Dynamically reconfigurable complex emulsions via tunable interfacial tensions

Precise morphology control of all-organic core-shell droplets for synthesis of microencapsulated phase change materials through AC electric fields

HYPOTHESIS

Complex emulsions usually consist of aqueous phases, like oil-in-water-in-oil (o/w/o) and water-in-oil-in-water (w/o/w), serving foundational roles in colloid science. Oil-in-oil-oil (o/o/o) emulsions offer new avenues for non-aqueous reagents but face challenges in balancing the forces between multiple organic phases.

EXPERIMENTS

In this work, we generate o/o/o emulsions by integrating an AC electric field with a double cross-junction microchannel. The characteristics of generating dynamics is observed and analyzed based on the interaction between the electric force, viscous force, and interfacial tension.

FINDINGS

We first establish an innovative evaluation theory to quantify the generation efficiency for complex emulsions. The results show that the electric effect improves the generation efficiency and monodispersity across a variety of high flow rates compared with conventional methods, enabling the flexibility in adjusting droplet sizes and core-shell structures. At low flow rates, the breakup of core-shell droplets can also be controlled by the electric force under different types of o/o/o emulsions. The inner phase could be substituted with alkane phase-change materials and processed into microencapsulated phase-change materials (MEPCMs). These organic MEPCMs could be integrated into electrolytes due to their ultra-low electric conductivity, which shows a significant temperature buffering effect in lithium batteries. This research not only enhances our understanding of colloidal systems but also fabricates core-shell structures with customized functionalities, paving the way for advancements in energy conversion and management, drug delivery, and materials engineering.

Fluorophilic boronic acid copolymer surfactant for stabilization of complex emulsion droplets with fluorinated oil

A fluorosurfactant is synthesized by the copolymerization of fluoroacrylate and boronic acid acrylamide monomers to stabilize fluorinated oil droplets. The copolymer surfactant couples with diols or polyols in adjacent aqueous phase to form an ultrathin elastic interfacial film to rigidify the interface, thereby preventing drop recoalescence and enabling new applications.

Asymmetric Self-Assembly of Colloidal Superstructures in Nested Transient Emulsion Aerosols

Emulsions are versatile and robust platforms for colloidal self-assembly, but their ability to create complex and functional superstructures is hindered by the inherent symmetry of droplets. Here the creation of an aerosol of nested transient emulsion droplets with inherent asymmetry is reported, achieved by converging beams of water and 1-butanol mists. Self-assembly of nanoparticles occurs within such emulsion droplets as driven by the rapid two-phase interface diffusion, producing anisotropic superstructures. A unique hollowing process is observed due to the asymmetric diffusion of solvents, akin to the Kirkendall effect. This novel assembly platform offers several advantages, including asymmetric self-assembly in air, surfactant-free operation, and tunable droplet size. It enables the creation of clean, functional nanoparticle superstructures that can be easily disassembled when needed. These advancements pave the way for exploring intricate, anisotropic superstructures with diverse applications that are unavailable in conventional superstructures of spherical symmetry.

Bio-Based Multicompartment Photonic Pigments: Unlocking Non-Iridescent Pure RGB Structural Colors for Versatile Chromatic Engineering

Non-iridescent photonic glass pigments of block copolymers show great potential for sustainable structural coloration. However, the ability to create accurate RGB color mixtures for real-world applications is limited by the prevalent use of non-degradable, fossil oil-derived components and the difficulty in achieving pure red hues. This work presents an alternative strategy for achieving more sustainable structural coloration by fabricating composite photonic pigments through controlled self-assembly of water, vegetable oil, and biodegradable bottlebrush block copolymers (BBCPs) in a complex emulsion system. The obtained photonic balls feature unprecedented multicompartment structures characterized by a short-range ordered assembly of water nanodroplets stabilized by the BBCPs, along with oil droplets stabilized by these nanodroplets, which substantially enhances resistance to Ostwald ripening. Furthermore, a new structural model is introduced to eliminate disordered scattering, successfully creating a pure red structural color and overcoming a long-standing limitation in versatile chromatic engineering.

Responsive Surfactant-Driven Morphology Transformation of Block Copolymer Microparticles

Block copolymer (BCP) microparticles, which exhibit rapid change of morphology and physicochemical property in response to external stimuli, represent a promising avenue for the development of programmable smart materials. Among the methods available for generating BCP microparticles with adjustable morphologies, the confined assembly of BCPs within emulsions has emerged as a particularly facile and versatile approach. This review provides a comprehensive overview of the role of responsive surfactants in modulating interfacial interactions at the oil-water interface, which facilitates controlled BCP microparticle morphology. We elucidate how variations in the properties of responsive surfactants, activated by external stimuli, influence BCP chain arrangement and interfacial selectivity. Additionally, this review explores the applications of shape-switchable microparticles in advanced technologies such as smart display, fluorescence modulation, magnetic resonance imaging, drug delivery, and photonic crystal. Finally, the challenges and prospective future directions in this rapidly evolving field are discussed.

Peptide-mediated liquid-liquid phase separation and biomolecular condensates

Liquid-liquid phase separation (LLPS) is a cornerstone of cellular organization, driving the formation of biomolecular condensates that regulate diverse biological processes and inspire innovative applications. This review explores the molecular mechanisms underlying peptide-mediated LLPS, emphasizing the roles of intermolecular interactions such as hydrophobic effects, electrostatic interactions, and π-π stacking in phase separation. The influence of environmental factors, such as pH, temperature, ionic strength, and molecular crowding on the stability and dynamics of peptide coacervates is examined, highlighting their tunable properties. Additionally, the unique physicochemical properties of peptide coacervates, including their viscoelastic behavior, interfacial dynamics, and stimuli-responsiveness, are discussed in the context of their biological relevance and engineering potential. Peptide coacervates are emerging as versatile platforms in biotechnology and medicine, particularly in drug delivery, tissue engineering, and synthetic biology. By integrating fundamental insights with practical applications, this review underscores the potential of peptide-mediated LLPS as a transformative tool for advancing science and healthcare.

