Continuous Fabrication of Hierarchical and Asymmetric Bijel Microparticles, Fibers, and Membranes by Solvent Transfer-Induced Phase Separation (STRIPS)

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.

A single-particle mechanofluorescent sensor

Monitoring mechanical stresses in microchannels is challenging. Herein, we report the development of a mechanofluorescence sensor system featuring a fluorogenic single polydiacetylene (PDA) particle, fabricated using a co-flow microfluidic method. We construct a stenotic vessel-mimicking capillary channel, in which the hydrodynamically captured PDA particle is subjected to controlled fluid flows. Fluorescence responses of the PDA particle are directly monitored in real time using fluorescent microscopy. The PDA particle displays significant nonlinear fluorescence emissions influenced by fluid viscosity and the presence of nanoparticles and biomolecules in the fluid. This nonlinear response is likely attributed to the torsion energy along the PDA's main chain backbone. Computational fluid dynamic simulations indicate that the complete blue-to-red transition necessitates ~307 μJ, aligning with prior research. We believe this study offers a unique advantage for simulating specific problematic regions of the human body in an in vitro environment, potentially paving the way for future exploration of difficult-to-access areas within the body.

Magnetically Active Bicontinuous Polymer Structures for Multiple Controlled Drug Delivery

The targeted delivery of drugs using wireless navigable magnetic robots allows the delivery of drug molecules to be controlled non only in time but also in space, improving medical outcomes. The main disadvantages behind their use lies in the low amount of drug that can be transported and the single nature of drug that can be loaded (hydrophilic or hydrophobic). These considerations limit their use in co-delivery systems, now recognized to be very promising for many different pathologies. A magnetic bijel-like structure is developed to load and release different types of molecules (hydrophilic and hydrophobic). In this work, the use of ε-caprolactone is explored, which can polymerize, forming hydrophobic domains (oil phase). After mixing with iron oxide nanoparticles (NPs), the water dispersion creates a magnetic biphasic porous structure without phase separation. The resulting device shows good performance both in magnetic actuation and as a drug delivery system.

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.

Mixed Nanosphere Assemblies at a Liquid-Liquid Interface

The in-plane packing of gold (Au), polystyrene (PS), and silica (SiO2) spherical nanoparticle (NP) mixtures at a water-oil interface is investigated in situ by UV-vis reflection spectroscopy. All NPs are functionalized with carboxylic acid such that they strongly interact with amine-functionalized ligands dissolved in an immiscible oil phase at the fluid interface. This interaction markedly increases the binding energy of these nanoparticle surfactants (NPSs). The separation distance between the Au NPSs and Au surface coverage are measured by the maximum plasmonic wavelength (λmax) and integrated intensities as the assemblies saturate for different concentrations of non-plasmonic (PS/SiO2) NPs. As the PS/SiO2 content increases, the time to reach intimate Au NP contact also increases, resulting from their hindered mobility. λmax changes within the first few minutes of adsorption due to weak attractive inter-NP forces. Additionally, a sharper peak in the reflection spectrum at NP saturation reveals tighter Au NP packing for assemblies with intermediate non-plasmonic NP content. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) measurements confirm a decrease in Au NP domain size for mixtures with larger non-plasmonic NP content. The results demonstrate a simple means to probe interfacial phase separation behavior using in situ spectroscopy as interfacial structures densify into jammed, phase-separated NP films.

Bicontinuous Interfacially Jammed Emulsion Gels (Bijels): Preparation, Control Strategies, and Derived Porous Materials

Bicontinuous interfacially jammed emulsion gels, also known as Bijels, are a new type of soft condensed matter. Over the last decade, Bijels have attracted considerable attention because of their unique morphology, property, and broad application prospects. In the present review, we summarize the preparation methods and main control strategies of Bijels, focusing on the research progress and application of Bijels as templates for porous materials preparation in recent years. The potential future directions and applications of Bijels are also envisaged.

Evidence for Enhanced Tracer Diffusion in Densely Packed Interfacial Assemblies of Hairy Nanoparticles

Nearly monodisperse nanoparticle (NP) spheres attached to a nonvolatile ionic liquid surface were tracked by in situ scanning electron microscopy to obtain the tracer diffusion coefficient Dtr as a function of the areal fraction ϕ. The in situ technique resolved both tracer (gold) and background (silica) particles for ∼1-2 min, highlighting their mechanisms of diffusion, which were strongly dependent on ϕ. Structure and dynamics at low and moderate ϕ paralleled those reported for larger colloidal spheres, showing an increase in order and a decrease in Dtr by over 4 orders of magnitude. However, ligand interactions were more important near jamming, leading to different caging and jamming dynamics for smaller NPs. The normalized Dtr at ultrahigh ϕ depended on particle diameter and ligand molecular weight. Increasing the PEG molecular weight by a factor of 4 increased Dtr by 2 orders of magnitude at ultrahigh ϕ, indicating stronger ligand lubrication for smaller particles.

