Structure-Property Relationship in Capillary Foams

Fabrication of Porous Adsorbents Templated from Capillary Foam Stabilized with Chlorella for Highly Efficient Removal of Cationic Dyes

The thermodynamic instability of conventional aqueous foam-stabilized surfactants is a critical bottleneck in the construction of porous materials. Herein, a novel strategy is proposed for preparing a capillary foam based on Chlorella and utilizing it as a template for constructing porous materials with high-efficiency adsorption. The capillary foam was stabilized by Chlorella particles enclosed within a gel network of oil bridges connecting the particles (capillary suspension). Chlorella particles, which act as stable particles, form oil bridges and are distributed at the phase interface of the capillary foam. These particles exhibited resistance to shear forces, allowing the formation of a long-term stable capillary foam. Using this foam as a template, a porous material with outstanding adsorption performance for Methylene Blue (MB) and Brilliant Green (BG) dyes was successfully constructed. Additionally, the material exhibited a sustained high adsorption performance even after five thermal regeneration-adsorption cycles. In conclusion, this study presents a green and straightforward method for constructing capillary foams with high stability, which is a promising approach for developing porous materials with exceptional adsorption and regeneration properties for dyes.

Cellulose fiber-based and engineered capillary foam toward a sustainable, recyclable, and high-performance cushioning structural material

Foam materials have been widely used in cushioning packaging to ensure the integrity of products inside by absorbing energy and preventing collision. However, the extensive use of petroleum-based plastic foams may exacerbate environmental pollution and consume large amounts of energy. Therefore, there has been an increasing focus on producing high-performance and environmentally friendly foams in recent years. In this study, we developed a simple approach for manufacturing cellulose fiber-based capillary foams featuring superior stability and three-dimensional (3D) backbone network cross-linking structure composed of polyvinyl alcohol (PVA) and cationic starch (CS). The resultant capillary foam showed low density (0.154 g/cm3), superior mechanical properties (elastic modulus ranging from 77 to 501 kPa), high energy absorbing efficiency (32.8 %), and low cushioning coefficient (3.0). Besides, the end-of-life cellulose fiber-based capillary foam can be easily recycled for use, showing an attractive closed-loop cycle process. This study presents a unique option for creating affordable, eco-friendly, and malleable foams, demonstrating the potential to substitute the currently used petroleum-based foams in the packaging, food, and transport industries.

Bubble-particle dynamics in multiphase flow of capillary foams in a porous micromodel

Surfactant-free capillary foams (CFs) are known to be remarkably tolerant to oil, and possess unique stability and flow properties. These properties result from the presence of oil-and-particle-coated bubbles that are interconnected by a dense particle-oil capillary network. In this work, we present a study of the dynamics of capillary foams flowing through a porous micromodel. We determine that despite the presence of oil-particle networks, CFs can flow through a microporous environment and that above a threshold flowrate, >80% of foam pumped through the micromodel can be recovered. In addition, we highlight the absence of steady state in CF flow and identify the underlying phenomena including the increasing apparent viscosity, reconfigurable flow paths, and intermittent clogging of the micromodel from an oil-particle composite and bubbles trapped in pores. We also characterize bubble dynamics and show that CFs surprisingly exhibit the same bubble generation and destruction mechanisms as classical foams despite the absence of surfactants. Our observations suggest that the porous medium plays a key role in generating uniformly sized bubbles and that capillary foams in a microporous environment tend to reconfigure their flow paths in a manner that may provide opportunities for increased sweep efficiency in enhanced oil recovery.

Effect of Shear on Pumped Capillary Foams

Foam flow in many applications, like firefighting and oil recovery, requires stable foams that can withstand the stress and aging that result from both shear and thermodynamic instability. Events of drainage and coarsening drive the collapse of foams and greatly affect foam efficacy in processes relying on foam transport. Recently, it was discovered that foams can be stabilized by the synergistic action of colloidal particles and a small amount of a water-immiscible liquid that mediates capillary forces. The so-called capillary foams contain gas bubbles that are coated by a thin oil-particle film and integrated in a network of oil-bridged particles; the present study explores how this unique architecture impacts the foams' flow dynamics. We pumped capillary foams through millimeter-sized tubing (ID: 790 μm) at different flow rates and analyzed the influence of stress and aging on capillary foam stability. We find that the foams remain stable when pumped at higher flow rates but undergo phase separation when pumped at low flow rates. Our observations further show that the particle network is responsible for the observed stability in capillary foams and that network strength and stability of an existing foam can be increased by shearing.

A holistic view on the role of egg yolk in Old Masters' oil paints

Old Masters like Botticelli used paints containing mixtures of oils and proteins, but "how" and "why" this was done is still not understood. Here, egg yolk is used in combination with two pigments to evaluate how different repartition of proteinaceous binder can be used to control the flow behavior as well as drying kinetics and chemistry of oil paints. Stiff paints enabling pronounced impasto can be achieved, but paint stiffening due to undesired uptake of humidity from the environment can also be suppressed, depending on proteinaceous binder distribution and colloidal paint microstructure. Brushability at high pigment loading is improved via reduction of high shear viscosity and wrinkling can be suppressed adjusting a high yield stress. Egg acts as antioxidant, slowing down the onset of curing, and promoting the formation of cross-linked networks less prone to oxidative degradation compared to oil alone, which might improve the preservation of invaluable artworks.

Synthesis of d-Gluconic Acetal Surfactants and Their Foaming Behaviors

Sugars are natural and environmentally benign substances, which can offer various hydroxyl groups. The understanding of details of the hydroxyl interactions in the hydrophilic groups of sugar-based surfactants, as well as the related properties, is still indistinct. Here, novel d-gluconic acetal surfactants with bicyclic and monocyclic structures in the head group were designed and synthesized. The obtained surfactant with a bicyclic architecture exhibited excellent foamability and a multistimulus-responsive behavior toward foam stabilization. In addition, the control of foamability from defoaming and foaming could be achieved by changing pH values or bubbling gas of CO₂/N₂. To explore the structural effects such as hydroxyl groups and rigidity of the head group on the properties of sugar-based surfactants, another kind of amphiphilic molecule with various OH- groups and a monocycle in the head group was designed for comparison. These two series of amphiphilic molecules both exhibited good surface activity. However, only the d-gluconic acetal surfactant with a bicyclic structure and a smaller number of OH- groups exhibited excellent foamability. Further studies showed that the foam behaviors were attributed to the conformation and arrangement of the surfactant molecule at the surface layer with the assistance of hydrogen bonds formed by hydroxyl groups and H₂O molecules. In addition, the surfactant could provide an environmentally friendly foamer in many potential applications.