Nanostructured, Fluid-Bicontinuous Gels for Continuous-Flow Liquid-Liquid Extraction

Dual access to the fluid networks of colloid-stabilized bicontinuous emulsions through uninterrupted connections

Large surface areas are important for enhancing mass and energy transfer in biological and technological processes. Bicontinuous interfacially jammed emulsion gels (bijels) increase the surface area between two fluids by intertwining them into particle stabilized networks. To facilitate efficient mass and energy exchange via the bijels' high surface area, the fluid networks need to be connected to their respective bulk phases. Here, we generate bijels between two bulk fluids and investigate the connections the bijel makes. We analyze these connections by investigating the colloidal stability, interfacial rheology and mass transfer dynamics during bijel formation. To this end, we employ confocal and electron microscopy, as well as dynamic light scattering, pendant drop analysis, electrophoretic mobility measurements and diffusion simulations. We find that the connections the bijel makes to the bulk fluid can be disrupted by severe colloidal aggregation and interruptions of the bicontinuous fluid network. However, the addition of alcohol to the bulk fluid moderates aggregation and allows undisturbed fluid network formation, facilitating open connections between bijel and bulk fluid. The unprecedented control of bijel pore connections from this research will be crucial for the application of bijels as separation membranes, electrochemical energy storage materials and chemical reactors.

Formation of bijels stabilized by magnetic ellipsoidal particles in external magnetic fields

Bicontinuous interfacially-jammed emulsion gels (bijels) are increasingly used as emulsion templates for the fabrication of functional porous materials including membranes, electrodes, and biomaterials. Control over the domain size and structure is highly desirable in these applications. For bijels stabilized by spherical particles, particle size and volume fraction are the main parameters that determine the emulsion structure. Here, we investigate the use of ellipsoidal magnetic particles and study the effect of external magnetic fields on the formation of bijels. Using hybrid Lattice Boltzmann-molecular dynamics simulations, we analyze the effect of the magnetic field on emulsion dynamics and the structural properties of the resulting bijel. We find that the formation of bijels remains robust in the presence of magnetic fields, and that the domain size and tortuosity become anisotropic when ellipsoidal particles are used. We show that the magnetic fields lead to orientational ordering of the particles which in turn leads to alignment of the interfaces. The orientational order facilitates enhanced packing of particles in the interface which leads to different jamming times in the directions parallel and perpendicular to the field. Our results highlight the potential of magnetic particles for fabrication and processing of emulsion systems with tunable properties.

Chemistry, applications, and future prospects of structured liquids

Structured liquids are emerging functional soft materials that combine liquid flowability with solid-like structural stability and spatial organization. Here, we delve into the chemistry and underlying principles of structured liquids, ranging from nanoparticle surfactants (NPSs) to supramolecular assemblies and interfacial jamming. We then highlight recent advancements related to the design of intricate all-liquid 3D structures and examine their reconfigurability. Additionally, we demonstrate the versatility of these soft functional materials through innovative applications, such as all-liquid microfluidic devices and liquid microreactors. We envision that in the future, the vast potential of the liquid-liquid interface combined with human creativity will pave the way for innovative platforms, exemplified by current developments like liquid batteries and circuits. Although still in its nascent stages, the field of structured liquids holds immense promise, with future applications across various sectors poised to harness their transformative capabilities.

The effect of charge screening for cationic surfactants on the rigidity of interfacial nanoparticle assemblies

HYPOTHESIS

Functionalizing colloidal particles with oppositely charged surfactants is crucial for stabilizing emulsions, foams, all-liquid structures, and bijels. However, surfactants can reduce the attachment energy, the driving force for colloidal self-assembly at interfaces. An open question remains on how the inherent interfacial activity of cationic surfactants influences the interfacial rigidity of particle-laden interfaces. We hypothesize that charge screening among cationic surfactants regulates the rigidity of oil/water interfaces by reducing the attachment energy of nanoparticles.