Bioinspired Switchable Passive Daytime Radiative Cooling Coatings

Passive daytime radiative cooling (PDRC) relies on simultaneous reflection of sunlight and radiation toward cold outer space. Current designs of PDRC coatings have demonstrated potential as eco-friendly alternatives to traditional electrical air conditioning (AC). While many features of PDRC have been individually optimized in different studies, for practical impact, it is essential for a system to demonstrate excellence in all essential aspects, like the materials that nature has created. We propose a bioinspired PDRC structure templated by bicontinuous interfacially jammed emulsion gels (bijels) that possesses excellent cooling, thinness, tunability, scalability, and mechanical robustness. The unique bicontinuous disordered structure captures key features of Cyphochilus beetle scales, enabling a thin (130 μm) bijel PDRC coating to achieve high solar reflectance (≳0.97) and high longwave-infrared (LWIR) emissivity (≳0.93), resulting in a subambient temperature drop of ∼5.6 °C under direct sunlight. We further demonstrate switchable cooling inspired by the exoskeleton of the Hercules beetle and mechanical robustness in analogy to spongy bone structures.

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.

Spiral grass inspired eco-friendly zein fibrous membrane for multi-efficient air purification

Air pollution, one of the severest threats to public health, may lead to cardiovascular and respiratory illnesses. In order to cope with the deteriorating air pollutant, there is an increasing demand for filters with high purification efficiency, but it's tough to strike a balance between efficiency and resistance. Fabricating an eco-friendly fibrous filter which can capture both PM2.5 and gaseous chemical hazards with high efficiency but under ultra-low resistance is a long-term challenge. Herein, inspired by the interesting ribbon shape of spiral grass, a green and robust 3D nonwoven membrane with controllable hierarchical structure made of self-curved zein nanofibers modified by zeolitic imidazolate framework-8 (ZIF-8) via bi-solvent electrospinning and fumigation welding method was fabricated. The obtained ZIF-8 modified zein membranes showed extraordinary overall performance with high PM2.5 removal efficiency (99.04 %) at a low stress drop (54.87 Pa), first-rate formaldehyde removal efficiency (98.8 %) and excellent photocatalytic antibacterial. In addition, the relatively weak mechanical properties of zein fibrous membranes have been improved via solvent fumigation welding of the joint zein fibers. This study provides a green and convenient insight to the manufacturing of environmentally-friendly zein fibrous membranes with high filtration efficiency, low air resistance and high formaldehyde removal for sustainable air remediation.

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.

Modulation of Oil/Polymer Nanocapsule Size via Phase Diagram-Guided Microfluidic Coprecipitation

Flow-based nanoprecipitation of different solutes via rapid mixing of two miscible liquids is a scalable strategy for manufacturing nanoparticles with various shapes and morphologies. Controlling the size of nanoparticles in flow-based nanoprecipitation, however, is often left to empirical variations in the flow rate ratios or the total flow rate of the two streams. In this work, we investigate the coprecipitations of oil and polymer to form nanocapsules via the Ouzo effect using glass capillary microfluidics across a range of mixing conditions. In the range of flow rates studied, the two streams mix convectively in micro-vortices formed at the junction of the two stream inlets. Using computational fluid dynamics simulations and glass capillary microfluidic nanoprecipitation, we establish a relationship between the precipitation conditions occurring experimentally in situ and the location on the ternary Ouzo phase diagram where precipitation is taking place. We find that a key variable in the resulting average diameter of the fabricated capsules is the degree of supersaturation experienced by both the oil and the polymer in the vortex zone of the device, showing a strong correlation between the two values. The control over the nanocapsule size by varying the extent of supersaturation of both precipitants is demonstrated by using two oils having distinct phase diagrams. This work provides a systematic approach to controlling the size of nanoparticles fabricated via continuous nanoprecipitation by linking the in situ flow conditions to ternary phase diagram behavior, enabling accurate control over nanocapsule size.

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.

