Dynamically reconfigurable complex emulsions via tunable interfacial tensions

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.

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.

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.

Ultrahigh efficient emulsification with drag-reducing liquid gating interfacial behavior

Emulsification is a crucial technique for mixing immiscible liquids into droplets in numerous areas ranging from food to medicine to chemical synthesis. Commercial emulsification methods are promising for high production, but suffer from high energy input. Here, we report a very simple and scalable emulsification method that employs the drag-reducing liquid gating structure to create a smooth liquid-liquid interface for the reduction of resistance and tunable generation of droplets with good uniformity. Theoretical modeling and experimental results demonstrate that our method exhibits ultrahigh efficiency, which can reach up to more than 4 orders of magnitude greater energy-saving compared to commercial methods. For temperature-sensitive biological components, such as enzymes, proteins, and bacteria, it can offer a comfortable environment to avoid exposure to high temperatures during emulsifying, and the interface also enables the suppression of fouling. This unique drag-reducing liquid gating interfacial emulsification mechanism promotes the efficiency of droplet generation and provides fresh insight into the innovation of emulsifications that can be applied in many fields, including the food industry, the daily chemical industry, biomedicine, material fabrication, the petrochemical industry, and beyond.

Systematic characterization of effect of flow rates and buffer compositions on double emulsion droplet volumes and stability

Double emulsion droplets (DEs) are water/oil/water droplets that can be sorted via fluorescence-activated cell sorting (FACS), allowing for new opportunities in high-throughput cellular analysis, enzymatic screening, and synthetic biology. These applications require stable, uniform droplets with predictable microreactor volumes. However, predicting DE droplet size, shell thickness, and stability as a function of flow rate has remained challenging for monodisperse single core droplets and those containing biologically-relevant buffers, which influence bulk and interfacial properties. As a result, developing novel DE-based bioassays has typically required extensive initial optimization of flow rates to find conditions that produce stable droplets of the desired size and shell thickness. To address this challenge, we conducted systematic size parameterization quantifying how differences in flow rates and buffer properties (viscosity and interfacial tension at water/oil interfaces) alter droplet size and stability, across 6 inner aqueous buffers used across applications such as cellular lysis, microbial growth, and drug delivery, quantifying the size and shell thickness of >22 000 droplets overall. We restricted our study to stable single core droplets generated in a 2-step dripping-dripping formation regime in a straightforward PDMS device. Using data from 138 unique conditions (flow rates and buffer composition), we also demonstrated that a recent physically-derived size law of Wang et al. can accurately predict double emulsion shell thickness for >95% of observations. Finally, we validated the utility of this size law by using it to accurately predict droplet sizes for a novel bioassay that requires encapsulating growth media for bacteria in droplets. This work has the potential to enable new screening-based biological applications by simplifying novel DE bioassay development.

Janus Droplet Formation via Thermally Induced Phase Separation: A Numerical Model with Diffusion and Convection

Microscale Janus particles have versatile potential applications in many physical and biomedical fields, such as microsensor, micromotor, and drug delivery. Here, we present a phase-field approach of multicomponent and multiphase to investigate the Janus droplet formation via thermally induced phase separation. The crucial kinetics for the formation of Janus droplets consisting of two polymer species and a solvent component via an interplay of both diffusion and convection is considered in the Cahn-Hilliard-Navier-Stokes equation. The simulation results of the phase-field model show that unequal interfacial tensions between the two polymer species and the solvent result in asymmetric phase separation in the formation process of Janus droplets. This asymmetric phase separation plays a vital role in the establishment of the so-called core-shell structure that has been observed in previous experiments. By varying the droplet size, the surface tension, and the molecular interaction between the polymer species, several novel droplet morphologies are predicted in the development process of Janus droplets. Moreover, we stress that the hydrodynamics should be reckoned as a non-negligible mechanism that not only accelerates the Janus droplet evolution but also has great impacts on the coarsening and coalescence of the Janus droplets.

