerged during this work: the adsorption of fluorocarbons at gas–liquid interfaces was found to have a profound effect on foam structure—an aspect previously overlooked in the field. This finding led us to investigate the underlying mechanisms and their implications on morphology and stability.
Another important line of investigation focused on the evolution of thin films between bubbles/drops during solidification. These films are critical in determining final foam structure, especially for mechanical and acoustic properties. We thus studied individual films under realistic foam-like conditions, gaining insights into pore opening and deformation processes during solidification.
To precisely tune bubble/drop surface elasticity, we developed two novel strategies: (1) Layer-by-layer deposition of polymer multilayers, and (2) Interfacial cross-linking reactions. Characterizing the resulting complex interfaces, and the interactions between stabilized drops or bubbles, proved more difficult than anticipated due to a lack of suitable methods in the literature. We therefore invested in the development of custom tools and models, which now form a unique experimental platform and expertise. These efforts revealed that novel foam morphologies can be achieved by combining classical capillary effects with elastic forces from polymeric skins or embedded soft inclusions.
METAFOAM has resulted in 30 peer-reviewed publications (with four more underway), and was widely disseminated through 9 invited lectures, 26 oral presentations, and 9 posters. The project has also intiated numerous scientific outreach activities, often at the art/science interface.
Though the project has formally concluded, the scientific momentum and visibility it generated continue to support new research directions and collaborations.
The project resulted in the development of three original technological platforms:
1. A multi-stage microfluidic chip featuring a novel flexible geometry for bubble/drop generation. This system harnesses gravity to sequentially transport bubbles/drops through distinct fluid layers, enabling the deposition of multi-layer coatings.
2. A microfluidic Thin Film Pressure Balance, allowing for the controlled creation, manipulation, and observation of freestanding thin films undergoing solidification. This tool enables the study of film stability and transformations in real time.
3. A LabView-controlled dual-droplet device designed to: (i) Generate and hold two drops or bubbles with complex interfacial properties under well-controlled conditions; (ii) Apply a novel pressure-based protocol to extract viscoelastic interfacial properties; (iii) Analyse interactions between the two droplets/bubbles.
Using these and other techniques, METAFOAM tackled several key scientific challenges in parallel:
A central goal was to establish model formulations for generating bubbles and drops with complex viscoelastic interfaces, capable of undergoing mechanical self-assembly in a liquid matrix that could then be solidified into architected materials. We achieved significant advances in four material systems: (i) Hydrogel foams, (ii) Polyurethane foams, (iii) Polystyrene foams, and (iv) Silicone emulsions. Across all systems, we attained unprecedented control over solidification kinetics and final foam/emulsion morphologies. Notably, we developed the first solidification route for hydrogels from the gas phase, and the first UV-curable polyurethane foam system, enabling on-demand solidification.
A key side discovery emerged during this work: the adsorption of fluorocarbons at gas–liquid interfaces was found to have a profound effect on foam structure—an aspect previously overlooked in the field. This finding led us to investigate the underlying mechanisms and their implications on morphology and stability.
Another important line of investigation focused on the evolution of thin films between bubbles/drops during solidification. These films are critical in determining final foam structure, especially for mechanical and acoustic properties. We thus studied individual films under realistic foam-like conditions, gaining insights into pore opening and deformation processes during solidification.
To precisely tune bubble/drop surface elasticity, we developed two novel strategies: (1) Layer-by-layer deposition of polymer multilayers, and (2) Interfacial cross-linking reactions. Characterizing the resulting complex interfaces, and the interactions between stabilized drops or bubbles, proved more difficult than anticipated due to a lack of suitable methods in the literature. We therefore invested in the development of custom tools and models, which now form a unique experimental platform and expertise. These efforts revealed that novel foam morphologies can be achieved by combining classical capillary effects with elastic forces from polymeric skins or embedded soft inclusions.
METAFOAM has resulted in 30 peer-reviewed publications (with four more underway), and was widely disseminated through 9 invited lectures, 26 oral presentations, and 9 posters. The project has also intiated numerous scientific outreach activities, often at the art/science interface.
Though the project has formally concluded, the scientific momentum and visibility it generated continue to support new research directions and collaborations.
The most significant achievements of the project are:
* Development of a novel microfluidic platform that enables precise control of bubble size while simultaneously allowing the deposition of polymeric skins on bubble surfaces.
* Establishment of new solidification methods for architected foams, including (i) gas-phase-initiated cross-linking of hydrogel foams, and (ii) UV-triggered curing of polyurethane foams.
* Discovery of a major interfacial phenomenon, demonstrating that fluorocarbon condensation at gas–liquid interfaces dramatically alters foam properties—an effect previously overlooked in the field.
* Creation of a robust experimental setup and protocol to measure the viscoelastic properties of solid-like skins grown on bubble and droplet interfaces.
* Design of an original device and methodology to quantitatively characterize the mechanical interaction between two individual bubbles or droplets with complex interfacial properties.
* Demonstration of a new mechanism for structural control, showing that the combination of classical capillary forces and elastic effects—whether from polymer-coated interfaces or embedded soft inclusions—can drive the mechanical self-assembly of novel foam and emulsion architectures.
* Development of a novel microfluidic platform that enables precise control of bubble size while simultaneously allowing the deposition of polymeric skins on bubble surfaces.
* Establishment of new solidification methods for architected foams, including (i) gas-phase-initiated cross-linking of hydrogel foams, and (ii) UV-triggered curing of polyurethane foams.
* Discovery of a major interfacial phenomenon, demonstrating that fluorocarbon condensation at gas–liquid interfaces dramatically alters foam properties—an effect previously overlooked in the field.
* Creation of a robust experimental setup and protocol to measure the viscoelastic properties of solid-like skins grown on bubble and droplet interfaces.
* Design of an original device and methodology to quantitatively characterize the mechanical interaction between two individual bubbles or droplets with complex interfacial properties.
* Demonstration of a new mechanism for structural control, showing that the combination of classical capillary forces and elastic effects—whether from polymer-coated interfaces or embedded soft inclusions—can drive the mechanical self-assembly of novel foam and emulsion architectures.