Fluid Flow Templating of Polymeric Soft Matter with Diverse Morphologies

It is challenging to find a conventional nanofabrication technique that can consistently produce soft polymeric matter of high surface area and nanoscale morphology in a way that is scalable, versatile, and easily tunable. Here, the capabilities of a universal method for fabricating diverse nano- and micro-scale morphologies based on polymer precipitation templated by the fluid streamlines in multiphasic flow are explored. It is shown that while the procedure is operationally simple, various combinations of its intertwined mechanisms can controllably and reproducibly lead to the formation of an extraordinary wide range of colloidal morphologies. By systematically investigating the process conditions, 12 distinct classes of polymer micro- and nano-structures including particles, rods, ribbons, nanosheets, and soft dendritic colloids (dendricolloids) are identified. The outcomes are interpreted by delineating the physical processes into three stages: hydrodynamic shear, capillary and mechanical breakup, and polymer precipitation rate. The insights into the underlying fundamental mechanisms provide guidance toward developing a versatile and scalable nanofabrication platform. It is verified that the liquid shear-based technique is versatile and works well with many chemically diverse polymers and biopolymers, showing potential as a universal tool for simple and scalable nanofabrication of many morphologically distinct soft matter classes.

Fabrication and Evaluation of Multiwalled Carbon Nanotube-Containing Bijels and Bijels-Derived Porous Nanocomposites

Structures having continuous porous networks are of great interest for applications in areas such as separation, energy storage, and tissue engineering. Bicontinuous interfacially jammed emulsion gels ("bijels") have been actively investigated as templates for fabricating useful structures for such applications. However, the fabrication of bijels-templated porous nanocomposites incorporated with reinforcing or functional nanoparticles (or nanofibers) to provide specific, targeted functions is still a challenge, stemming from the difficulties of fabricating functional nanoparticle-containing bijels. In this study, bijels-derived porous nanocomposites incorporated with multiwalled carbon nanotubes (MWCNTs), which possessed interconnected channels inside the structures, were made via a facile phase inversion technique for bijels fabrication. For the composite manufacture, in the first step of bijels fabrication, MWCNT adsorption into the oil phase of bijels was observed. It was revealed that MWCNTs were physically absorbed into the oil-rich phase without disrupting the bicontinuous structure of bijels. The successful fabrication of non-crosslinked and crosslinked porous structures containing MWCNTs was evidenced through imaging by confocal laser scanning microscopy and scanning electron microscopy, respectively. For potential controlled release applications, an anticancer drug, doxorubicin hydrochloride (DOX), was incorporated into bijels-derived structures and nanocomposites. The in vitro DOX release profiles from drug delivery systems based on bijels-derived MWCNT-containing nanocomposites suggested that the photothermal effect of MWCNTs initiated by near-infrared irradiation could modulate the drug release behavior. Overall, this study has developed a facile approach to fabricate bijels-templated bicontinuous porous structures incorporated with functional nanoparticles (or nanofibers) and opened an avenue for making MWCNT-containing porous nanocomposites for controlled drug release applications.

Natural Amphiphilic Shellac Nanoparticle-Stabilized Novel Pickering Emulsions with Droplets and Bi-continuous Structures

Shellac is a natural amphiphilic substance, and its nanoparticles can be used to stabilize Pickering emulsions with droplets and bi-continuous structures. In this study, shellac nanoparticles (SNPs) were produced through the anti-solvent method, and these SNPs were used to produce a series of Pickering emulsions. Fourier transform infrared results showed that SNPs were generated through hydrogen bonding and hydrophobic effects. The contact angle of SNPs was 122.3°, indicating that hydrophobicity was their dominant characteristic. According to the results of confocal laser scanning microscopy, the Pickering emulsions stabilized by SNPs showed oil-in-water, bi-continuous structure, and water-in-oil characteristics, which were dependent on the oil-phase content. The resistance value of the emulsified part of these Pickering emulsion systems significantly increased at an oil-phase ratio of 80-90% (more than 10⁵ MΩ), as compared with the 10-70% oil-phase content (around 1 MΩ). The viscosity of SNP-stabilized Pickering emulsions with bi-continuous structures was highest at 40% oil-phase content. The porous material prepared by using Pickering emulsions with bi-continuous structures as a template had an interconnected structure and was able to absorb both water and oil. This study indicated that these amphiphilic SNPs readily form bi-continuous structures and effectively stabilize Pickering emulsions with droplets. These SNPs are expected to have increased application in food and pharmaceutical industries.