Droplet-supported liquid-liquid lateral phase separation as a step to floating protein heterostructures

Liquid-liquid phase separation plays an important role in many natural and technological processes. Herein, we implement lateral microphase separation at the surface of oil micro-droplets suspended in water to prepare a range of discrete floating protein/polymer continuous two-dimensional (2D) heterostructures with variable interfacial domain structures and dynamics. We show that gel-like domains of bovine serum albumin (BSA) co-exist with fluid-like polyvinyl alcohol (PVA) regions at the oil droplet surface to produce floating heterostructures comprising a 2D phase-separated protein mesh or an array of discrete mobile protein rafts depending on the conditions employed. Enzymes are embedded in the discontinuous BSA domains to produce droplet-supported microphase-separated 2D reaction scaffolds that can be tuned for interfacial catalysis. Taken together, our work has general implications for the structural and functional augmentation of oil droplet interfaces and contributes to the surface engineering and functionality of droplet-based micro-reactors.

Competing Hexagonal and Square Lattices on a Spherical Surface

The structural properties of packed soft-core particles provide a platform to understand the cross-pollinated physical concepts in solid-state and soft-matter physics. Confined on a spherical surface, the traditional differential geometry also dictates the overall defect properties in otherwise regular crystal lattices. Using molecular dynamics simulation of the Hertzian model as a tool, we report here the emergence of new types of disclination patterns: domain and counter-domain defects, when hexagonal and square patterns coexist. A new angle is presented to understand the incompatibility between tiling lattice shapes and the available spherical areal shapes, which is common in nature─from molecular systems in biology to backbone construction in architectures.

Bridged emulsion gels from polymer-nanoparticle enabling large-amount biomedical encapsulation and functionalization

Large-amount encapsulation and subsequent expressing are common characteristics for many biomedical applications, such as cosmetic creams and medical ointments. Emulsion gels can accomplish that, but often undergo exclusive, complex, multiple synthesis steps, showing extremely laborious and non-universal. The method here is simple via precisely interfacial engineering in homogenizing a nanoparticle aqueous dispersion and a polymer oil solution, gaining interfacial 45° three-phase-contact-angle for the nanoparticle that can bridge across oil emulsions' interfaces and ultimately form interconnected macroscopic networks. Their bridged skeletons and rheology are tunable over a vast range and deterministic on the basis of components' inputs. Furthermore, emulsion gels with high encapsulation and storage ability encapsulating active sunscreen ingredients, as a proof-of-concept, outperform commercial products. The ease (only seconds by strongly mixing two solutions) and the versatile chemical selection of our synthetic emulsion gels suggest an exciting general, scalable strategy for the next-generation cosmetic, ointment or otherwise food gel systems.

Non-Ionic Fluorosurfactants for Droplet-Based in vivo Applications

Fluorocarbon oils are uniquely suited for many biomedical applications due to their inert, bioorthogonal properties. In order to interface fluorocarbon oils with biological systems, non-ionic fluorosurfactants are necessary. However, there is a paucity of non-ionic fluorosurfactants with low interfacial tension (IFT) to stabilize fluorocarbon phases in aqueous environments (such as oil-in-water emulsions). We developed non-ionic fluorosurfactants composed of a polyethylene glycol (PEG) segment covalently bonded to a flexible perfluoropolyether (PFPE) segment that confer low IFTs between a fluorocarbon oil (HFE-7700) and water. The synthesis of a panel of surfactants spanning a molecular weight range of 0.64-66 kDa with various hydrophilic-lipophilic balances allowed for identification of minimal IFTs, ranging from 1.4 to 17.8 mN m-1. The majority of these custom fluorosurfactants display poor solubility in water, allowing their co-introduction with fluorocarbon oils and minimal leaching. We applied the PEG5PFPE1 surfactant for mechanical force measurements in zebrafish, enabling exceptional sensitivity.

Near-identical macromolecules spontaneously partition into concentric circles

Although separation is entropically unfavourable, it is often essential for our life1,2. The separation of very similar macromolecules such as deoxyribonucleic acids (DNAs) and their single nucleotide variants is difficult but holds great advantage for the progress of life science3. Here we report that a particular liquid-liquid phase separation (LLPS) at a solid-liquid interface led to the partitioning of DNAs with nearly identical structures. We found this intriguing phenomenon when we did drop-casting onto a glass plate an aqueous ammonium sulfate dispersion of phase-separated droplets comprising a homogeneous mixture of poly(ethylene glycol) (PEG) samples with different termini. Even when the molecular weights of their PEG parts were identical to each other, terminally different PEGs spread competitively at the solid-liquid interface and partitioned into micrometre-scale concentric circles. We found that this competitive spreading was induced by an ammonium sulfate layer spontaneously formed on the glass surface. We successfully extended the above mechanism to partitioning a mixture of nearly identical DNAs into concentric circles followed by their selective extraction using the salting-in effect. We could isolate a human cancer-causing single nucleotide variant in 97% purity from its 1:1 mixture with the original DNA.

Preparation of bovine serum albumin-arabinoxylan cold-set gels by glucono-δ-lactone and salt ions double induction

In order to investigate the effect of glucono-δ-lactone (GDL) and different salt ions (Na+ and Ca2+) induction on the cold-set gels of bovine serum albumin (BSA)-arabinoxylan (AX), the gel properties and structure of BSA-AX cold-set gels were evaluated by analyzing the gel strength, water-holding capacity, thermal properties, and Fourier Transform Infrared (FTIR) spectra. It was shown that the best gel strength (109.15 g) was obtained when the ratio of BSA to AX was 15:1. The addition of 1 % GDL significantly improved the water-holding capacity, gel strength and thermal stability of the cold-set gels (p < 0.05), and the microstructure was smoother. Low concentrations of Na+ (3 mM) and Ca2+ (6 mM) significantly enhanced the hydrophobic interaction and hydrogen bonding between BSA and AX after acid induction, and the Na+-induced formation of a denser microstructure with a higher water-holding capacity (75.51 %). However, the excess salt ions disrupted the stable network structure of the cold-set gels and reduced their thermal stability and crystalline structure. The results of this study contribute to the understanding of the interactions between BSA and AX induced by GDL and salt ions, and provide a basis for designing hydrogels with different properties.

