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Layer-by-Layer coating technology is of great importance for many applications of microparticles whereby exposure of the particles to various reagents is needed. Mutual contamination of the reagents during this process is a key challenge, and this undesired effect should be avoided. Here we introduce a device that provides subsequent exposure of particles to various liquids and minimizes mixing of the liquids at the same time. The key element of the device is a rail (groove) at the bottom of a microfluidic channel. The rail forms an angle (between 0 and 90 degrees) and thus enables passive transport of particles through the intact co-flows of the different fluids. To avoid the undesirable effect of reagent stream mixing, internal walls are introduced to separate the different flows. Various designs of the proposed device are considered, and their performance is experimentally analyzed. more...

In microfluidics studies, it is often (implicitly) assumed that particles follow the surrounding flow. The effect of boundaries is, therefore, neglected. In the present paper, we investigate—experimentally and theoretically—the situation wherein the presence of boundaries affects the dynamics of particles. We designed and fabricated a tool to detect particle positions and measure their displacement and velocity to study the effect of wall on magnetically driven superparamagnetic microparticles (SMP). Two experimental setups are employed. A vertical setup is operated in free particle migration mode where the particle motion is not affected by the chip boundaries. We here measure the density and the magnetic susceptibility of the particle. This setup allows to characterize particle properties and to calibrate the interaction of the particles with the external magnetic field. In a horizontal setup, we track migration with the particle motion affected by the presence of channel bottom. more...

In this study, we present a novel microfluidic device for high-throughput production of monodisperse droplets. The proposed 3D-emulsifier consists of multiple parallel droplet generators coupled to only two inlets (continuous and dispersed phase). The three-dimensional nature of our device allows for a maximal density of droplet generators per surface area. We produce oil droplets in water using a device containing a single and four droplet generators. The droplet size and throughput were experimentally determined. In the four nozzle chip, we observe an important effect of the flow rate on the size distribution between the different droplet generators. Importantly, the shift between the squeezing and the transition regime shows the highest monodispersed production rate. For the four-nozzle chip, we show a 4-fold increase in the production throughput, while maintaining a high monodispersity of the droplets. A theoretical scale-up of our device is performed, demonstrating a possible throughput of 8.2 L/h, opening the door for (industrial) applications requiring much larger flow rates than what is typically achievable with microfluidic devices. more...

A novel microfluidic device that subjects a solution to a constant shear flow was developed. By taking advantage of the linear velocity profile in a lid driven flow configuration, small volumes (10-5 L) can be subjected to a constant shear profile with a shear rate between 0.1 and 100 s-1 at accurately controlled temperatures between 20 and 50 °C. The tunable shear can be maintained for extensive and fully controlled times. A dedicated microscope setup for visualization enables the on-chip detection of micron-sized crystals, particles, and aggregates. The influence of shear on the crystallization process of the reference protein lysozyme was studied. The results indicate that shear rates between 1 and 10 s-1 decrease solubility and promote nucleation not only in the supersaturated and metastable zones of the phase diagram, but also in the undersaturated zone. A monotonically increasing nucleation rate was observed for shear rates between 1 and 10 s-1. It is anticipated that the presented methodology can shed light on a variety of phase transitions that are influenced by flow., info:eu-repo/semantics/published more...

Various cobalt silicate precipitation patterns can be observed when an aqueous solution of cobalt ions gets into contact with a solution of silicate ions upon injection of one solution into the other in the confined geometry of a Hele-Shaw cell. The properties of these precipitation patterns are studied here as a function of the injection flow rate, densities and viscosities of the solutions, and the choice of which solution is injected into the other one. Our results show that the structure of the precipitation pattern depends on the local velocity as well as on the difference in viscosities between the injected and the displaced solutions. Specifically, decreasing the injection flow rate and/or decreasing the density jump while increasing the difference in viscosities between the reactant solutions results in more circular patterns. Moreover, we show that some structures are robustly observed in given ranges of the local flow velocity in the cell. Locally, precipitation can then transition from one type of pattern to another during injection, according to that preferred structure at the given local velocity. We also show that injection of the cobalt solution into the silicate solution results in the so-called direct patterns that are different from the reverse patterns obtained when the silicate solution is injected in the solution of cobalt ions. Our results help in understanding the production of precipitate structures under nonequilibrium flow conditions. more...