Bijels as a Fluid Labyrinth for Drugs: The Effect of Nanoparticles on the Release Kinetics of Ethosuximide and Dimethyl Fumarate

Bijels (bicontinuous interfacially jammed emulsion gels) raised an increasing interest as biomaterials for controlled drug delivery due to their biphasic nature organized in mesoscopic tortuous domains. Two bijel formulations were prepared and explored as delivery systems for both hydrophilic and lipophilic drugs, ethosuximide and dimethyl fumarate. The two bijel-like structures, based on polymerized ε-caprolactone/water, differ in the stabilizing nanoparticle hydroxyapatite (inorganic) and nanogel-based nanoparticles (organic). Diffusion nuclear magnetic resonance spectroscopy has been used to characterize the bijel structure and the transport behavior of the drug molecules confined within the water/organic interconnected domains. A reduced diffusion coefficient is observed for several concentrations of the drugs and both bijel formulations. Moreover, in vitro release profiles also reveal the effect of the microstructure and drug-nanoparticle interactions.

In situ imaging of the three-dimensional shape of soft responsive particles at fluid interfaces by atomic force microscopy

The reconfiguration of individual soft and deformable particles upon adsorption at a fluid interface underpins many aspects of their dynamics and interactions, ultimately regulating the properties of monolayers of relevance for applications. In this work, we demonstrate that atomic force microscopy can be used for the in situ reconstruction of the three-dimensional conformation of model poly(N-isopropylacrylamide) microgels adsorbed at an oil-water interface. We image the particle topography from both sides of the interface to characterize its in-plane deformation and to visualize the occurrence of asymmetric swelling in the two fluids. In addition, the technique enables investigating different fluid phases and particle architectures, as well as studying the effect of temperature variations on particle conformation in situ. We envisage that these results open up an exciting range of possibilities to provide microscopic insights into the single-particle behavior of soft objects at fluid interfaces and into the resulting macroscopic material properties.

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.

Aquabots

Soft robots, made from elastomers, easily bend and flex, but deformability constraints severely limit navigation through and within narrow, confined spaces. Using aqueous two-phase systems we print water-in-water constructs that, by aqueous phase-separation-induced self-assembly, produce ultrasoft liquid robots, termed aquabots, comprised of hierarchical structures that span in length scale from the nanoscopic to microsciopic, that are beyond the resolution limits of printing and overcome the deformability barrier. The exterior of the compartmentalized membranes is easily functionalized, for example, by binding enzymes, catalytic nanoparticles, and magnetic nanoparticles that impart sensitive magnetic responsiveness. These ultrasoft aquabots can adapt their shape for gripping and transporting objects and can be used for targeted photocatalysis, delivery, and release in confined and tortuous spaces. These biocompatible, multicompartmental, and multifunctional aquabots can be readily applied to medical micromanipulation, targeted cargo delivery, tissue engineering, and biomimetics.

Spongy all-in-liquid materials by in-situ formation of emulsions at oil-water interfaces

Printing a structured network of functionalized droplets in a liquid medium enables engineering collectives of living cells for functional purposes and promises enormous applications in processes ranging from energy storage to tissue engineering. Current approaches are limited to drop-by-drop printing or face limitations in reproducing the sophisticated internal features of a structured material and its interactions with the surrounding media. Here, we report a simple approach for creating stable liquid filaments of silica nanoparticle dispersions and use them as inks to print all-in-liquid materials that consist of a network of droplets. Silica nanoparticles stabilize liquid filaments at Weber numbers two orders of magnitude smaller than previously reported in liquid-liquid systems by rapidly producing a concentrated emulsion zone at the oil-water interface. We experimentally demonstrate the printed aqueous phase is emulsified in-situ; consequently, a 3D structure is achieved with flexible walls consisting of layered emulsions. The tube-like printed features have a spongy texture resembling miniaturized versions of “tube sponges” found in the oceans. A scaling analysis based on the interplay between hydrodynamics and emulsification kinetics reveals that filaments are formed when emulsions are generated and remain at the interface during the printing period. Stabilized filaments are utilized for printing liquid-based fluidic channels.

All-in-liquid printing promises applications from energy storage to drug delivery and tissue engineering. Here, authors present the in-situ generation of layered emulsion in a fraction of a second at the oil-water interface forming 3D tube-like structures in a liquid medium.