Encapsulation of Hydrophobic-but-Not-Lipophilic Perfluoro Liquids Based on a Self-Assembled Double Emulsion Template via Solvent Evaporation Method

The facile encapsulation of perfluoro liquids that are hydrophobic but not lipophilic into liposomes or microcapsules presents a significant challenge in the fields of biomedicine, dynamic optics, functional chemical applications, etc. This is due to their chemical inertness and physical immiscibility, particularly those with low boiling points. In this study, a novel strategy based on a double emulsion template via solvent evaporation is proposed after investigating the mechanism of three-phase emulsion systems. The perfluoro liquid droplets can be easily emulsified into a polymer solution as the second emulsion layer, where the polymer shell is formed during solvent evaporation in the continuum medium under proper processing controls. The morphology of particles is predictable and fits well with the linear model derived from Neumann's triangle in three-phase systems. Furthermore, a comprehensive study on the encapsulation of perfluoro ketone, which is widely used as a green fire extinguisher agent, is conducted as an example. The encapsulated perfluoro ketone showed instant thermal response upon heating while maintaining a good shelf life at room temperature. The remarkable fire suppression performance exhibited great potential for practical applications. This work offers more insight into the encapsulation of "naughty" perfluorinated chemicals and provides more possibilities for extended applications.

Optical Detection of Interleukin-6 Using Liquid Janus Emulsions Using Hyperthermophilic Affinity Proteins

When equal volumes of two immiscible liquids are mixed (e.g., a hydrocarbon and a fluorocarbon), Janus droplets can form in an aqueous solution. In a gravity-aligned Janus droplet, the boundary between the two phases is flat and thus optically transparent when viewed from above. When tipped due to interactions with an analyte (i.e., agglutination), the resulting change in refraction and reflection yields an optical signal that can be detected and quantified. This study reports the detection and quantitation of interleukin-6 (IL-6) using emulsions functionalized at the hydrocarbon:aqueous interface with engineered proteins that bind IL-6 at high affinity and specificity. Hyperthermophilic affinity proteins (rcSso7d) are derived from thermophiles, giving them excellent thermal stability. Two rcSso7d affinity protein variants were synthesized with a noncanonical azide-functionalized amino acid to enable click chemistry to novel polymeric anchors embedded in the hydrocarbon phase. The two binding proteins recognize different epitopes, enabling the detection of both monomeric and dimeric IL-6 via agglutination. It is noteworthy that the rsSso7d protein variants, in addition to having superior thermal stability and facile recombinant synthesis in E. coli, show superior performance when compared to commercial antibodies for IL-6.

Nanoparticle Surfactants at Complex Emulsion Interfaces

Using nanoparticle surfactants to stabilize the liquid-liquid interface has attracted significant attention for developing all-liquid constructs including emulsions and liquid devices. Here, an efficient strategy is demonstrated to stabilize complex emulsions that consist of multiphase droplets by using the co-assembly between the cellulose nanocrystal and amine-functionalized polystyrene. Cellulose nanocrystal surfactants (CNCSs) form and assembly in situ at the specified area of emulsion interface, showing a unique pH responsiveness due to their dynamic nature and allowing the reconfiguration of complex emulsion from encapsulated to Janus structures. Such complex emulsions can be further used as the templates to fabricate polymeric particles with hollow, semi-spherical, and spherical shapes on large scale. These findings establish a promising platform for designing intelligent soft matter that can be used in microreactors, sensors, and anisotropic materials.

Transparency-changing elastomers by controlling of the refractive index of liquid inclusions

Complex materials that change their optical properties in response to changes in environmental conditions can find applications in displays, smart windows, and optical sensors. Here a class of biphasic composites with stimuli-adaptive optical transmittance is introduced. The biphasic composites comprise aqueous droplets (a mixture of water, glycerol, and surfactant) embedded in an elastomeric matrix. The biphasic composites are tuned to be optically transparent through a careful match of the refractive indices between the aqueous droplets and the elastomeric matrix. We demonstrate that stimuli (e.g., salinity and temperature change) can trigger variations in the optical transmittance of the biphasic composite. The introduction of such transparency-changing soft matter with liquid inclusions offers a novel approach to designing advanced optical devices, optical sensors, and metamaterials.

Bursting of Underwater Oil Drops

For decades, two main facets of underwater oil spills have been explored extensively-the rise of oil drops and resulting evolution of the oil slick at the air-water interface. We report on the bursting of rising oil drops at an air-liquid interface which precedes slick formation and reveal a counterintuitive bulge reversal that releases a daughter oil droplet inside the bulk as opposed to upward-shooting jets observed in bursting air bubbles. By unraveling the underlying physics we show that daughter droplet size and bulk liquid properties are correlated and their formation can be suppressed by an increase in the bulk viscosity.

Microgels for Cell Delivery in Tissue Engineering and Regenerative Medicine

Microgels prepared from natural or synthetic hydrogel materials have aroused extensive attention as multifunctional cells or drug carriers, that are promising for tissue engineering and regenerative medicine. Microgels can also be aggregated into microporous scaffolds, promoting cell infiltration and proliferation for tissue repair. This review gives an overview of recent developments in the fabrication techniques and applications of microgels. A series of conventional and novel strategies including emulsification, microfluidic, lithography, electrospray, centrifugation, gas-shearing, three-dimensional bioprinting, etc. are discussed in depth. The characteristics and applications of microgels and microgel-based scaffolds for cell culture and delivery are elaborated with an emphasis on the advantages of these carriers in cell therapy. Additionally, we expound on the ongoing and foreseeable applications and current limitations of microgels and their aggregate in the field of biomedical engineering. Through stimulating innovative ideas, the present review paves new avenues for expanding the application of microgels in cell delivery techniques.

Controllable enzymatic hydrolysis in reverse Janus emulsion microreactors

HYPOTHESIS

The key feature of living cells is multicompartmentalization for enzymatic reactions. Artificial cell-like multicompartments with micro domains are appealing to mimic the biological counterparts. In addition, establishing a sustainable, efficient, and controllable reaction system for enzymatic hydrolysis is imperative for the production of natural fatty acids from animal and plant-based fats.

EXPERIMENTS

Reverse Janus emulsion microreactors, i.e. (W1 + W2)/O, is constructed through directly using natural fats as continuous phase and aqueous two-phase solutions (ATPS) as inner phases. Enzyme is confined in the compartmented aqueous droplets dominated by the salt of Na2SO4 and polyethylene glycol (PEG). Enzyme catalyzed ester hydrolysis employed as a model reaction is performed under the conditions of agitation-free and mild temperature. Regulation of reaction kinetics is investigated by diverse droplet topology, composition of inner ATPS, and on-demand emulsification.