Recent Progress toward Physical Stimuli-Responsive Emulsions

Emulsion as a fine dispersion of immiscible liquids has involved widespread applications in industry, pharmaceuticals, agriculture and personal care. Stimuli-responsive emulsions capable of on-demand demulsification or changing their properties are required in many cases such as controllable release cargo, oil recovery, emulsifiers recycle and product separation, great progress has been achieved in these areas. Among these various triggers, much effort has been made to develop physical stimuli, due to the noninvasive and environmentally friendly characteristics. Physical stimuli-responsive emulsions provide a plenty of valuable practical applications in the fields of sustainable industry, biomedical reaction, drug delivery. Here, we summarize the recent development in the field of emulsions in response to physical stimuli consisting of temperature, light, magnetic field, electrical field, etc. The preparation methods and mechanisms of physical stimuli-responsive emulsions and their applications of catalysis reaction, drug delivery, and oil recovery are highlighted in this review. The future directions and outstanding problems of the physical stimuli-responsive emulsions are also discussed. This article is protected by copyright. All rights reserved.

Detection of PFAS and Fluorinated Surfactants Using Differential Behaviors at Interfaces of Complex Droplets

Contamination of per- and polyfluoroalkyl substances (PFAS) in water supplies will continue to have serious health and environmental consequences. Despite the importance of monitoring the concentrations of PFAS at potential sites of contamination and at treatment plants, there are few suitable and rapid on-site methods. Many nonconventional techniques do not possess the necessary selectivity and sensitivity to distinguish PFAS from other surface-active components and to quantify the low concentrations in real-world conditions. Herein, we report a novel and rapid method for the detection of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) by leveraging their differential behaviors at the interfaces of emissive complex droplets. Measurement of surface and interfacial tensions via a force tensiometer reveals that PFAS preferentially self-assemble at the water-fluorocarbon oil interface (F/W) rather than the water-hydrocarbon oil interface (H/W). We also observe an opposite behavior for hydrocarbon surfactants. This difference in interfacial behavior produces distinct effects on the morphological change and optical emission of biphasic oil-in-water droplets. The change in the intensity of fluorescence emission, measured with a simple spectroscopic setup, correlates with the concentrations of PFAS. We also demonstrate that the range of detection and sensitivity can be tuned by adjusting the initial composition of the complex droplets. Our results illustrate an alternative mode of sensors that may provide a rapid and on-site detection of PFAS.

Reversible morphology-resolved chemotactic actuation and motion of Janus emulsion droplets

We report, for the first time, a chemotactic motion of emulsion droplets that can be controllably and reversibly altered. Our approach is based on using biphasic Janus emulsion droplets, where each phase responds differently to chemically induced interfacial tension gradients. By permanently breaking the symmetry of the droplets’ geometry and composition, externally evoked gradients in surfactant concentration or effectiveness induce anisotropic Marangoni-type fluid flows adjacent to each of the two different exposed interfaces. Regulation of the competitive fluid convections then enables a controllable alteration of the speed and the direction of the droplets’ chemotactic motion. Our findings provide insight into how compositional anisotropy can affect the chemotactic behavior of purely liquid-based microswimmers. This has implications for the design of smart and adaptive soft microrobots that can autonomously regulate their response to changes in their chemical environment by chemotactically moving towards or away from a certain target, such as a bacterium.

Artificial microswimmers can emulate the autonomous regulation of chemotactic motility of living organisms. Frank et al. realize a chemotactic locomotion of emulsion droplets, composed of two phase-separated fluids, that can be reversibly directed up or down a chemical concentration gradient.

Fluorinated surfactants: A review on recent progress on synthesis and oilfield applications

The selection of appropriate chemicals and the synthetic method plays an important role in oilfield application. The objectives of this study are to describe the various synthetic route for the preparation of fluorinated surfactants and highlight their oilfield applications. Fluorinated surfactants are the type of surfactants where the hydrophobic tail is either partially fluorinated or replaced totally with fluorine molecules. Fluorinated surfactants have distinct properties compared to corresponding hydrocarbon surfactants such as lower surface tension, better efficiency in lowering the interfacial tension, both oleophobic and hydrophobic nature, high thermal stability, and better chemical tolerance. These properties make them a material of choice for several applications which include but are not limited to fire-fighting, household items, foaming, coating, and paints. Despite these attractive properties, environmental concerns associated with fluorinated surfactants is a major hurdle in extending the application of such surfactants. This review discusses the various synthetic routes for the synthesis of different classes of surfactants such as cationic, anionic, non-ionic, and zwitterionic surfactants. The fundamental surface/interface properties of the synthesized surfactants are also highlighted. In addition, the review highlights the application of fluorinated surfactants in the oil & gas industry.