EXPERIMENTS

We investigate the interfacial rigidity of cetyltrimethylammonium bromide (CTAB) functionalized silica nanoparticles (Ludox® TMA) by analyzing the shape deformation of 1,4-butanediol diacrylate (BDA) droplets under varying salt and alcohol concentrations. The nanoparticle packing density is assessed using scanning electron microscopy. Attachment energy is characterized through interfacial tension measurements, three-phase contact angle analysis, and CTAB adsorption studies. We also examine the effects of interfacial rigidities on the structure of bijel films formed via roll-to-roll solvent transfer-induced phase separation (R2R-STrIPS) using confocal laser scanning microscopy.

FINDINGS

Increasing salt and alcohol concentrations decrease the interfacial rigidity of CTAB-functionalized nanoparticle films by reducing the interfacial tension. The contact angle has a minor influence on the rigidity. These results indicate that CTAB charge screening weakens the nanoparticle attachment energy to the interface. Controlling the rigidity enables the mass production of bijel sheets with consistent flatness, which is crucial for their potential applications in catalysis, energy storage, tissue engineering, and filtration membranes.

Engineering Interconnected Open-Porous Particles via Microfluidics Using Bijel Droplets as Structural Templates

Designing porous structures is key in materials science, particularly for separation, catalysis, and cell culture systems. Bicontinuous interfacially jammed emulsion gels represent a unique class of soft matter formed by kinetically arresting the separation of the spinodal decomposition phase, which is stabilized by colloidal particles with neutral wetting. This study introduces a microfluidic technique to create highly interconnected open-porous particles using bijel droplets stabilized with hexadecyltrimethylammonium bromide (CTAB)-modified silica particles. Monodisperse droplets comprising a hydrophobic monomer, water, ethanol, silica particles, and CTAB were initially formed in the microfluidic device. The diffusion of ethanol from these droplets into the continuous cyclohexane phase triggered spinodal decomposition within the droplets. The phase-separated structure within the droplets was stabilized by the CTAB-modified silica particles, and subsequent photopolymerization yielded microparticles with highly interconnected, open pores. Moreover, the influence of the ratio of the CTAB and silica particles, fluid composition, and microchannel direction on the final structure of the microparticles was explored. Our findings indicated that the phase-separated structure of the particles transitioned from oil-in-water to water-in-oil as the CTAB/silica ratio was increased. At intermediate CTAB/silica ratios, microparticles with bicontinuous structures were formed. Regardless of the fluid composition, the pore size of the particles increased with time after phase separation. However, this coarsening was arrested 15 s after droplet formation in the CTAB-modified silica particles, accompanied by a change in the particle shape from spherical to ellipsoidal. In situ observations of the bijel droplet formation revealed that the particle shape deformation is caused by the rolling of elastic bijel droplets at the bottom of the microchannel. As such, the channel setup was altered from horizontal to vertical to prevent the deformation of bijel droplets, resulting in spherical particles with open pores.

Development of a capillary electrophoresis method based on magnetic solid-phase extraction for simultaneous and sensitive detection of eight biogenic amines in foods

BACKGROUND

Biogenic amines (BAs) in high concentrations are toxic and may cause a series of health symptoms. A sensitive measurement of BA levels is essential for human health. Capillary electrophoresis (CE) has emerged for the separation of eight BAs due to simple sample preparation and highly efficient separation. However, an important drawback for CE is low sensitivity. Magnetic solid-phase extraction (MSPE) has become a technique of interest owing to its brief operation and low solvent consumption. Hence, MSPE as a pretreatment has great potential to improve CE sensitivity for the analysis of BAs in complex food.

RESULTS

Results showed that the Pt-Co-MWCNTs-COOH possessed strong magnetism, good reusability, and high adsorptive ability toward eight biogenic amines based on the hydrogen bonding between the -COOH of Pt-Co-MWCNTs-COOH and -NH2 groups of BAs. Using it as an adsorbent, a magnetic solid-phase extraction coupled with capillary electrophoresis (MSPE-CE) method was developed to effectively extract and sensitively analyze eight BAs. Under optimal conditions, the MSPE-CE method has wide linearities (10.0-1000.0 μg L-1 ) and low limits of detection (1.0-6.1 μg L-1 ). The accuracy of the developed method yielded recovery values from 82.07% to 102.58%. Meanwhile, the BAs contents in two samples were analyzed using the MSPE-CE method, with the results consistent with those detected by a high-performance liquid chromatography method.