FINDINGS

Excellent enzymatic activity toward hydrolysis of plant and animal oils achieves 88.5 % conversion after 3 h. Compartmented micro domains contribute to condense and organize the enzymes spatially. Timely removal of the products away from reaction sites of oil/water interface "pushed" the reaction forward. Distribution and transfer of enzyme in two aqueous lobes provide extra freedom in the regulation of hydrolysis kinetics, with equilibrium conversion controlled freely from 14.5 % to 88.5 %. Reversible "open" and "shut" of hydrolysis is acheived by on-demand emulsification and spontaneous demulsification. This paper paves the way to advancing progress in compartmentalized emulsion as a sustainable and high-efficiency platform for biocatalytic applications.

Dynamic Electrostatic Interfacial Engineering for Block Copolymer Microparticles with Reversible Structures

Responsive nanoparticle surfactants (NPSs) can dynamically and reversibly modulate the interfacial interactions between incompatible components, which are essential in the interfacial catalysis, corrosion, and self-assembly of block copolymers (BCPs). However, NPSs with stimuli-responsive behavior often involve tedious chemical synthesis and surface modifications. Herein, we propose a strategy to in situ construct a kind of dynamic and reversible NPSs by the interfacial electrostatic interaction between the negatively charged nanoparticles (NPs) and the positively charged homopolymers. The NPSs assembled at the oil/water interface reduce the interfacial tension and direct the confined assembly of BCP. Meanwhile, the dynamic NPSs can be disassembled by increasing the pH value or introducing competitive electrostatic attractions, which can dynamically and reversibly change the interfacial properties as well as the alignment of polymer chains, enabling BCP microparticles with reversibly switchable lamellar and cylindrical structures. Furthermore, by the introduction of aggregation-induced emission luminogens as tails to the NPSs, the reversible transformation of BCP microparticles can be visualized by fluorescence emission, which is dependent on the nanostructures of microparticles. This work establishes a concept for dynamically manipulating interfacial interactions and reversibly switching BCP microparticles without time-consuming NPS synthesis, showing promising applications in the fabrication of smart materials with switchable structures and properties.

Dynamic covalent surfactants and their uses in the development of smart materials

Dynamic covalent chemistry, which leverages the dynamic nature of reversible covalent bonds controlled by the conditions of reaction equilibrium, has demonstrated great potential in diverse applications related to both the stability of covalent bonds and the possibility of exchanging building blocks, imparting to the systems the possibility of "error checking" and "proof-reading". By incorporating dynamic covalent bonds into surfactant molecular architectures, combinatorial libraries of surfactants with bespoke functionalities can be readily fabricated through a facile strategy, with minimum effort in organic synthesis. Consequently, a multidisciplinary field of research involving the creation and application of dynamic covalent surfactants has recently emerged, which has aroused great attention in surfactant and colloid science, supramolecular chemistry, self-assembly, smart materials, drug delivery, and nanotechnology. This review reports results in this field published over recent years, discusses the possibilities presented by dynamic covalent surfactants and their applications in developing smart self-assembled materials, and outlines some future perspectives.

Dynamic Manipulation of Droplets on Liquid-Infused Surfaces Using Photoresponsive Surfactant

Fast and programmable transport of droplets on a substrate is desirable in microfluidic, thermal, biomedical, and energy devices. Photoresponsive surfactants are promising candidates to manipulate droplet motion due to their ability to modify interfacial tension and generate "photo-Marangoni" flow under light stimuli. Previous works have demonstrated photo-Marangoni droplet migration in liquid media; however, migration on other substrates, including solid and liquid-infused surfaces (LIS), remains an outstanding challenge. Moreover, models of photo-Marangoni migration are still needed to identify optimal photoswitches and assess the feasibility of new applications. In this work, we demonstrate 2D droplet motion on liquid surfaces and on LIS, as well as rectilinear motion in solid capillary tubes. We synthesize photoswitches based on spiropyran and merocyanine, capable of tension changes of up to 5.5 mN/m across time scales as short as 1.7 s. A millimeter-sized droplet migrates at up to 5.5 mm/s on a liquid, and 0.25 mm/s on LIS. We observe an optimal droplet size for fast migration, which we explain by developing a scaling model. The model also predicts that faster migration is enabled by surfactants that maximize the ratio between the tension change and the photoswitching time. To better understand migration on LIS, we visualize the droplet flow using tracer particles, and we develop corresponding numerical simulations, finding reasonable agreement. The methods and insights demonstrated in this study enable advances for manipulation of droplets for microfluidic, thermal and water harvesting devices.

Temperature-Sensitive Janus Particles PEG/SiO2/PNIPAM-PEA: Applications in Foam Stabilization and Defoaming

The current study presents a scalable approach for the preparation of temperature-responsive PEG/SiO2/PNIPAM-PEA Janus particles and, for the first time, investigates their potential applications in stabilizing foam and defoaming by adjusting the temperature. The method utilizes a (W1 + O)/W2 emulsion system, which incorporates appropriate surfactants to stabilize the emulsion and prevent rapid dissolution of the hydrophilic triblock polymer PEG-b-PTEPM-b-PNIPAM in water. The PEG/SiO2/PNIPAM-PEA Janus particles with temperature-responsive characteristics were synthesized in a single step that combined the sol-gel reaction and photoinduced free radical polymerization. The contact angle of the hydrophilic PEG/SiO2/PNIPAM surface was measured to be 54.7 ± 0.1°, while the contact angle of the hydrophobic PEA surface was found to be 122.4 ± 0.1°. By incorporating PEG/SiO2/PNIPAM-PEA Janus particles at a temperature of 25 °C, the foam's half-life is significantly prolonged from 42 s to nearly 30 min. However, with an increase in temperature to 50 °C, the foam's half-life rapidly diminished to only 44 s. This innovative application effectively enhances foam stabilization at low temperatures and facilitates the rapid dissipation of foam at high temperatures.

Design automation of microfluidic single and double emulsion droplets with machine learning

Droplet microfluidics enables kHz screening of picoliter samples at a fraction of the cost of other high-throughput approaches. However, generating stable droplets with desired characteristics typically requires labor-intensive empirical optimization of device designs and flow conditions that limit adoption to specialist labs. Here, we compile a comprehensive droplet dataset and use it to train machine learning models capable of accurately predicting device geometries and flow conditions required to generate stable aqueous-in-oil and oil-in-aqueous single and double emulsions from 15 to 250 μm at rates up to 12000 Hz for different fluids commonly used in life sciences. Blind predictions by our models for as-yet-unseen fluids, geometries, and device materials yield accurate results, establishing their generalizability. Finally, we generate an easy-to-use design automation tool that yield droplets within 3 μm (<8%) of the desired diameter, facilitating tailored droplet-based platforms and accelerating their utility in life sciences.