Bijel rheology reveals a 2D colloidal glass wrapped in 3D

We present rheological evidence demonstrating the glass-like nature of bicontinuous interfacially jammed emulsion gels (bijels). Under small amplitude oscillatory shear, bijels exhibited rheological signatures akin to α and β relaxation that are also invariable to interfacial tension changes, behaviors which are reminiscent of caged particle dynamics found in colloidal glasses, and well described by a previously reported adaptation of mode-coupling theory for colloidal glass rheology. Guided by their rheological signatures and supported by particle detachment and attraction energy approximations, we rationalize that bijels can be represented as 2-dimensional (2D) colloidal glasses that percolate in 3-dimensional (3D) space, and attractive interactions are not required for their stability. To provide further support for this conjecture, we qualitatively compare the rheology of bijels and a capillary suspension that is stabilized by strong, rigid capillary bridges between the particles, beyond their limit of linear viscoelasticity. Our results demonstrate that the strong adsorption of particles to the continuous interface and the lack of strong attractive interparticle forces enable recovery by interfacial tension into new jammed configurations after shear deformation. These behaviors are qualitatively different from those in the capillary suspension, where the breaking of attractive interparticle bonds results in dramatic changes to the microstructure and rheology over a narrow range of shear amplitudes. Our findings unveil bijels as 2D colloidal glasses weaving in 3D space and establish that interparticle attractions are not required for stability in bijels, and interfacial jamming alone is sufficient to impart viscoelasticity and gel-like rheology to these materials.

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

Fluid-bicontinuous gels are unique materials that allow two distinct fluids to interact through a percolating, rigid scaffold. Current restrictions for their use are the large fluid-channel sizes (>5 µm), limiting the fluid-fluid interaction surface-area, and the inability to flow liquids through the channels. In this work a scalable synthesis route of nanoparticle stabilized fluid-bicontinuous gels with channels sizes below 500 nm and specific surface areas of 2 m² cm^-3 is introduced. Moreover, it is demonstrated that liquids can be pumped through the fluid-bicontinuous gels via electroosmosis. The fast liquid flow in the fluid-bicontinuous gel facilitates their use for molecular separations in continuous-flow liquid-liquid extraction. Together with the high surface areas, liquid flow through fluid-bicontinuous gels enhances their potential as highly permeable porous materials with possible uses as microreaction media, fuel-cell components, and separation membranes.

Centrifugal Assembly of Helical Bijel Fibers for pH Responsive Composite Hydrogels

In microfluidics, centrifugal forces are important for centrifugal microfluidic chips and curved microchannels. Here, an unrecognized use of the centrifugal effect in microfluidics is introduced. The assembly of helical soft matter fibers in a rotating microcapillary is investigated. During assembly, the fibers undergo phase separation, generating particle stabilized bicontinuous interfacially jammed emulsions gels. This process is accompanied by a transition of the fiber density over time. As a result, the direction of the centrifugal force in the rotating microcapillary changes. The authors analyze this effect systematically with high-speed video microscopy and complementary computer simulations. The resulting understanding enables the control of the helical fiber assembly into microropes. These microropes can be converted into pH responsive hydrogels that swell and shrink with potential applications in tissue engineering, soft robotics, controlled release, and sensing. More generally, the knowledge gained from this work shows that centrifugal forces potentially enable directed self-assembly or separation of colloids, biological cells, and emulsions in microfluidics.

Multicompartment Hydrogels

Hydrogels belong to the most promising materials in polymer and materials science at the moment. As they feature soft and tissue-like character as well as high water-content, a broad range of applications are addressed with hydrogels, e.g. tissue engineering and wound dressings but also soft robotics, drug delivery, actuators and catalysis. Ways to tailor hydrogel properties are crosslinking mechanism, hydrogel shape and reinforcement, but new features can be introduced by variation of hydrogel composition as well, e.g. via monomer choice, functionalization or compartmentalization. Especially, multicompartment hydrogels drive progress towards complex and highly functional soft materials. In the present review the latest developments in multicompartment hydrogels are highlighted with a focus on three types of compartments, i.e. micellar/vesicular, droplets or multi-layers including various sub-categories. Furthermore, several morphologies of compartmentalized hydrogels and applications of multicompartment hydrogels will be discussed as well. Finally, an outlook towards future developments of the field will be given. The further development of multicompartment hydrogels is highly relevant for a broad range of applications and will have a significant impact on biomedicine and organic devices. This article is protected by copyright. All rights reserved.

Microfluidics-Enabled Soft Manufacture of Materials with Tailorable Wettability

Microfluidics and wettability are interrelated and mutually reinforcing fields, experiencing synergistic growth. Surface wettability is paramount in regulating microfluidic flows for processing and manipulating fluids at the microscale. Microfluidics, in turn, has emerged as a versatile platform for tailoring the wettability of materials. We present a critical review on the microfluidics-enabled soft manufacture (MESM) of materials with well-controlled wettability and their multidisciplinary applications. Microfluidics provides a variety of liquid templates for engineering materials with exquisite composition and morphology, laying the foundation for precisely controlling the wettability. Depending on the degree of ordering, liquid templates are divided into individual droplets, one-dimensional (1D) arrays, and two-dimensional (2D) or three-dimensional (3D) assemblies for the modular fabrication of microparticles, microfibers, and monolithic porous materials, respectively. Future exploration of MESM will enrich the diversity of chemical composition and physical structure for wettability control and thus markedly broaden the application horizons across engineering, physics, chemistry, biology, and medicine. This review aims to systematize this emerging yet robust technology, with the hope of aiding the realization of its full potential.