CONCLUSION

Given those advantages, the established MSPE-CE method promises the practical guidance of monitoring a variety of BAs and provides a foundation for the detection of other food hazards. © 2023 Society of Chemical Industry.

Elastic microphase separation produces robust bicontinuous materials

Bicontinuous microstructures are essential to the function of diverse natural and synthetic systems. Their synthesis has been based on two approaches: arrested phase separation or self-assembly of block copolymers. The former is attractive for its chemical simplicity and the latter, for its thermodynamic robustness. Here we introduce elastic microphase separation (EMPS) as an alternative approach to make bicontinuous microstructures. Conceptually, EMPS balances the molecular-scale forces that drive demixing with large-scale elasticity to encode a thermodynamic length scale. This process features a continuous phase transition, reversible without hysteresis. Practically, EMPS is triggered by simply supersaturating an elastomeric matrix with a liquid, resulting in uniform bicontinuous materials with a well-defined microscopic length scale tuned by the matrix stiffness. The versatility of EMPS is further demonstrated by fabricating bicontinuous materials with superior mechanical properties and controlled anisotropy and microstructural gradients. Overall, EMPS presents a robust alternative for the bulk fabrication of homogeneous bicontinuous materials.

Diffusion across particle-laden interfaces in Pickering droplets

Emulsions stabilized by nanoparticles, known as Pickering emulsions, exhibit remarkable stability, which enables applications ranging from encapsulation, to advanced materials, to chemical conversion. The layer of nanoparticles at the interface of Pickering droplets is a semi-permeable barrier between the two liquid phases, which can affect the rate of release of encapsulates, and the interfacial transfer of reactants and products in biphasic chemical conversion. A gap in our fundamental understanding of diffusion in multiphase systems with particle-laden interfaces currently limits the optimal development of these applications. To address this gap, we developed an experimental approach for in situ, real-time quantification of concentration fields in Pickering droplets in a Hele-Shaw geometry and investigated the effect of the layer of nanoparticles on diffusion of solute across a liquid-liquid interface. The experiments did not reveal a significant hindrance on the diffusion of solute across an interface densely covered by nanoparticles. We interpret this result using an unsteady diffusion model to predict the spatio-temporal evolution of the concentration of solute with a particle-laden interface. We find that the concentration field is only affected in the immediate vicinity of the layer of particles, where the area available for diffusion is affected by the particles. This defines a characteristic time scale for the problem, which is the time for diffusion across the layer of particles. The far-field concentration profile evolves towards that of a bare interface. This localized effect of the particle hindrance is not measurable in our experiments, which take place over a much longer time scale. Our model also predicts that the hindrance by particles can be more pronounced depending on the particle size and physicochemical properties of the liquids and can ultimately affect performance in applications.

Super-Macroporous Lightweight Materials Templated from Bicontinuous Intra-Phase Jammed Emulsion Gels Based on Nanochitin

Non-equilibrium multiphase systems are formed by mixing two immiscible nanoparticle dispersions, leading to bicontinuous emulsions that template cryogels with interconnected, tortuous channels. Herein, a renewable, rod-like biocolloid (chitin nanocrystals, ChNC) is used to kinetically arrest bicontinuous morphologies. Specifically, it is found that ChNC stabilizes intra-phase jammed bicontinuous systems at an ultra-low particle concentration (as low as 0.6 wt.%), leading to tailorable morphologies. The synergistic effects of ChNC high aspect ratio, intrinsic stiffness, and interparticle interactions produce hydrogelation and, upon drying, lead to open channels bearing dual characteristic sizes, suitably integrated into robust bicontinuous ultra-lightweight solids. Overall, it demonstrates the successful formation of ChNC-jammed bicontinuous emulsions and a facile emulsion templating route to synthesize chitin cryogels that form unique super-macroporous networks.

