Aggregation and clogging phenomena of rigid microparticles in microfluidics: Comparison of a discrete element method (DEM) and CFD-DEM coupling method

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.

Investigating reactive transport and precipitation patterns of calcium carbonate in fractured porous media

HYPOTHESIS

Understanding calcium carbonate (CaCO3) precipitation in various polymorphs from nanoparticle size (amorphous calcium carbonate) to microparticle size (vaterite, aragonite, dendrite, calcite) is important for practical applications, including carbon geo-storage (e.g., basalt formations), hydrogen storage, groundwater management, and soil stabilization. Our hypothesis suggests that the interplay of Péclet numbers (Pe), Damköhler numbers (Da), and Supersaturation Index (SI) significantly impacts the evolution of CaCO3 precipitation in fractured porous media in terms of mixing patterns, spatiotemporal evolution, crystal morphology, crystal size, and clogging behavior.

EXPERIMENTS

This study takes a novel approach to explore the colloidal formation and precipitation dynamics of CaCO3 within a fractured microfluidic system. Here, calcium chloride (CaCl2) and sodium bicarbonate (NaHCO3) solutions were injected and reacted under varied Pe (0-11), Da (0-1), and SI (2-5).

FINDINGS

Our analysis revealed distinct precipitation patterns and mixing types, such as transverse, longitudinal, and incomplete mixing, providing insights into the behavior in fractured porous media. We systematically analyzed the temporal and spatial evolution of precipitation, demonstrating how Pe, Da, and SI dictate precipitation rates and spatial distribution. Additionally, the study uncovered a range of CaCO3 polymorphic forms, illustrating their evolution and coexistence. Morphological changes and crystal sizes were examined to decode nucleation and growth processes. Significantly, our findings highlight the relationship between precipitation and clogging in the fractured medium, offering a deeper understanding of reactive transport in complex porous environments. These insights are crucial for enhancing carbon containment security and storage efficiency in underground formations, improving groundwater remediation techniques, and developing novel construction materials through controlled precipitation processes.

Full-cycle study on developing a novel structured micromixer and evaluating the nanoparticle products as mRNA delivery carriers

Achieving precise control of nanoparticle size while maintaining consistency and high uniformity is of paramount importance for improving the efficacy of nanoparticle-based therapies and minimizing potential side effects. Although microfluidic technologies are widely used for reliable nanoparticle synthesis, they face challenges in meeting critical homogeneity requirements, mainly due to imperfect mixing efficiency. Furthermore, channel clogging during continuous operation presents a significant obstacle in terms of quality control, as it progressively impedes the mixing behavior necessary for consistent nanoparticle production for therapeutic delivery and complicates the scaling-up process. This study entailed the development of a 3D-printed novel micromixer embedded with hemispherical baffle microstructures, a dual vortex mixer (DVM), which integrates Dean vortices to generate two symmetrical counter-rotating intensified secondary flows. The DVM with a relatively large mixer volume showed rapid mixing characteristics even at a flow rate of several mL min-1 and produced highly uniform lipids, liposomes, and polymer nanoparticles in a size range (50-130 nm) and polydispersity index (PDI) values below 0.15. For the evaluation of products, SARS-CoV-2 Spike mRNA-loaded lipid nanoparticles were examined to verify protein expression in vitro and in vivo using firefly luciferase (FLuc) mRNA. This showed that the performance of the system is comparable to that of a commercial toroidal mixer. Moreover, the vigorous in-situ dispersion of nanoparticles by harnessing the power of vortex physically minimizes the occurrence of aggregation, ensuring consistent production performance without internal clogging of a half-day operation and facilitating quality control of the nanoparticles at desired scales.

Versatile and Remotely Controllable Light-Induced Coagulation of Particles Under Flow in a 2D Channel

On-demand switch on/off blood clogging is of paramount importance for the survival of mammals, for example as a quick response to seal damage wounds to minimize their bleeding rate. This mechanism is a complex chain process from initiated red blood cell aggregation at the target location (open wound) that quickly seals on a macroscopic scale the damaged flash. Inspired by nature an on-demand switchable particle clogging mechanism is developed with high spatial resolution down to micrometer size using light as an external non-invasive stimulation. Particle clogging can be adjusted on demand strong enough to even withstand pressure-driven fluid flow, additionally building up walls of aggregated particles, which stop the momentum of big particles under shear. The principle relies on a photosensitive surfactant, which induces under light illumination a long-ranged lateral attractive phoretic-osmotic activity of silica microparticles forcing them to aggregate. The strength of aggregation and therefore motion reduction or even stop of the particles against the fluid flow depends on the ratio between the aggregation strength and the velocity of the particles. The aggregation strength can be precisely controlled by the applied light intensity and adjusted particle concentration. Increasing both parameters results in a stronger aggregation tendency.

