The effects of droplet stabilization by surfactants and nanoparticles on leakage, cross-talk, droplet stability, and cell adhesion

Partially fluorinated nanoparticles (FNPs) have been proposed as a promising alternative for stabilising aqueous droplets in fluorinated oils. The exceptional energetic stability of FNPs at the droplet interface holds the potential for minimising leakage, enhancing stability, and promoting improved cell adhesion. However, their lower diffusion coefficient compared to surfactants presents challenges in achieving rapid droplet stabilisation, which is important in microfluidics applications. While several studies have focused on some of these aspects, a comprehensive study and direct comparison with conventional fluorosurfactants is still missing. In this manuscript, we undertake an examination and comparison of four crucial facets of both FNP- and surfactant-stabilised droplets: leakage of compounds, emulsion stability, droplet formation dynamics and cell adhesion. Contrary to what has previously been claimed, our findings demonstrate that FNPs only reduce leakage and cross-talk in very specific cases (e.g., resorufin), failing to provide enhanced compartmentalisation for highly hydrophobic dyes (e.g., rhodamine dyes). On the other hand, FNP-stabilised droplets indeed exhibit greater long-term stability compared to their surfactant-stabilised counterparts. Regarding the size of droplets generated via a diversity of microfluidic methods, no significant differences were observed between FNP-stabilised and surfactant-stabilised droplets. Finally, the previously reported improvements in cell adhesion and spreading on FNP-stabilised interfaces is limited to flat oil/water (o/w) interfaces and could not be observed within droplets. These comprehensive analyses shed light on the nuanced performance of FNPs and commercial fluorosurfactants as stabilising agents for aqueous droplets in fluorinated oils, contributing valuable insights for choosing the correct formulation for specific droplet-based microfluidics applications.

Optical pH Monitoring in Microdroplet Platforms for Live Cell Experiments Using Colloidal Surfactants

Droplet microfluidic technology facilitates the development of high-throughput screening applications in nanoliter volumes. Surfactants provide stability for emulsified monodisperse droplets to carry out compartmentalization. Fluorinated silica-based nanoparticles are used; they can minimize crosstalk in microdroplets and provide further functionalities by surface labeling. Here we describe a protocol for monitoring pH changes in live single cells by fluorinated silica nanoparticles, for their synthesis, chip fabrication, and optical monitoring on the microscale. The nanoparticles are doped with ruthenium-tris-1,10-phenanthroline dichloride on the inside and conjugated with fluorescein isothiocyanate on the surface. This protocol may be used more generally to detect pH changes in microdroplets. The fluorinated silica nanoparticles can also be used as droplet stabilizers with an integrated luminescent sensor for other applications.

Particle Stabilization of Oil-Fluorocarbon Interfaces and Effects on Multiphase Oil-in-Water Complex Emulsion Morphology and Reconfigurability

Stabilization of oil-oil interfaces is important for nonaqueous emulsions as well as for multiphase oil-in-water emulsions, with relevance to a variety of fields ranging from emulsion polymerization to sensors and optics. Here, we focus on examining the ability of functionalized silica particles to stabilize interfaces between fluorinated oils and other immiscible oils (such as hydrocarbons and silicones) in nonaqueous emulsions and also on the particles' ability to affect the morphology and reconfigurability of complex, biphasic oil-in-water emulsions. We compare the effectiveness of fluorophilic, lipophilic, and bifunctional fluorophilic-lipophilic coated nanoparticles to stabilize these oil-oil interfaces. Sequential bulk emulsification steps by vortex mixing, or emulsification by microfluidics, can be used to create complex droplets in which particles stabilize the oil-oil interfaces and surfactants stabilize the oil-water interfaces. We examine the influence of particles adsorbed at the internal oil-oil interface in complex droplets to hinder the reconfiguration of these complex emulsions upon addition of aqueous surfactants, creating "metastable" droplets that resist changes in morphology. Such metastable droplets can be triggered to reconfigure when heated above their upper critical solution temperature. Thus, not only do these bifunctional silica particles enable the stabilization of a broad array of oil-fluorocarbon nonaqueous emulsions, but the ability to address the oil-oil interface within complex O/O/W droplets expands the diversity of oil chemical choices available and the accessibility of droplet morphologies and sensitivity.

