Fusion and sorting of two parallel trains of droplets using a railroad-like channel network and guiding tracks

We propose a robust droplet fusion and sorting method for two parallel trains of droplets that is relatively insensitive to frequency and phase mismatch. Conventional methods of droplet fusion require an extremely precise control of aqueous/oil flows for perfect frequency matching between two trains of droplets. In this work, by combining our previous two methods (i.e., droplet synchronization using railroad-like channels and manipulation of shape-dependent droplets using guiding tracks), we realized an error-free droplet fusion/sorting device for the two parallel trains of droplets. If droplet pairs are synchronized through a railroad-like channel, they are electrically fused and the fused droplets transit to a middle guiding track to flow in a middle channel; otherwise non-synchronized non-fused droplets will be discarded into the side waste channels by flowing through their own guiding tracks. The simple droplet synchronization, fusion, and sorting technology will have widespread application in droplet-based chemical or biological experiments, where two trains of the chemically or biologically treated or pre-formed droplets yield a train of 100% one-to-one fused droplets at the desired outlet channel by sorting all the non-synchronized non-fused droplets into waste outlets.     

Guiding, distribution, and storage of trains of shape-dependent droplets

We present a simple method of guiding, distributing, and storing of a train of shape-dependent droplets by using side flows, cavity guiding tracks, and storage chambers. The squeezing flow makes a train of flattened droplets to align to one side of the wall and the pushing flow guides it to one of the designated guiding tracks. Then the guided droplets move along the guiding track due to the lowered surface energy when they flow along the track. In addition, simultaneous droplet guiding and storing process has been demonstrated. An array of storage chambers placed in each track could store each train containing differently concentrated droplets. The proposed method will be useful for distribution of droplets for further processes or storing for multiplex, large-scale, dynamic assays over time.     

Effects of channel sizes on traffic of solid in water in oil compound droplets through a vertical channel

The purpose of this article is to investigate the effects of channel sizes on the traffic of S/W compound droplets through a vertical channel. Compared with the horizontal channel, a vertical channel can effectively inhabit the contact of compound droplets with the channel wall, thus improving the survival rate. It is also found that the effects of tube length on droplet traffic are always dependent on the oil phase flow rate. In a short tube (L?=?2.0?cm), the survival rate increases as the oil phase flow rate increases. This may be due to significant prevention of coalescence among S/W compound droplets under a high oil phase flow rate. However, in a long tube (L?=?7.5?cm), the survival rate decreases with increasing oil phase flow rate, because disturbance of a water droplet can peel off the water phase coated on the surface of the solid particles. During the traffic process, the distance between water droplets and S/W compound droplets decreases linearly with time because of the larger diameter of the compound droplets. These study results can provide a useful guide for the preparation of high-throughput S/W compound droplets in a controllable and reproducible manner.     

Linear stability of a draining film squeezed between two approaching droplets

In the present paper we analyze the effect of infinitesimal non-axisymmetric perturbations in determining the critical gap thickness at which a draining, finite radius thin-film becomes unstable. The film is part of the suspending fluid trapped between two approaching deformable drops under the action of a flow field. We carry out a linear stability analysis in the context of a quasi-static approximation where the rate of growth of the disturbances is assumed to be much faster than the rate of film drainage. An analytical solution is derived for the model in the special case of a uniformly thick film, for two types of perturbation: fixed-end and free-end. It is shown, for this special case, when the hydrodynamic force pushing the drops together from the external flow is constant, that the four most unstable disturbances are of the free-end kind, associated with the lowest frequency modes of azimuthal variation in the film thickness. Higher modes are stabilized by surface tension. Our analysis also shows that adopting the unretarded form of the van der Waals disjoining pressure yields results similar to the analysis when electromagnetic retardation effects are included in the calculation. A second case is analyzed where the film is also of uniform thickness but its lateral extent and the gap thickness are both time-dependent. This case was included to extend the predictions to glancing drop-collisions where the external hydrodynamic force is time-dependent. We find that there is a maximum capillary number below which the film becomes unstable, and that there is range of angles in the trajectory where the film becomes unstable, but that outside this range the film is stable.     

Transport and deformation of droplets in a microdevice using dielectrophoresis

In microfluidic devices the fluid can be manipulated either as continuous streams or droplets. The latter is particularly attractive as individual droplets can not only move but also split and fuse, thus offering great flexibility for applications such as laboratory-on-a-chip. We consider the transport of liquid drops immersed in a surrounding liquid by means of the dielectrophoretic force generated by electrodes mounted at the bottom of a microdevice. The direct numerical simulation (DNS) approach is used to study the motion of droplets subjected to both hydrodynamic and electrostatic forces. Our technique is based on a finite element scheme using the fundamental equations of motion for both the droplets and surrounding fluid. The interface is tracked by the level set method and the electrostatic forces are computed using the Maxwell stress tensor. The DNS results show that the droplets move, and deform, under the action of nonuniform electric stresses on their surfaces. The deformation increases as the drop moves closer to the electrodes. The extent to which the isolated drops deform depends on the electric Weber number. When the electric Weber number is small, the drops remain spherical; otherwise, the drops stretch. Two droplets, however, that are sufficiently close to each other, can deform and coalesce, even if the electric Weber number is small. This phenomenon does not rely on the magnitude of the electric stresses generated by the bulk electric field, but instead is due to the attractive electrostatic drop-drop interaction overcoming the surface tension force. Experimental results are also presented and found to be in agreement with the DNS results.     