Raman and Surface-Enhanced Raman Scattering Detection in Flowing Solutions for Complex Mixture Analysis

Raman scattering provides a chemical-specific and label-free method for identifying and quantifying molecules in flowing solutions. This review provides a comprehensive examination of the application of Raman spectroscopy and surface-enhanced Raman scattering (SERS) to flowing liquid samples. We summarize developments in online and at-line detection using Raman and SERS analysis, including the design of microfluidic devices, the development of unique SERS substrates, novel sampling interfaces, and coupling these approaches to fluid-based chemical separations (e.g., chromatography and electrophoresis). The article highlights the challenges and limitations associated with these techniques and provides examples of their applications in a variety of fields, including chemistry, biology, and environmental science. Overall, this review demonstrates the utility of Raman and SERS for analysis of complex mixtures and highlights the potential for further development and optimization of these techniques.

Acoustofluidic large-scale mixing for enhanced microfluidic immunostaining for tissue diagnostics

The usage of microfluidics for automated and fast immunoassays has gained a lot of interest in the last decades. This integration comes with certain challenges, like the reconciliation of laminar flow patterns of micro-scale systems with diffusion-limited mass transport. Several methods have been investigated to enhance microfluidic mixing in microsystems, including acoustic-based fluidic streaming. Here, we report both by numerical simulation and experiments on the beneficiary effect of acoustic agitation on the uniformity of immunostaining in large-size and thin microfluidic chambers. Moreover, we investigate by numerical simulation the impact of reducing the incubation times and the concentrations of the biochemical detection reagents on the obtained immunoassay signal. Finally, acoustofluidic mixing was successfully used to reduce by 80% the incubation time of the Her2 (human epidermal growth factor receptor 2) and CK (cytokeratins) biomarkers for the spatial immunostaining of breast cancer cell pellets, or reducing their concentration by 66% and achieving a higher signal-to-background ratio than comparable spatially resolved immunostaining with static incubation.

Graphene Oxide Paper Manipulation of Micro-Reactor Drops

Digital microfluidics, which relies on the movement of drops, is relatively immune to clogging problems, making it suited for micro-reactor applications. Here, graphene oxide paper of 100 μm thickness, fabricated by blade coating sedimented dispersions onto roughened substrates, followed by drying and mechanical exfoliation, was found to be relatively free of cracks and curling. It also exhibited high wettability and elasto-capillary characteristics. Possessing low enough stiffness, it could rapidly and totally self-wrap water drops of 20 μL volume placed 2 mm from its edge when oriented between 0 and 60° to the horizontal. This complete wrapping behavior allowed drops to be translated via movement of the paper over long distances without dislodgement notwithstanding accelerations and decelerations. An amount of 2 drops that were wrapped with separate papers, when collided with each other at speeds up to 0.64 m/s, were found to eschew coalescence. This portends the development of robust digital microfluidic approaches for micro-reactors.

Design of functional nanoparticles by microfluidic platforms as advanced drug delivery systems for cancer therapy

Nanoparticle systems are functional carriers that can be used in the cancer therapy field for the delivery of a variety of hydrophobic and/or hydrophilic drugs. Recently, the advent of microfluidic platforms represents an advanced approach to the development of new nanoparticle-based drug delivery systems. Particularly, microfluidics can simplify the design of new nanoparticle-based systems with tunable physicochemical properties such as size, size distribution and morphology, ensuring high batch-to-batch reproducibility and consequently, an enhanced therapeutic effect in vitro and in vivo. In this perspective, we present accurate state-of-the-art microfluidic platforms focusing on the fabrication of polymer-based, lipid-based, lipid/polymer-based, inorganic-based and metal-based nanoparticles for biomedical applications.

Shape-Controlled Self-Assembly of Light-Powered Microrobots into Ordered Microchains for Cells Transport and Water Remediation

Nature presents the collective behavior of living organisms aiming to accomplish complex tasks, inspiring the development of cooperative micro/nanorobots. Herein, the spontaneous assembly of hematite-based microrobots with different shapes is presented. Autonomous motile light-driven hematite/Pt microrobots with cubic and walnut-like shapes are prepared by hydrothermal synthesis, followed by the deposition of a Pt layer to design Janus structures. Both microrobots show a fuel-free motion ability under light irradiation. Because of the asymmetric orientation of the magnetic dipole moment in the crystal, cubic hematite/Pt microrobots can self-assemble into ordered microchains, contrary to the random aggregation observed for walnut-like microrobots. The microchains exhibit different synchronized motions under light irradiation depending on the mutual orientation of the individual microrobots during the assembly, which allows them to accomplish multiple tasks, including capturing, picking up, and transporting microscale objects, such as yeast cells and suspended matter in water extracted from personal care products, as well as degrading polymeric materials. Such light-powered self-assembled microchains demonstrate an innovative cooperative behavior for small-scale multitasking artificial robotic systems, holding great potential toward cargo capture, transport, and delivery, and wastewater remediation.