Fluid dynamics alters liquid-liquid phase separation in confined aqueous two-phase systems

Liquid-liquid phase separation is key to understanding aqueous two-phase systems (ATPS) arising throughout cell biology, medical science, and the pharmaceutical industry. Controlling the detailed morphology of phase-separating compound droplets leads to new technologies for efficient single-cell analysis, targeted drug delivery, and effective cell scaffolds for wound healing. We present a computational model of liquid-liquid phase separation relevant to recent laboratory experiments with gelatin-polyethylene glycol mixtures. We include buoyancy and surface-tension-driven finite viscosity fluid dynamics with thermally induced phase separation. We show that the fluid dynamics greatly alters the evolution and equilibria of the phase separation problem. Notably, buoyancy plays a critical role in driving the ATPS to energy-minimizing crescent-shaped morphologies, and shear flows can generate a tenfold speedup in particle formation. Neglecting fluid dynamics produces incorrect minimum-energy droplet shapes. The model allows for optimization of current manufacturing procedures for structured microparticles and improves understanding of ATPS evolution in confined and flowing settings important in biology and biotechnology.

In situ Tracking of Exoenzyme Activity Using Droplet Luminescence Concentrators for Ratiometric Detection of Bacteria

We demonstrate a novel, rapid, and cost-effective biosensing paradigm that is based on an in situ visualization of bacterial exoenzyme activity using biphasic Janus emulsion droplets. Sensitization of the droplets toward dominant extracellular enzymes of bacterial pathogens is realized via selective functionalization of one hemisphere of Janus droplets with enzyme-cleavable surfactants. Surfactant cleavage results in an interfacial tension increase at the respective droplet interface, which readily transduces into a microscopically detectable change of the internal droplet morphologies. A macroscopic fluorescence read-out of such morphological transitions is obtained via ratiometrically recording the angle-dependent anisotropic emission signatures of perylene-containing droplets from two different angles. The optical read-out method facilitates detection of marginal morphological responses of polydisperse droplet samples that can be easily produced in any environment. The performance of Janus droplets as powerful optical transducers and signal amplifiers is highlighted by rapid (<4 h) and cost-effective antibody and DNA-free identification of three major foodborne pathogens, with detection thresholds of below 10 CFU mL-1 for Salmonella and <102 to 103 CFU mL-1 for Listeria and Escherichia coli.

Breaking the photoswitch speed limit

The forthcoming generation of materials, including artificial muscles, recyclable and healable systems, photochromic heterogeneous catalysts, or tailorable supercapacitors, relies on the fundamental concept of rapid switching between two or more discrete forms in the solid state. Herein, we report a breakthrough in the "speed limit" of photochromic molecules on the example of sterically-demanding spiropyran derivatives through their integration within solvent-free confined space, allowing for engineering of the photoresponsive moiety environment and tailoring their photoisomerization rates. The presented conceptual approach realized through construction of the spiropyran environment results in ~1000 times switching enhancement even in the solid state compared to its behavior in solution, setting a record in the field of photochromic compounds. Moreover, integration of two distinct photochromic moieties in the same framework provided access to a dynamic range of rates as well as complementary switching in the material's optical profile, uncovering a previously inaccessible pathway for interstate rapid photoisomerization.

Photocharging of Carbon Nitride Thin Films for Controllable Manipulation of Droplet Force Gradient Sensors

Intentional generation, amplification, and discharging of chemical gradients is central to many nano- and micromanipulative technologies. We describe a straightforward strategy to direct chemical gradients inside a solution via local photoelectric surface charging of organic semiconducting thin films. We observed that the irradiation of carbon nitride thin films with ultraviolet light generates local and sustained surface charges in illuminated regions, inducing chemical gradients in adjacent solutions via charge-selective immobilization of surfactants onto the substrate. We studied these gradients using droplet force gradient sensors, complex emulsions with simultaneous and independent responsive modalities to transduce information on transient gradients in temperature, chemistry, and concentration via tilting, morphological reconfiguration, and chemotaxis. Fine control over the interaction between local, photoelectrically patterned, semiconducting carbon nitride thin films and their environment yields a new method to design chemomechanically responsive materials, potentially applicable to micromanipulative technologies including microfluidics, lab-on-a-chip devices, soft robotics, biochemical assays, and the sorting of colloids and cells.

Functionalized Amphiphilic Block Copolymers and Complex Emulsions for Selective Sensing of Dissolved Metals at Liquid-Liquid Interfaces

Increasing contamination in potable water supplies necessitates the development of sensing methods that provide the speed and selectivity necessary for safety. One promising method relies on recognition and detection at the liquid-liquid interface of dynamic complex emulsions. These all-liquid materials transduce changes in interfacial tensions into optical signals via the coupling of their chemical, physical, and optical properties. Thus, to introduce selectivity, it is necessary to modify the liquid-liquid interface with an interfacially stable and selective recognition unit. To this end, we report the synthesis and characterization of amphiphilic block copolymers modified with metal chelators to selectively measure the concentrations of dissolved metal ions. We find that significant reduction in interfacial tensions arises upon quantitative addition of metal ions with high affinity toward functionalized chelators. Furthermore, measurements from UV-vis spectroscopy reveal that complexation of the block copolymers with metal ions leads to an increase in surface excess and surfactant effectiveness. We also demonstrate selective detection of iron(III) cations (Fe3+) on the μM levels even through interference from other mono-, di-, or trivalent cations in complex matrices of synthetic groundwater. Our results provide a unique platform that couples selective recognition and modulation of interfacial behaviors and demonstrates a step forward in the development of the multiplexed sensing device needed to deconvolute the complicated array of contaminants that comprise real-world environmental samples.

Stabilizing Liquids Using Interfacial Supramolecular Assemblies

Stabilizing liquids based on supramolecular assembly (non-covalent intermolecular interactions) has attracted significant interest, due to the increasing demand for soft, liquid-based devices where the shape of the liquid is far from the equilibrium spherical shape. The components comprising these interfacial assemblies must have sufficient binding energies to the interface to prevent their ejection from the interface when the assemblies are compressed. Here, we highlight recent advances in structuring liquids based on non-covalent intermolecular interactions. We describe some of the progress made that reveals structure-property relationships. In addition to treating advances, we discuss some of the limitations and provide a perspective on future directions to inspire further studies on structured liquids based on supramolecular assembly.