Effect of Thermal History and Hydrocarbon Core Size on Perfluorocarbon Endoskeletal Droplet Vaporization

Vaporizable hydrocarbon-in-fluorocarbon endoskeletal droplets are a unique category of phase-change emulsions with interesting physical and thermodynamic features. Here, we show microfluidic fabrication of various morphologies, such as solid-in-liquid, liquid-in-solid, and Janus-type, of complex solid n-C₂₀H₄₂ or n-C₂₁H₄₄ and liquid n-C₅F₁₂ droplets. Furthermore, we investigated the vaporization behavior of these endoskeletal droplets, focusing on the effects of heat treatment and core size. Comparison of vaporization and differential scanning calorimetry results indicated that vaporization occurs prior to melting of the bulk hydrocarbon phase for C₂₀H₄₂/C₅F₁₀ droplets and near the rotator phase for C₂₁H₄₄/C₅F₁₀ droplets. We found that heat treatment of the droplets increased the fraction of droplets that vaporized and also increased the vaporization temperature of the droplets, although the effect was temporary. Furthermore, we found that changing the relative size of the solid hydrocarbon core compared to the surrounding liquid shell increased the vaporization temperature and the vaporizing fraction. Taken together, these data support the hypothesis that surface melting behavior exhibited by the linear alkane may trigger the fluorocarbon vaporization event. These results may aid in the understanding of the interfacial thermodynamics and transport and the engineering of novel vaporizable endoskeletal droplets for biomedical imaging and other applications.

Acoustically manipulating internal structure of disk-in-sphere endoskeletal droplets

Manipulation of micro/nano particles has been well studied and demonstrated by optical, electromagnetic, and acoustic approaches, or their combinations. Manipulation of internal structure of droplet/particle is rarely explored and remains challenging due to its complicated nature. Here we demonstrated the manipulation of internal structure of disk-in-sphere endoskeletal droplets using acoustic wave. We developed a model to investigate the physical mechanisms behind this interesting phenomenon. Theoretical analysis of the acoustic interactions indicated that these assembly dynamics arise from a balance of the primary and secondary radiation forces. Additionally, the disk orientation was found to change with acoustic driving frequency, which allowed on-demand, reversible adjustment of the disk orientations with respect to the substrate. This dynamic behavior leads to unique reversible arrangements of the endoskeletal droplets and their internal architecture, which may provide an avenue for directed assembly of novel hierarchical colloidal architectures and intracellular organelles or intra-organoid structures.

Endoskeletal droplets are a class of complex colloids containing a solid internal phase cast within a liquid emulsion droplet. Here, authors show acoustic manipulation of solid disks inside liquid droplets whose orientation can be externally controlled with the frequency.

Recyclable and re-usable smart surfactant for stabilization of various multi-responsive emulsions alone or with nanoparticles

A novel multi-responsive surfactant (abbreviated as N⁺-8P8-N) was synthesized, in which one octyl trimethylamine group (quaternary ammonium) and one octyl dimethylamine group are connected to a benzene ring through ether bonds. This novel surfactant can stabilize conventional oil-in-water (O/W) emulsions alone, and O/W Pickering emulsions and novel oil-in-dispersion emulsions together with oppositely and similarly charged nanoparticles, respectively. In all cases rapid demulsification can be achieved through either pH or CO₂/N₂ triggers, by which the surfactant is reversibly converted between a normal cationic surfactant form (N⁺-8P8-N) and a strongly hydrophilic and surface-inactive bola form (N⁺-8P8-NH⁺). Notably, the bola form N⁺-8P8-NH⁺ dissolves in the aqueous phase alone or together with nanoparticles after demulsification without contamination of the oil phase, and the aqueous phase can be recycled many times triggered by pH or CO₂/N₂ in accordance with the principle of green chemistry. This newly designed re-usable smart surfactant is significant for the development of various temporarily stable emulsions, which are extensively applied in emulsion polymerization, new material synthesis, heterogeneous catalysis and oil transportation.