Dual-functional Ag3PO4@palygorskite composite for efficient photodegradation of alkane by in situ forming Pickering emulsion photocatalytic system

Removal of oil from water is highly imperative, because of the worldwide oil-contaminated water caused by industrial development and oil spill accidents. As a solution to meet the demand for clean energy technology, photocatalysis has drawn great attention recently. However, a major problem encountered in photodegrading oil is the difficult availability of oil by photocatalyst. To overcome this problem, a novel concept of integrating Pickering emulsification of palygorskite (PAL) clay particles with photocatalytic activity of Ag₃PO₄ is proposed in this work. By a simple co-precipitation method, Ag₃PO₄@PAL composite was prepared and used for the simultaneous emulsification and decomposition of tetradecane. Via a simple Pickering emulsion-based photocatalytic system, Ag₃PO₄ could contact with tetradecane directly, which effectively overcomes the agglomeration and settlement of Ag₃PO₄ in aqueous phase. This in situ photocatalytic system shows a higher efficiency for photodegradation of tetradecane, comparing with traditional solution-dispersed photocatalytic system. Under visible-light irradiation, the removal efficiency of tetradecane is 4.9 times higher than Ag₃PO₄ alone. Direct contact of Ag₃PO₄ with oil pollutes and sufficiently large active surface area greatly improve the efficiency of photodegrading oil. This study provides a new and simple strategy for oil photodegradation via an in situ Pickering emulsion system.

Nanoparticles Actively Fragment Armored Droplets

Understanding the complexity of fragmentation processes is essential for regulating intercellular communication in mechanistic biology and developing bottom-up approaches in a large range of multiphase flow processes. In this context, self-fragmentation proceeds without any external mechanical energy input, allowing one to create efficiently micro- and nanodroplets. Here we examine self-fragmentation in emulsion nanodroplets stabilized by solid particles with different surface features. Mesoscopic modeling and accelerated dynamics simulations allow us to overcome the limitations of atomistic simulations and offer detailed insight into the interplay between the evolution of the droplet shape and the particle finite-size effects at the interface. We show that finite-size nanoparticles play an active role in the necking breakup, behaving like nanoscale razors, and affect strongly the thermodynamic properties of the system. The role played by the particles during self-fragmentation might be of relevance to multifunctional biomaterial design and tuning of signaling pathways in mechanistic biology.

Quantifying phenotypes in single cells using droplet microfluidics

This book chapter describes the use of droplet microfluidics to phenotype single cells. The basic process flow includes the encapsulation of single cells with a specific probe into aqueous micro-droplets suspended in a biocompatible oil. The probe is chosen to measure the phenotype of interest. After incubation, the encapsulated cell turns the probe fluorescent and renders the entire droplet fluorescent. Enumerating drops that are fluorescent quantifies the concentration of cells possessing the phenotype of interest. Examining the distribution of fluorescence further allows one to quantify the heterogeneity among the cell population.

Bifunctional Solid Catalyst for Organic Reactions in Water: Simultaneous Anchoring of Acetylacetone Ligands and Amphiphilic Ionic Liquid "Tags" by Using a Dihydropyran Linker

The use of solid catalysts to promote organic reactions in water faces the inherent difficulty of the poor mass-transfer efficiency of organic substances in water, which is often responsible for insufficient reaction and low yields. To solve this problem, the solid surface can be manipulated to become amphiphilic. However, the introduction of surfactant-like moieties onto the surface of silica-based materials is not easy. By using an accessible dihydropyran derivative as a grafting linker, a surfactant-combined bifunctional silica-based solid catalyst that possessed an ionic liquid tail and a metal acetylacetonate moiety was prepared through a mild Lewis-acid-catalyzed ring-opening reaction with a thiol-functionalized silica. The surfactant-combined silica-supported metal acetylacetone catalysts displayed excellent catalytic activity in water for a range of reactions. The solid catalyst was also shown to be recyclable, and was reused several times without significant loss in activity.