Responsive Janus droplets as modular sensory layers for the optical detection of bacteria

The field of biosensor development is fueled by innovations in new functional transduction materials and technologies. Material innovations promise to extend current sensor hardware limitations, reduce analysis costs, and ensure broad application of sensor methods. Optical sensors are particularly attractive because they enable sensitive and noninvasive analyte detection in near real-time. Optical transducers convert physical, chemical, or biological events into detectable changes in fluorescence, refractive index, or spectroscopic shifts. Thus, in addition to sophisticated biochemical selector designs, smart transducers can improve signal transmission and amplification, thereby greatly facilitating the practical applicability of biosensors, which, to date, is often hampered by complications such as difficult replication of reproducible selector-analyte interactions within a uniform and consistent sensing area. In this context, stimuli-responsive and optically active Janus emulsions, which are dispersions of kinetically stabilized biphasic fluid droplets, have emerged as a novel triggerable material platform that provides as a versatile and cost-effective alternative for the generation of reproducible, highly sensitive, and modular optical sensing layers. The intrinsic and unprecedented chemical-morphological-optical coupling inside Janus droplets has facilitated optical signal transduction and amplification in various chemo- and biosensor paradigms, which include examples for the rapid and cost-effective detection of major foodborne pathogens. These initial demonstrations resulted in detection limits that rival the capabilities of current commercial platforms. This trend article aims to present a conceptual summary of these initial efforts and to provide a concise and comprehensive overview of the pivotal kinetic and thermodynamic principles that govern the ability of Janus droplets to sensitively and selectively respond to and interact with bacteria.

Recent advances in micro-sized oxygen carriers inspired by red blood cells

Supplementing sufficient oxygen to cells is always challenging in biomedical engineering fields such as tissue engineering. Originating from the concept of a 'blood substitute', nano-sized artificial oxygen carriers (AOCs) have been studied for a long time for the optimization of the oxygen supplementation and improvement of hypoxia environments in vitro and in vivo. When circulating in our bodies, micro-sized human red blood cells (hRBCs) feature a high oxygen capacity, a unique biconcave shape, biomechanical and rheological properties, and low frictional surfaces, making them efficient natural oxygen carriers. Inspired by hRBCs, recent studies have focused on evolving different AOCs into microparticles more feasibly able to achieve desired architectures and morphologies and to obtain the corresponding advantages. Recent micro-sized AOCs have been developed into additional categories based on their principal oxygen-carrying or oxygen-releasing materials. Various biomaterials such as lipids, proteins, and polymers have also been used to prepare oxygen carriers owing to their rapid oxygen transfer, high oxygen capacity, excellent colloidal stability, biocompatibility, suitable biodegradability, and long storage. In this review, we concentrated on the fabrication techniques, applied biomaterials, and design considerations of micro-sized AOCs to illustrate the advances in their performances. We also compared certain recent micro-sized AOCs with hRBCs where applicable and appropriate. Furthermore, we discussed existing and potential applications of different types of micro-sized AOCs.

Non-equilibrium ordering of liquid crystalline (LC) films driven by external gradients in surfactant concentration

HYPOTHESIS

Gradients in the concentration of amphiphiles play an important role in many non-equilibrium processes involving complex fluids. Here we explore if non-equilibrium interfacial behaviors of thermotropic (oily) liquid crystals (LCs) can amplify microscopic gradients in surfactant concentration into macroscopic optical signals.

EXPERIMENTS

We use a milli-fluidic system to generate gradients in aqueous sodium dodecyl sulfate (SDS) concentration and optically quantify the dynamic ordering of micrometer-thick nematic LC films that contact the gradients.

FINDINGS

We find that the reordering of the LCs is dominated by interfacial shearing by Marangoni flows, thus providing simple methods for rapid mapping of interfacial velocities from a single optical image and investigating the effects of confinement of surfactant-driven interfacial flows. Additionally, we establish that surface advection and surfactant desorption are the two key processes that regulate the interfacial flows, revealing that the dynamic response of the LC can provide rapid and potentially high throughput approaches to measurement of non-equilibrium interfacial properties of amphiphiles. We also observe flow-induced assemblies of microparticles to form at the LC interface, hinting at new non-equilibrium approaches to microparticle assembly. We conclude that dynamic states adopted by LCs in the presence of surfactant concentration gradients provide new opportunities for engineering complex fluids beyond equilibrium.

Liquid-liquid phase separation of immiscible polymers at double emulsion interfaces for configurable microcapsules

Liquid-liquid phase separation at complex interfaces is a common phenomenon in biological systems and is also a fundamental basis to create synthetic materials in multicomponent mixtures. Understanding the liquid-liquid phase separation in well-defined macromolecular systems is anticipated to shed light on similar behaviors in cross-disciplinary areas. Here we study a series of immiscible polymers and reveal a generic phase diagram of liquid-liquid phase separation at double emulsion interfaces, which depicts the equilibrium structures by interfacial tension and polymer fraction. We further reveal that the interfacial tensions in various systems fall on a linear relationship with spreading coefficients. Based on this theoretical guideline, the liquid-liquid phase separation can be modulated by a low fraction of amphiphilic block copolymers, leading the double emulsion droplets configurable between compartments and anisotropic shapes. The solidified anisotropic microcapsules could provide unique orientation-sensitive optical properties and thermomechanical responses. The theoretical analysis and experimental protocol in this study yield a generalizable strategy to prepare multiphase double emulsions with controlled structures and desired properties.

Multiplexed and continuous microfluidic sensors using dynamic complex droplets

Emissive complex droplets with reconfigurable morphology and dynamic optical properties offer exciting opportunities as chemical sensors due to their stimuli-responsive characteristics. In this work, we demonstrated a real-time optical sensing platform that combines poly(dimethylsiloxane) (PDMS) microfluidics and complex droplets as sensing materials. We utilized a mechanism, called directional emission, to transduce changes in interfacial tension into optical signals. We discuss the fabrication and integration of PDMS microfluidics with complex emulsions to facilitate continuous measurement of fluorescent emission and, ultimately, the interfacial tensions. Furthermore, by varying the interfacial functionalization and fluorescent dye with characteristic wavelength, we generate multiple formulations of droplets and obtain differential responses to stimuli that alter interfacial tensions (i.e., composition of surfactants, pH). Our results illustrate a proof-of-concept multiplexed and continuous sensing platform with potential applications in miniaturized, on-site environmental monitoring and biosensing.