The selection of polymorphs of the organic compound 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, ROY, is studied experimentally in the confined space between two horizontal glass plates when an acetone solution of ROY of variable concentration is injected at a variable flow rate into water. Depending on the local concentration within the radial flow, a polymorph selection is observed such that red prisms are favored close to the injection center while yellow needles are the preferred polymorph close to the edge of the injected ROY domain. At larger flow rates, a buoyancy-driven instability induces stripes at the outer edge of the displacement pattern, in which specific polymorphs are seen to crystallize. Our results evidence the possibility of a selection of ROY polymorph structures in out-of-equilibrium flow conditions. more...

Manipulation of high-density materials, such as crystals and liquid condensates, is of great importance for many applications, including serial crystallography, structural and molecular biology, chemistry, and medicine. In this work, we describe an acoustic technique to focus and harvest flowing crystals and liquid condensates. Moreover, we show, based on numerical simulations, that the acoustic waves can be used for size-based particle (crystals, droplets, etc.) separation. This is an essential technological step in biological research, medical applications, and industrial processes. The presented technology offers high precision, biocompatibility, ease of use and additionally, is non-invasive and inexpensive. With the recent advent of X-ray Free Electron Laser (XFEL) technology and the associated enormous importance of a thin jet of crystals, this technology might pave the way to a novel type of XFEL injector. more...

The general objective of this paper is to investigate the separation, with microfluidics, of the components of a ternary mixture, when using vacuum or purge gas pervaporation. The ternary mixture considered is a mixture of methanol (MeOH), water (H2O) and hydrogen peroxide (H2O2). In a previous work (Ziemecka in Lab Chip 15:504–511, 2015), we presented the proof of concept of a microfluidic device, which was able to partially separate MeOH from the other components of such a mixture, by using vacuum pervaporation. Here, our goal is to optimize the operation of this device, by considering vacuum pervaporation, but also purge gas pervaporation. First, we provide a mathematical model of the device. This model is used to discuss the influence of the operating parameters on the device operation. To apply this model to the considered mixture, we determined the MeOH and H2O permeability coefficients of PDMS membranes prepared from different concentrations of the curing agent. The model is then successfully compared to experimental data. The model and the experiments show that high efficiencies can be reached for both vacuum and purge gas pervaporation, provided a fine-tuning of the operating parameters. For instance, a good efficiency of the vacuum pervaporation is reached at high temperature and low pressure. For purge gas pervaporation, it is reached for low temperature and high pressure. more...

Catalyst-assisted self-assembly is widespread in nature to achieve spatial control over structure formation. Reported herein is the formation of hydrogel micropatterns on catalytic surfaces. Gelator precursors react on catalytic sites to form building blocks which can self-assemble into nanofibers. The resulting structures preferentially grow where the catalyst is present. Not only is a first level of organization, allowing the construction of hydrogel micropatterns, achieved but a second level of organization is observed among fibers. Indeed, fibers grow with their main axis perpendicular to the substrate. This feature is directly linked to a unique mechanism of fiber formation for a synthetic system. Building blocks are added to fibers in a confined space at the solid–liquid interface. more...

We report the continuous production of microcapsules composed of an aqueous core and permeable hydrogel shell, made stable by the controlled photo-cross-linking of the shell of an all-aqueous double emulsion. While most previous work on water-based emulsions focused on active droplet formation, here double emulsion droplets were spontaneously generated at a three-dimensional flow-focusing junction through the break-up of a double jet formed by immiscible aqueous solutions of polyethylene glycol and cross-linkable dextrans. The capsules obtained with this lipid-free, organic-solvent-free, and surfactant-free approach displayed excellent stability under a variety of harsh conditions (3 more...