Fabrication of Multi-Layered Microspheres Based on Phase Separation for Drug Delivery

In this work, we used a co-flow microfluidic device with an injection and a collection tube to generate droplets with different layers due to phase separation. The phase separation system consisted of poly(ethylene glycol) diacrylate 700 (PEGDA 700), PEGDA 250, and sodium alginate aqueous solution. When the mixture droplets formed in the outer phase, PEGDA 700 in the droplets would transfer into the outer aqueous solution, while PEGDA 250 still stayed in the initial droplet, breaking the miscibility equilibrium of the mixture and triggering the phase separation. As the phase separation proceeded, new cores emerged in the droplets, gradually forming the second and third layers. Emulsion droplets with different layers were polymerized under ultraviolet (UV) irradiation at different stages of phase separation to obtain microspheres. Microspheres with different layers showed various release behaviors in simulated gastric fluid (SGF) and simulated intestinal fluid (SIF). The release rate decreased with the increase in the number of layers, which showed a potential application in sustained drug release.

Exploring New Horizons in Liquid Compartmentalization via Microfluidics

Spatial organization of cellular processes is crucial to efficiently regulate life's essential reactions. Nature does this by compartmentalization, either using membranes, such as the cell and nuclear membrane, or by liquid-like droplets formed by aqueous liquid-liquid phase separation. Aqueous liquid-liquid phase separation can be divided in two different phenomena, associative and segregative phase separation, of which both are studied for their membraneless compartmentalization abilities. For centuries, segregative phase separation has been used for the extraction and purification of biomolecules. With the emergence of microfluidic techniques, further exciting possibilities were explored because of their ability to fine-tune phase separation within emulsions of various compositions and morphologies and achieve one of the simplest forms of compartmentalization. Lately, interest in aqueous liquid-liquid phase separation has been revived due to the discovery of membraneless phases within the cell. In this Perspective we focus on segregative aqueous phase separation, discuss the theory of this interesting phenomenon, and give an overview of the evolution of aqueous phase separation in microfluidics.

Phase-Field Modeling of Multiple Emulsions Via Spinodal Decomposition

Currently, multiple emulsions via liquid-liquid phase separation in ternary polymer solutions have sparked considerable interest because of its remarkable potential in physical, medical, and biological applications. The transient "onion-like" multilayers are highly dependent on the evolution kinetics, which is challenging to be scrutinized in experiments and has not yet been fully understood. Here, we report a thermodynamically consistent multicomponent Cahn-Hilliard model to investigate the kinetics of multiple emulsions by tracing the temporal evolution of the local compositions inside the emulsion droplets. We reveal that the mechanism governing the kinetics is attributed to the competition between surface energy minimization and phase separation. Based on this concept, a generalized morphology diagram for different emulsion patterns is achieved, showing a good accordance with previous experiments. Moreover, combining the analysis for the kinetics and the morphology diagram, we predict new emulsion structures that provide general guidelines to discovery, design, and manipulation of complex multiphase emulsions.

Microspheres viscous drag at a deformed fluid interface: particle's weight and electrical charges effects

When a microparticle is trapped at a fluid interface, particle's electrical charge and weight combine to deform the interface. Such deformation is expected to affect the particle diffusion via hydrodynamics boundary conditions. Using available models of particle-induced electrostatic deformation of the interface and particle dynamics at the interface, we are able to analytically predict particle diffusion coefficient values in a large range of particle's contact angle and size. This might offer a solid background of numerical values to compare with for future experimental studies in the field of particle diffusion at a fluid interface.

Stabilizing liquid drops in nonequilibrium shapes by the interfacial crosslinking of nanoparticles

Droplets are spherical due to the principle of interfacial energy minimization. Here, we show that nonequilibrium droplet shapes can be stabilized via the interfacial self-assembly and crosslinking of nanoparticles. This principle allows for the stability of practically infinitely long liquid tubules and monodisperse cylindrical droplets. Droplets of oil-in-water are elongated via gravitational or hydrodynamic forces at a reduced interfacial tension. Silica nanoparticles self-assemble and cross-link on the interface triggered by the synergistic surface modification with hexyltrimethylammonium- and trivalent lanthanum-cations. The droplet length dependence is described by a scaling relationship and the rate of nanoparticle deposition on the droplets is estimated. Our approach potentially enables the 3D-printing of Newtonian Fluids, broadening the array of material options for additive manufacturing techniques.