Tailoring Pickering Double Emulsions by in Situ Particle Surface Modification

Fundamental studies on the formation and stability of Pickering double emulsions are crucial for their industrial applications. Available methods of double emulsion preparation involve multiple tedious steps and can formulate a particular type of double emulsion, that is, water-in-oil-in-water (w/o/w) or oil-in-water-in-oil (o/w/o). In this work, we proposed a simple single-step in situ surface modification method to stabilize different types of double emulsions using hematite and silica particle systems which involves the addition of oleic acid. In the emulsification studies, we use (i) a combination of hematite and oleic acid, which is termed the binary system, and (ii) a mixture of hematite and silica particles together with oleic acid, which is designated as the ternary system. The wettability of hematite particles is tuned by direct or sequential addition of oleic acid to the water-decane medium. The direct surface modification (which involves the addition of a known quantity of oleic acid to the oil-water mixtures at once) of hematite particles in both binary and ternary systems shows transitional phase inversion from oil-in-water (o/w) to water-in-oil (w/o) emulsions. However, sequential surface modification results in the transition of a single emulsion to double emulsions. In the case of the binary system, the sequential surface modification of the hematite-particle-stabilized o/w emulsion can be converted into double emulsions of o/w/o type. However, in the case of the ternary system, i.e., in the presence of silica particles, sequential surface modification of hematite particles stabilizes both single (o/w) and double (w/o/w and o/w/o) emulsions. The critical concentration of oleic acid required to form a double emulsion is observed to be dependent on the ratio of the surface area of the silica particle to the total surface area of particles (S) and mixing protocols. A study of the size distribution of oil and water droplets of double emulsions shows that droplet size can be controlled by oleic acid concentration and magnitude of S. The arrangements of the particles at interfaces are visualized by SEM imaging. In this way, we developed an easy and novel single-step method of double emulsion preparation and provide a strategy to tailor the formation of different types of emulsions with a single/binary particle system by sequential in situ surface modification of the particles.

Thermodynamic origins of two-component multiphase condensates of proteins

Intracellular condensates are highly multi-component systems in which complex phase behaviour can ensue, including the formation of architectures comprising multiple immiscible condensed phases. Relying solely on physical intuition to manipulate such condensates is difficult because of the complexity of their composition, and systematically learning the underlying rules experimentally would be extremely costly. We address this challenge by developing a computational approach to design pairs of protein sequences that result in well-separated multilayered condensates and elucidate the molecular origins of these compartments. Our method couples a genetic algorithm to a residue-resolution coarse-grained protein model. We demonstrate that we can design protein partners to form multiphase condensates containing naturally occurring proteins, such as the low-complexity domain of hnRNPA1 and its mutants, and show how homo- and heterotypic interactions must differ between proteins to result in multiphasicity. We also show that in some cases the specific pattern of amino-acid residues plays an important role. Our findings have wide-ranging implications for understanding and controlling the organisation, functions and material properties of biomolecular condensates.

Dynamic In Situ Monitoring of the Salt Counter-ion Effect on Surfactant Effectiveness Using Reconfigurable Janus Emulsions

A straightforward method for visualization and quantification of surfactant effectiveness within different electrolyte environments based on using reconfigurable Janus emulsions as novel optical probes is reported. More specifically, we investigated the effect of different types and concentrations of salt counter-ions on the surfactant surface excess of commercial ionic and non-ionic surfactants, namely sodium dodecyl sulfate (SDS) and Tween 80 via in situ monitoring the morphological reconfigurations of biphasic Janus emulsions comprising hydrocarbon and fluorocarbon oils. We find that significant variations in interfacial tensions of SDS-stabilized interfaces (up to 15 mN·m^-1) can be evoked by titrating mono-, di-, and trivalent cationic counter-ions, which is coherent with the lyotropic (Hofmeister) series. In contrast, the salt counter-ion effect on the surfactant effectiveness was less pronounced for the non-ionic surfactant Tween 80 (∼3 mN·m^-1). Our results reveal a facile in situ method for monitoring the central role of electrolyte type and concentration on surfactant effectiveness and, more broadly, illustrate that Janus emulsions serve as powerful optical probes to dynamically study the properties of surfactants at liquid interfaces. We demonstrate the utility of our findings for an electro-induced morphological reconfiguration of Janus droplet morphologies by dynamically tuning Cu²⁺ concentration in solution using an electrode setup. The latter provides a unique platform for liquid-phase, real-time, and continuous tuning of Janus droplet morphologies, e.g., for their application in sensing and dynamic optical device platforms.

Nanostructure-free crescent-shaped microparticles as full-color reflective pigments

Structural colors provide a promising visualization with high color saturation, iridescent characteristics, and fade resistance. However, pragmatic uses are frequently impeded by complex manufacturing processes for sophisticated nanostructures. Here, we report a facile emulsion-templating strategy to produce crescent-shaped microparticles as structural color pigments. The micro-crescents exhibit brilliant colors under directional light originating from total internal reflections and optical interferences in the absence of periodic nanostructures while being transparent under ambient light. The colors are finely tunable by adjusting the size of the micro-crescents, which can be further mixed to enrich the variety. Importantly, the pre-defined convex surface secures high stability of colors and enables structural coloration on target surfaces through direct deposition as inks. We anticipate this class of nanostructure-free structural colorants is pragmatic as invisible inks in particular for anti-counterfeiting patches and color cosmetics with distinctive impressions due to low-cost, scalable manufacturing, unique optical properties, and versatility.

Behaviors of self-delivery lidocaine nano systems affected by intermolecular interaction

Lidocaine salts self-assembled into different nano systems in water at a clinical dosage (2%, 0.2 mL) without excipient addition, and led to different sensory block durations and acute systemic toxicities, which were affected by the self-delivery and self-release behaviors of the salts in vivo. These differences were mainly caused by intermolecular π-π stacking under different conditions, which was proved by the unique supramolecular arrangement of gourd-shaped Janus particles. π-π stacking in lidocaine nano systems can be enhanced by carbon dioxide addition or the exchange of counter ions from Br^- to Cl^-. Without π-π stacking, nano systems self-assembled by lidocaine hydrobromide achieved 7.8 h sensory block by intradermal administration on rats, which is much longer than the efficacy of classical local anesthetics and more suitable for postoperative treatment.