We demonstrate and investigate how a propagating acidic wave in a solution of dibenzoyl-L-cystine (DBC) can lead to the orientation and alignment of self-assembled fibers constituting a hydrogel. Taking advantage of its pH-induced self-assembly, a hydrogel composed of parallel fibers can be formed by propagating a pH gradient front from one part of a cell to the other. The diffusing species (typically the ion pair H+–Cl−) control the velocity of the pH-gradient front and, therefore, their diffusion rate should match the gelation rate so as to attain complete orientation. more...

This paper presents a method to form micron-sized droplets in an aqueous two-phase system (ATPS) and to subsequently polymerize the droplets to produce hydrogel beads. Owing to the low interfacial tension in ATPS, droplets do not easily form spontaneously. We enforce the formation of drops by perturbing an otherwise stable jet that forms at the junction where the two aqueous streams meet. This is done by actuating a piezo-electric bending disc integrated in our device. The influence of forcing amplitude and frequency on jet breakup is described and related to the size of monodisperse droplets with a diameter in the range between 30 and 60 μm. Rapid on-chip polymerization of derivatized dextran inside the droplets created monodisperse hydrogel particles. This work shows how droplet-based microfluidics can be used in all-aqueous, surfactant-free, organic-solvent-free biocompatible two-phase environment. more...

This paper studies the Rayleigh-Plateau instability for co-flowing immiscible aqueous polymer solutions in a microfluidic channel. Careful vibration-free experiments with controlled actuation of the flow allowed direct measurement of the growth rate of this instability. Experiments for the well-known aqueous two phase system (ATPS, or aqueous biphasic systems) of dextran and polyethylene glycol solutions exhibited a growth rate of 1 s(-1), which was more than an order of magnitude slower than an analogous experiment with two immiscible Newtonian fluids with viscosities and interfacial tension that closely matched the ATPS experiment. Viscoelastic effects and adhesion to the walls were ruled out as explanations for the observed behavior. The results are remarkable because all current theory suggests that such dilute polymer solutions should break up faster, not slower, than the analogous Newtonian case. Microfluidic uses of aqueous two phase systems include separation of labile biomolecules but have hitherto be limited because of the difficulty in making droplets. The results of this work teach how to design devices for biological microfluidic ATPS platforms. more...

We present a microfluidic method to compartmentalize aqueous polymer solutions within water-in-water microdroplets, which are continuously generated without using organic solvents or surfactants. Phase separation inside the drops yields all-aqueous core-shell structures (water-in-water-in-water), as we demonstrate using the aqueous two phase system of polyethylene glycol and dextran. more...

Microfluidic devices offer a well-controlled environment for crystal nucleation and growth. Important parameters are mixing and the associated shear. Incomplete mixing results in a wide crystal size distribution and affect the morphology of the product. Bulk acoustic waves offer the possibility to quickly mix two liquid streams and to manipulate particles in the microfluidic channel. In this contribution, fast mixing of two liquid streams by acoustic streaming is demonstrated. Miconazole nitrate crystals were grown in the presence of the acoustic field, and focused on the pressure node. Growth and focusing of amyloid fibrils in the microfluidic channel has also been demonstrated. more...

This paper presents a rail guided method to apply Layer-by-Layer (LbL) coating on particles in a microfluidic device. The passive, microfluidic approach allows handling suspensions of particles to be coated in the system. The trajectory of the particles is controlled using engraved rails, inducing lateral movement of particles while keeping the axially oriented liquid flow (and the interface of different liquids) undisturbed. The depth and angle of the rails together with the liquid velocity were studied to determine a workable geometry of the device. A discontinuous LbL coating procedure was converted into one continuous process, demonstrating that the chip can perform seven consecutive steps normally conducted in batch operation, further easily extendable to larger cycle numbers. Coating of the particles with two bilayers was confirmed by fluorescent microscopy. more...