Nanoparticle-stabilised emulsions: droplet armouring vs. droplet bridging

We speculate that the ability of small nanoparticles to stabilise emulsions via droplet bridging is controlled by the balance between an energy increase, due to the interaction between two oil-water interfaces in close proximity through surface forces, and an energy decrease due to the adsorption of a bridging particle on two oil-water interfaces. For nanoparticles of diameter greater than about 10 nm, tuning this interaction may render bridging possible, whereas nanoparticles smaller than 10 nm may stabilise emulsions only via the formation of dense layers of adsorbed particles on droplets. Both predictions are experimentally testable.

Mode Transition of Droplet Formation in a Semi-3D Flow-Focusing Microfluidic Droplet System

Uniform droplets have significant potential in many biological applications due to their higher surface area to volume ratio. This paper proposed a semi-three-dimensional (sime-3D) flow-focusing microfluidic system, which was fabricated using the multi-layer soft lithography method. Based on the semi-3D structure, we focus on droplets formation modes and droplet uniformity at different bulk concentration of surfactant. The relationships between droplets uniformity, droplets breakup processes (jetting mode, dripping mode and tip-streaming mode) and surfactant concentration was investigated. It was found that three droplet generation modes occur through adjusting the pressure ratio in two inlet channels and the concentration of surfactant in continuous phase liquid. The jetting mode would transform to the dripping mode or the tip-streaming mode as the pressure ratio in different surfactant concentrations increased. Furthermore, the uniformity of droplets could be improved through the transition of jetting to dripping mode. We assumed that the uniformity declined through the transition of jetting to tip-streaming, and explored the specific transitions from jetting to dripping mode and tip-streaming mode. Dripping mode leads to high droplet uniformity, and generation frequency decreases with increasing pressure ratio. Tip-streaming mode is considered as an extreme state of jetting mode, leading to higher formation frequency and smaller droplet size at low uniformity.

Preparation and Investigation of Foaming Amphiphilic Fluorinated Nanoparticles for Enhanced Oil Recovery

Amphiphilic nanoparticles have attracted increasing interest as Pickering emulsifiers owing to the combined advantages of both traditional surfactants and homogeneous particles. Here, foaming amphiphilic fluorinated nanoparticles were prepared for enhanced oil recovery by the toposelective surface modification method. The structure and properties of amphiphilic nanoparticles were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, a laser diffraction method, fluorescence microscopy, a pendant drop tensiometer, and foamscan. It was found that the amphiphilic fluorinated nanoparticles exhibited significant interfacial activity at the air-water interface and generated stabilized aqueous foams against coalescence and drainage even in the absence of surfactants. When the particle concentration reached 0.6 wt %, the adsorption of the amphiphilic nanoparticles at the interface was saturated and the equilibrium surface tension dropped to around 32.7 mN/m. When the particle concentration reached 0.4 wt %, the Gibbs stability criterion was fulfilled. The amphiphilic nanoparticles foam system has a better plugging capacity and enhanced oil recovery capacity. The results obtained provide fundamental insights into the understanding of the self-assembly behavior and foam properties of amphiphilic fluorinated nanoparticles and further demonstrate the future potential of the amphiphilic nanoparticles used as colloid surfactants for enhanced oil recovery applications.

Encapsulation of Single Nanoparticle in Fast-Evaporating Micro-droplets Prevents Particle Agglomeration in Nanocomposites

This work describes the use of fast-evaporating micro-droplets to finely disperse nanoparticles (NPs) in a polymer matrix for the fabrication of nanocomposites. Agglomeration of particles is a key obstacle for broad applications of nanocomposites. The classical approach to ensure the dispersibility of NPs is to modify the surface chemistry of NPs with ligands. The surface properties of NPs are inevitably altered, however. To overcome the trade-off between dispersibility and surface-functionality of NPs, we develop a new approach by dispersing NPs in a volatile solvent, followed by mixing with uncured polymer precursors to form micro-droplet emulsions. Most of these micro-droplets contain no more than one NP per drop, and they evaporate rapidly to prevent the agglomeration of NPs during the polymer curing process. As a proof of concept, we demonstrate the design and fabrication of TiO₂ NP@PDMS nanocomposites for solar fuel generation reactions with high photocatalytic efficiency and recyclability arising from the fine dispersion of TiO₂. Our simple method eliminates the need for surface functionalization of NPs. Our approach is applicable to prepare nanocomposites comprising a wide range of polymers embedded with NPs of different composition, sizes, and shapes. It has the potential for creating nanocomposites with novel functions.