Flows of a nonequilibrated aqueous two-phase system in a microchannel

Liquid-liquid phase separation is a rich and dynamic process, which recently has gained new interest, especially in biology and for material synthesis. In this work, we experimentally show that co-flow of a nonequilibrated aqueous two-phase system within a planar flow-focusing microfluidic device results in a three-dimensional flow, as the two nonequilibrated solutions move downstream along the length of the microchannel. After the system reaches steady-state, invasion fronts from the outer stream are formed along the top and bottom walls of the microfluidic device. The invasion fronts advance towards the center of the channel, until they merge. We first show by tuning the concentration of polymer species within the system that the formation of these fronts is due to liquid-liquid phase separation. Moreover, the rate of invasion from the outer stream increases with increasing polymer concentrations in the streams. We hypothesize the invasion front formation and growth is driven by Marangoni flow induced by the polymer concentration gradient along the width of the channel, as the system is undergoing phase separation. In addition, we show how at various downstream positions the system reaches its steady-state configuration once the two fluid streams flow side-by-side in the channel.

Bicontinuous interfacially jammed emulsion gels with nearly uniform sub-micrometer domains via regulated co-solvent removal

Porous materials possess numerous useful functions because of their high surface area and ability to modulate the transport of heat, mass, fluids, and electromagnetic waves. Unlike highly ordered structures, disordered porous structures offer the advantages of ease of fabrication and high fault tolerance. Bicontinuous interfacially jammed emulsion gels (bijels) are kinetically trapped disordered biphasic materials that can be converted to porous materials with tunable features. Current methods of bijel fabrication result in domains that are micrometers or larger, and non-uniform in size, limiting their surface area, mechanical strength, and interaction with electromagnetic waves. In this work, scalable synthesis of bijels with uniform and sub-micrometer domains is achieved via a two-step solvent removal process. The resulting bijels are characterized quantitatively to verify the uniformity and sub-micrometer scale of the domains. Moreover, these bijels have structures that resemble the microstructure of the scale of the white beetle Cyphochilus. We find that such bijel films with relatively small thicknesses (<150 μm) exhibit strong solar reflectance as well as high brightness and whiteness in the visible range. Considering their scalability in manufacturing, we believe that VIPS-STRIPS bijels have great potential in large-scale applications including passive cooling, solar cells, and light emitting diodes (LEDs).

Associative Liquid-In-Liquid 3D Printing Techniques for Freeform Fabrication of Soft Matter

Shaping soft materials into prescribed 3D complex designs has been challenging yet feasible using various 3D printing technologies. For a broader range of soft matters to be printable, liquid-in-liquid 3D printing techniques have emerged in which an ink phase is printed into 3D constructs within a bath. Most of the attention in this field has been focused on using a support bath with favorable rheology (i.e., shear-thinning behavior) which limits the selection of materials, impeding the broad application of such techniques. However, a growing body of work has begun to leverage the interaction or association of the two involved phases (specifically at the liquid-liquid interface) to fabricate complex constructs from a myriad of soft materials with practical structural, mechanical, optical, magnetic, and communicative properties. This review article has provided an overview of the studies on such associative liquid-in-liquid 3D printing techniques along with their fundamentals, underlying mechanisms, various characterization techniques used for ensuring the structural stability, and practical properties of prints. Also, the future paths with the potential applications are discussed.

Synthesis and Polyelectrolyte Functionalization of Hollow Fiber Membranes Formed by Solvent Transfer Induced Phase Separation

Ultrafiltration membranes are important porous materials to produce freshwater in an increasingly water-scarce world. A recent approach to generate porous membranes is solvent transfer induced phase separation (STrIPS). During STrIPS, the interplay of liquid-liquid phase separation and nanoparticle self-assembly results in hollow fibers with small surface pores, ideal structures for applications as filtration membranes. However, the underlying mechanisms of the membrane formation are still poorly understood, limiting the control over structure and properties. To address this knowledge gap, we study the nonequilibrium dynamics of hollow fiber structure evolution. Confocal microscopy reveals the distribution of nanoparticles and monomers during STrIPS. Diffusion simulations are combined with measurements of the interfacial elasticity to investigate the effect of the solvent concentration on nanoparticle stabilization. Furthermore, we demonstrate the separation performance of the membrane during ultrafiltration. To this end, polyelectrolyte multilayers are deposited on the membrane, leading to tunable pores that enable the removal of dextran molecules of different molecular weights (>360 kDa, >60 kDa, >18 kDa) from a feed water stream. The resulting understanding of STrIPS and the simplicity of the synthesis process open avenues to design novel membranes for advanced separation applications.