Tunable thermo-reversible bicontinuous nanoparticle gel driven by the binary solvent segregation

Bicontinuous porous structures through colloidal assembly realized by non-equilibrium process is crucial to various applications, including water treatment, catalysis and energy storage. However, as non-equilibrium structures are process-dependent, it is very challenging to simultaneously achieve reversibility, reproducibility, scalability, and tunability over material structures and properties. Here, a novel solvent segregation driven gel (SeedGel) is proposed and demonstrated to arrest bicontinuous structures with excellent thermal structural reversibility and reproducibility, tunable domain size, adjustable gel transition temperature, and amazing optical properties. It is achieved by trapping nanoparticles into one of the solvent domains upon the phase separation of the binary solvent. Due to the universality of the solvent driven particle phase separation, SeedGel is thus potentially a generic method for a wide range of colloidal systems.

Bicontinuous porous materials made by colloidal self-assemblies have many applications. Xi et al. utilize colloidal particles dispersed in a binary solvent to form thermo-reversible bicontinuous gel structures with good reproducibility and scalability, and tunable structural and optical properties.

Phase Inversion-Based Technique for Fabricating Bijels and Bijels-Derived Structures with Tunable Microstructures

Bicontinuous interfacially jammed emulsion gels ("bijels") are a new class of soft matter containing two interpenetrating continuous phases. They have great potential for applications in many areas. However, difficulties in fabricating bijels and controlling structural features of interest have posed severe barriers to their wide applications. In this study, a phase inversion-based technique was developed for fabricating bijels and bijels-derived structures. The effects of varying the composition of casting solutions for the fabrication of bijels on the porosity, oil-to-water percentage, and domain size of bijels were investigated. Composite bijels prepared from two organic monomers were also made, demonstrating the flexibility of the phase inversion-based technique for the fabrication of bijels. Interestingly, the incorporation of a second monomer into the casting solution also affected the porosity and domain size of bijels formed, which may provide a new strategy for the controlled fabrication of bijels. Doxorubicin hydrochloride (DOX, as a model drug)-loaded bijels-derived hybrid hydrogels comprising two continuous phases were successfully made, with one phase being cross-linked alginate that carried the drug. Controlled release of DOX from the bijels-derived structures could be achieved. In vitro degradation study indicated that cross-linking of alginate in bijels-derived hybrid hydrogels controlled alginate degradation, thereby affecting the DOX release behavior. Our current work has provided a facile and reproducible protocol for the controlled fabrication of bijels and bijels-derived structures, which facilitates expanding their applications in the biomedical field.

Interpretation of interfacial interactions between lenticular particles

HYPOTHESIS

The geometric features of charged particles at a fluid-fluid interface substantially affect their interfacial configurations and interparticle interactions (electrostatic and capillary forces). Because lenticular particles exhibit both spherical and nonspherical surface characteristics, an investigation of their interfacial phenomena can provide in-depth understanding of the relationship between the configuration and the interactions of these particles at the interface.

EXPERIMENTS

Three types of lenticular particles are prepared using a seeded emulsion polymerization method. Pair interactions at the oil-water interface are directly measured with optical laser tweezers. The numerical calculation of the attachment energy of the particle to the interface is used to predict their configuration behaviors at the interface.

FINDINGS

The lenticular particles are found to adopt either an upright or inverted configuration that can be determined stochastically. When the interface contacts the truncated boundary or the biconvex boundary, the local interface deformation-induced capillary attraction likely becomes dominant. The contact probability can be estimated on the basis of the attachment energy profile and related to the relative strengths of capillary attraction and electrostatic repulsion between two particles at the interface. Furthermore, possible artifacts in measurements of the pair interactions between nonspherical particles with optical laser tweezers are discussed, depending on their interfacial configurations.

Fabrication of solvent transfer-induced phase separation bijels with mixtures of hydrophilic and hydrophobic nanoparticles

Bicontinuous interfacially jammed emulsion gels (bijels), in which the oil and water phases are co-continuous throughout the structure, have potential for applications in separation, catalysis, tissue engineering and energy devices. Among the possible fabrication paths, the solvent transfer-induced phase separation (STRIPS) method has proven to be a powerful approach to produce bijels in a continuous fashion with a broad selection of liquids and nanoparticles. The successful formation of bicontinuous domains requires the use of neutrally wetting particles which was achieved by in situ modification of silica nanoparticles with an oppositely charged surfactant. This approach, however, is not ideal for applications that are adversely affected by the presence of surfactant. In this work, we use a pair of nanoparticles, one hydrophilic, and the other hydrophobic, to stabilize STRIPS bijels without any surfactants and show that the ratio of the hydrophilic to hydrophobic nanoparticles required to form stable bijels changes with the polarity of the oil phase. Highly non-polar oils require a smaller ratio than moderately polar oils. Furthermore, if a sufficiently polar oil is selected, STRIPS bijels can be stabilized using only the hydrophilic nanoparticle. Our results demonstrate the potential to imbue the interface of biphasic liquid mixtures such as bijels with multifunctionality by using two functional nanoparticles of opposite polarity.