Single Molecule Surface Enhanced Raman Scattering in a Single Gold Nanoparticle-Driven Thermoplasmonic Tweezer

Surface enhanced Raman scattering (SERS) is optically sensitive and chemically specific to detect single-molecule spectroscopic signatures. Facilitating this capability in optically trapped nanoparticles at low laser power remains a significant challenge. In this letter, we show single molecule SERS signatures in reversible assemblies of trapped plasmonic nanoparticles using a single laser excitation (633 nm). Importantly, this trap is facilitated by the thermoplasmonic field of a single gold nanoparticle dropcasted on a glass surface. We employ the bianalyte SERS technique to ascertain the single molecule statistical signatures and identify the critical parameters of the thermoplasmonic tweezer that provide this sensitivity. Furthermore, we show the utility of this low power (≈ 0.1 mW/μm²) tweezer platform to trap a single gold nanoparticle and transport assembly of nanoparticles. Given that our configuration is based on a dropcasted gold nanoparticle, we envisage its utility to create reconfigurable plasmonic metafluids in physiological and catalytic environments and to be potentially adapted as an in vivo plasmonic tweezer.

Mild Microfluidic Approaches to Oxide Nanoparticles Synthesis

Oxide nanoparticles (oxide NPs) are advanced materials with a wide variety of applications in different fields. The use of continuous flow methods is particularly appealing for their synthesis due to the high control achieved over the reaction conditions and the easy process scalability. The present review focuses on the preparation of oxide NPs using microfluidic setups at low temperature (≤80 °C), since the employment of mild reaction conditions is crucial for developing sustainable and cost-effective processes. A particular emphasis will be put on the improvement over the final product features (e. g., size, shape, and size distribution) given by flow methods with respect to conventional batch procedures. The main issues that arise by treating NPs suspensions in microfluidic systems are product deposition or channel clogging; mitigation strategies to overcome these drawbacks will also be presented and discussed.

Synthesis and Characterization of Polymeric Microspheres Template for a Homogeneous and Porous Monolith

Monolith is an emerging technology applicable for separation, filtration, and chromatography due to its interconnected pore structure. However, the current templates used to form monolith pores are associated with poor heat dissipation, uneven pore size distribution, and relatively low mechanical strength during monolith scale-up. Templates made from polymeric microsphere particles were synthesized via a solvent evaporation technique using different types of polymer (polystyrene, polycaprolactone, polypropylene, polyethylene, and poly (vinyl-alcohol) at varied polymer (10-40 wt%) and surfactant (5-10%) concentrations. The resulting microsphere particles were tested as a monolith template for the formation of homogenous pores. Among the tested polymers, polystyrene at 10 wt% concentration demonstrated good particle morphology determined to around 1.94-3.45 µm. The addition of surfactant at a concentration of 7-10 wt% during microsphere synthesis resulted in the formation of well-shaped and non-aggregating microsphere particles. In addition, the template has contributed to the production of porous monoliths with enhanced thermal stability. The thermogravimetric analysis (TGA) indicated monolith degradation between 230 °C and 450 °C, implying the material excellent mechanical strength. The findings of the study provide insightful knowledge on the feasibility of polymeric microsphere particles as a pore-directing template to fabricate monoliths with desired pore structures.

Polymer based nanoparticles for biomedical applications by microfluidic techniques: from design to biological evaluation

The development of microfluidic technologies represents a new strategy to produce and test drug delivery systems.

Deposition kinetics of bi- and tridisperse colloidal suspensions in microchannels under the van der Waals regime

We investigate the kinetics of irreversible adsorption under the van der Waals regime, i.e. weakly Brownian polydisperse colloidal suspensions injected into shallow microchannels at high ionic strengths, where each suspension is represented by populations of particles with different particle sizes. We find that each population size of the particle in the suspension can be treated independently using an analytical solution based on the advection-diffusion equation and that the distribution of the adsorbed particles along the channel axis behaves according to a power law. The experimental measurements agree with Langevin simulations and are well accounted for by theory valid in the van der Waals regime. Operating in the van der Waals regime permits the present study to confirm the use of microfluidics as an effective in situ method to measure the Hamaker constant of particles under aqueous conditions.