Amphiphilic nanoparticles suppress droplet break-up in a concentrated emulsion flowing through a narrow constriction

This paper describes the break-up behavior of a concentrated emulsion comprising drops stabilized by amphiphilic silica nanoparticles flowing in a tapered microchannel. Such geometry is often used in serial droplet interrogation and sorting processes in droplet microfluidics applications. When exposed to high viscous stresses, drops can undergo break-up and compromise their physical integrity. As these drops are used as micro-reactors, such compromise leads to a loss in the accuracy of droplet-based assays. Here, we show droplet break-up is suppressed by replacing the fluoro-surfactant similar to the one commonly used in current droplet microfluidics applications with amphiphilic nanoparticles as droplet stabilizer. We identify parameters that influence the break-up of these drops and demonstrate that break-up probability increases with increasing capillary number and confinement, decreasing nanoparticle size, and is insensitive to viscosity ratio within the range tested. Practically, our results reveal two key advantages of nanoparticles with direct applications to droplet microfluidics. First, replacing surfactants with nanoparticles suppresses break-up and increases the throughput of the serial interrogation process to 3 times higher than that in surfactant system under similar flow conditions. Second, the insensitivity of break-up to droplet viscosity makes it possible to process samples having different composition and viscosities without having to change the channel and droplet geometry in order to maintain the same degree of break-up and corresponding assay accuracy.

High-Efficiency and High-Throughput On-Chip Exchange of the Continuous Phase in Droplet Microfluidic Systems

This article describes an integrated platform for the on-chip exchange of the continuous phase in droplet microfluidic systems. The drops used in this work are stabilized by amphiphilic nanoparticles. For some characterizations and applications of these nanoparticle-stabilized drops, including the measurement of adsorption dynamics of nanoparticles to the droplet surface, it is necessary to change the composition of the continuous phase from that used during the droplet generation process. Thus far, no work has reported the exchange of the continuous phase for a large number (>1 million) of drops in a microfluidic system. This article describes the design and characterization of a high-efficiency and high-throughput on-chip exchanger of the continuous phase in a continuous-flow droplet microfluidic system. The efficiency of exchange was higher than 97%. The throughput was greater than 1 million drops/min, and this can be increased further by increasing the number of parallel exchangers used. Because drops are injected into the exchanger in a continuous-flow manner, the method is directly compatible with automation to further increase its reliability and potential scale-up.

A versatile platform for surface modification of microfluidic droplets

To advance emulsion droplet technology, we synthesize functional derivatives of Pluronic F127 that can simultaneously act as surfactants and as reactive sites for droplet surface decoration. The amine-, carboxyl-, N-hydroxysuccinimide ester-, maleimide- and biotin-terminated Pluronic F127 allows ligand immobilization on single-emulsion or double-emulsion droplets via electrostatic adsorption, covalent conjugation or site-specific avidin-biotin interaction.

Sequence specific sorting of DNA molecules with FACS using 3dPCR

Genetic heterogeneity is an important feature of many biological systems, but introduces technical challenges to their characterization. Even with the best modern instruments, only a small fraction of DNA molecules present in a sample can be read, and they are recovered in the form of short, hundred-base reads. In this paper, we introduce 3dPCR, a method to sort DNA molecules with sequence specificity. 3dPCR allows heterogeneous populations of DNA to be sorted to recover long targets for deep sequencing. It is valuable whenever a target sequence is rare in a mixed population, such as for characterizing mutations in heterogeneous cancer cell populations or identifying cells containing a specific genetic sequence or infected with a target virus.