Controlling Surfactant Adsorption on Highly Charged Nanoparticles to Stabilize Bijels

Bicontinuous particle-stabilized emulsions (bijels) are networks of interpenetrating oil/water channels with applications in catalysis, tissue engineering, and energy storage. Bijels can be generated by arresting solvent transfer induced phase separation (STrIPS) via interfacial jamming of nanoparticles. However, until now, STrIPS bijels have only been formed with silica nanoparticles of low surface charge densities, limiting their potential applications in catalysis and fluid transport. Here, we show how strongly charged silica nanoparticles can stabilize bijels. To this end, we carry out a systematic study employing dynamic light scattering, zeta potential, acid/base titrations, turbidimetry, surface tension, and confocal microscopy. We find that moderating the adsorption of oppositely charged surfactants on the particles is crucial to facilitate particle dispersibility in the bijel casting mixture and bijel stabilization. Our results potentially introduce a general understanding for bijel fabrication with different inorganic nanoparticle materials of variable charge density.

Autonomous analysis to identify bijels from two-dimensional images

Bicontinuous interfacially jammed emulsion gels (bijels) are novel composite materials that can be challenging to manufacture. As a step towards automating production, we have developed a machine learning tool to classify fabrication attempts. We use training and testing data in the form of confocal images from both successful and unsuccessful attempts at bijel fabrication. We then apply machine learning techniques to this data in order to classify whether an image is a bijel or a non-bijel. Our principal approach is to process the images to find their autocorrelation function and structure factor, and from these functions we identify variables that can be used for training a supervised machine learning model to identify a bijel image. We are able to categorise images with reasonable accuracies of 85.4% and 87.5% for two different approaches. We find that using both the liquid and particle channels helps to achieve optimal performance and that successful classification relies on the bijel samples sharing a characteristic length scale. Our second approach is to classify the shapes of the liquid domains directly; the shape descriptors are then used to classify fabrication attempts via a decision tree. We have used an adaptive design approach to find an image pre-processing step that yields the optimal classification results. Again, we find that the characteristic length scale of the images is crucial in performing the classification.

Creation of Nonspherical Microparticles through Osmosis-Driven Arrested Coalescence of Microfluidic Emulsions

Droplet-based microfluidics enable the production of emulsions and microparticles with spherical shapes, but the high-throughput fabrication of nonspherical emulsions and microparticles still remains challenging because interfacial tension plays a dominant role during preparation. Herein, ionic liquids (ILs) containing salts, which possess sufficient osmotic pressure to realize water transport and phase separation, are introduced as inner cores of oil-in-oil-in-water double emulsions and it is shown that nonspherical emulsions can be constructed by osmosis-driven arrested coalescence of inner cores. Subsequently, ultraviolet polymerization of the nonspherical emulsions leads to nonspherical microparticles. By tailoring the number, composition, and size of inner cores as well as coalescence time, a variety of nonspherical shapes such as dumbbell, rod, spindle, snowman, tumbler, three-pointed star, triangle, and scalene triangle are created. Importantly, benefitting from excellent solvency of ILs, this system can serve as a general platform to produce nonspherical microparticles made from different materials. Moreover, by controlling the osmotic pressure, programmed coalescence of inner cores in double emulsions is realizable, which indicates the potential to build microreactors. Thus, a simple and high-throughput strategy to create nonspherical microparticles with arrested coalescence shapes is developed for the first time and can be further used to construct novel materials and microreactors.

Functional Fibers, Composites and Textiles Utilizing Photothermal and Joule Heating

This review focuses on the mechanism of adjusting the thermal environment surrounding the human body via textiles. Recently highlighted technologies for thermal management are based on the photothermal conversion principle and Joule heating for wearable electronics. Recent innovations in this technology are described, with a focus on reports in the last three years and are categorized into three subjects: (1) thermal management technologies of a passive type using light irradiation of the outside environment (photothermal heating), (2) those of an active type employing external electrical circuits (Joule heating), and (3) biomimetic structures. Fibers and textiles from the design of fibers and textiles perspective are also discussed with suggestions for future directions to maximize thermal storage and to minimize heat loss.