Shortwave Infrared Fluorofluorophores for Multicolor In Vivo Imaging

Developing chemical tools to detect and influence biological processes is a cornerstone of chemical biology. Here we combine two tools which rely on orthogonality- perfluorocarbons and multiplexed shortwave infrared (SWIR) fluorescence imaging- to visualize nanoemulsions in real time in living mice. Drawing inspiration from fluorous and SWIR fluorophore development, we prepared two SWIR-emissive, fluorous-soluble chromenylium polymethine dyes. These are the most red-shifted fluorous fluorophores- "fluorofluorophores"-to date. After characterizing the dyes, their utility was demonstrated by tracking perfluorocarbon nanoemulsion biodistribution in vivo. Using an excitation-multiplexed approach to image two variables simultaneously, we gained insight into the importance of size and surfactant identity on biodistribution.

Morphology-Dependent Aggregation-Induced Emission of Janus Emulsion Surfactants

We report a novel stimuli-responsive fluorescent material platform that relies on an evocation of aggregation-induced emission (AIE) from tetraphenylethylene (TPE)-based surfactants localized at one hemisphere of biphasic micro-scale Janus emulsion droplets. Dynamic alterations in the available interfacial area were evoked through surfactant-induced dynamic changes of the internal droplet morphology that can be modulated as a function of the balance of interfacial tensions of the droplet constituent phases. Thus, by analogy with a Langmuir-Blodgett trough that enables selective concentration of surfactants at a liquid-gas interface, we demonstrate here a method for controllable modulation of the available interfacial area of surfactant-functionalized liquid-liquid interfaces. We show that a morphology-dependent alteration of the interfacial area can be used to evoke an optical signal, by selectively assembling synthesized TPE-based surfactants on the respective droplet interfaces. A trigger-induced increase in the concentration of TPE-based surfactants at the liquid-liquid interfaces results in an evocation of aggregation-induced emission (AIE), inducing an up to 3.9-fold increase in the measured emission intensity of the droplets.

Switchable Nanostructures Triggered by Noyori-Type Organometallics

Janus particles (JPs) self-assembled by a typical small organic gemini surfactant in water were reported by us. After the addition of a small amount Noyori-type organometallics to an organic solvent, these gourd-shaped JPs became new nanostructures, such as nanotubes (NTs), nanoribbons (NRs), and new types of JPs. Significant changes in specific rotation occurred on the solution-like samples, triggered by chiral organometallics in 20 μL of ethyl acetate. Almost all of these organometallics-triggered nanostructures can be conveniently detached and reversed within 5 min due to the easy-phase separation of ethyl acetate from the emulsion and the chemical-selective unstable binding between the organometallics and carbonate group on the surfactant.

Emerging Implications of Phase Separation in Cancer

In eukaryotic cells, biological activities are executed in distinct cellular compartments or organelles. Canonical organelles with membrane-bound structures are well understood. Cells also inherently contain versatile membrane-less organelles (MLOs) that feature liquid or gel-like bodies. A biophysical process termed liquid-liquid phase separation (LLPS) elucidates how MLOs form through dynamic biomolecule assembly. LLPS-related molecules often have multivalency, which is essential for low-affinity inter- or intra-molecule interactions to trigger phase separation. Accumulating evidence shows that LLPS concentrates and organizes desired molecules or segregates unneeded molecules in cells. Thus, MLOs have tunable functional specificity in response to environmental stimuli and metabolic processes. Aberrant LLPS is widely associated with several hallmarks of cancer, including sustained proliferative signaling, growth suppressor evasion, cell death resistance, telomere maintenance, DNA damage repair, etc. Insights into the molecular mechanisms of LLPS provide new insights into cancer therapeutics. Here, the current understanding of the emerging concepts of LLPS and its involvement in cancer are comprehensively reviewed.

Complex Suspended Janus Droplets Constructed through Solvent Evaporation-Induced Phase Separation at the Air-Liquid Interface

Phase separation technology has attracted extensive scientific interest because of its intriguing structure changes during the phase separation process. Phase separation inside emulsion droplets in continuous surroundings has been well studied in recent years. Many investigations have also been conducted to study the droplet phase separation phenomena in noncontinuous surroundings. However, studies on the phase separation phenomena and the spreading behavior of suspended droplets at the air-liquid interface were rarely reported. In this study, PEGDA-glycerol suspended Janus droplets with a patchy structure were produced by utilizing solvent evaporation-induced droplet phase separation at the air-liquid interface. By altering the glycerol/PEGDA volume ratio, the initial proportion of ethanol, and the concentration of surfactants, suspended droplets with different morphologies can be achieved, which include filbert-shaped droplets (FSDs), half lotus seedpod single-phase Janus droplets (HLSDs), lotus seedpod single-phase Janus droplets (LSDs), lotus seedpod-shaped droplets (LSSDs), multiple-bulge droplets (MBDs), and half gourd-shaped droplets (HGSDs). A patchy structure was generated at the air-droplet interface, which was attributed to the Marangoni stresses induced by nonuniform evaporation. Furthermore, a modified spreading coefficient theory was constructed and verified to illustrate the phase separation at the air-droplet interface, which was the first research to predict the phase separation phenomena at the air-liquid interface via spreading coefficients theory. Moreover, we studied the factors that led to the droplets being able to float by designing the combined parameters, including three interfacial tensions and the equilibrium contact angles. Therefore, a simple and versatile strategy for creating suspended Janus droplets has been developed for the first time, which holds significant potential in a variety of applications for material synthesis, such as the electrospinning solution behavior when sprayed from the nozzle into the air.

Capillary forces generated by biomolecular condensates

Liquid-liquid phase separation and related phase transitions have emerged as generic mechanisms in living cells for the formation of membraneless compartments or biomolecular condensates. The surface between two immiscible phases has an interfacial tension, generating capillary forces that can perform work on the surrounding environment. Here we present the physical principles of capillarity, including examples of how capillary forces structure multiphase condensates and remodel biological substrates. As with other mechanisms of intracellular force generation, for example, molecular motors, capillary forces can influence biological processes. Identifying the biomolecular determinants of condensate capillarity represents an exciting frontier, bridging soft matter physics and cell biology.