Evaporation of two solution droplets and a droplet located near a flat liquid surface

The mutual influence of two moderate-sized droplets of a dilute nonvolatile substance solution on the processes of their evaporation or condensation is theoretically analyzed under the assumption of a uniform concentration distribution inside the droplets. The conditions for the applicability of this approach are revealed. The evaporation or condensation of a droplet near a flat liquid surface is considered as a limiting case. The fluxes of water molecules to and from the surface of aqueous glycerol solution droplets occurring in air are numerically estimated depending on the droplet radii, distances between their surfaces, and air humidity. Analogous estimates are obtained for an aqueous glycerol solution droplet growing near a flat water surface.     

Hydrodynamical interaction of two emulsion droplets at small separations

The fluid's motion inside emulsion droplets is analysed when they mutually approach along their common axis and a thin liquid film is formed outside. A qualitative flow pattern is presented. Two particular cases are treated — a creeping motion and a boundary layer flow inside the droplets. Estimates are made for the tangential velocity at the droplet/film interface, for the drag force and for the energy dissipated in the respective phases.     

Prediction of hydrothermal behavior of a non-Newtonian nanofluid in a square channel by modeling of thermophysical properties using neural network

Journal of Thermal Analysis and Calorimetry - This paper assesses the contribution of TiO2 nanoparticles on thermal performance of a 0.5 mass% aqueous solution of carboxymethyl cellulose...     

Screening of protein crystallization conditions on a microfluidic chip using nanoliter-size droplets

Protein crystallization is a major bottleneck in determining tertiary protein structures from genomic sequence data. This paper describes a microfluidic system for screening hundreds of protein crystallization conditions using less than 4 nL of protein solution for each crystallization droplet. The droplets are formed by mixing protein, precipitant, and additive stock solutions in variable ratios in a flow of water-immiscible fluids inside microchannels. Each droplet represents a discrete trial testing different conditions. The system has been validated by crystallization of several water-soluble proteins.     

Parallel synthesis of a biased library of thiazolidinones as novel sodium channel antagonists

A biased chemical library containing 91 differentially substituted thiazolidinones was prepared in an effort to improve the pharmacology of a known anticonvulsant agent V102862. The collection was prepared in a single step multi-component condensation reaction that produced good yields and very high crude purity (75%-85%). Seven compounds, identified within the library were shown to be more potent than V102862, our parent reference compound, in an electrophysiological assay measuring sodium channel antagonism. The most potent compound, 3-(2-piperidinylethyl)-2-(3-(3-trifluoromethylphenoxy)phenyl)thiazolidinone, has a Ki of 90 nM.     

Hopping times of two hard disks diffusing in a channel

A finite difference method was used to solve numerically the multidimensional diffusion equation describing the time evolution of two hard disks diffusing in a narrow hard channel. The authors extract an estimate for the average time tauhop needed for the disks to hop pass each other. For narrow channels near the hopping threshold, tauhop diverges and is consistent with the scaling prediction of the transition state theory. This provides a much-needed rigorous benchmark to test an approximate solution to the diffusion problem.     

Controlled generation of droplets using an electric field in a flow-focusing paper-based device

Droplet-based microfluidics is a modular platform in high-throughput single-cell and small sample analyses. However, this droplet microfluidic system was widely fabricated using soft lithography or glass capillaries, which is expensive and technically demanding for various applications, limiting use in resource-poor settings. Besides, the variation in droplet size is also restricted due to the limitations on the operating forces that the paper-based platform is able to withstand. Herein, we develop a fully integrated paper-based droplet microfluidic platform for conducting droplet generation and cell encapsulation in independent aqueous droplets dispersed in a carrier oil by incorporating electric fields. Through imposing an electric field, the droplet size would decrease with increasing the electric field and smaller droplets can be produced at high applied voltage. The droplet diameter can be adjusted by the ratio of inner and outer flow velocities as well as the applied electric field. We also demonstrated the proof of concept encapsulation application of our paper device by encapsulating yeast cells under an electric field. Using a simple wax printing method, carbon electrodes can be integrated on the paper. The integrated paper-based microfluidic platform can be fabricated easily and conducted outside of centralized laboratories. This microfluidic system shows great potential in drug and cell investigations by encapsulating cells in resource-limited environments.     

Controlled generation of monodisperse discoid droplets using microchannel arrays

We report a novel technique for generating geometrically confined droplets using a unique microstructure composed of a microchannel (MC) array and a shallow well. Silicon MC array devices were successfully used to generate monodisperse discoid droplets of oil-in-water (O/W) and W/O types by forcing a to-be-dispersed phase through channels into a well filled with a continuous phase. Monodisperse discoid droplets with sizes down to several micrometers were obtained by controlling the channel and well dimensions. The resultant discoid droplets formed a mostly close-packed array in the well. Monodisperse discoid droplets consisting of a silicone oil/water/sodium dodecyl sulfate system did not coalesce during the storage time of seven days. Additionally, MC array plates with many channels can be useful for increasing the droplet productivity of a single microfluidic device.     

Sub-diffraction-limit cell imaging using a super-resolution microscope with simplified pulse synchronization

Stimulated emission depletion(STED) microscope is one of the most prominent super-resolution bio-imaging instruments, which holds great promise for ultrahigh-resolution imaging of cells. To construct a STED microscope, it is challenging to realize temporal synchronization between the excitation pulses and the depletion pulses. In this study, we present a simple and low-cost method to achieve pulse synchronization by using a condensed fluorescent dye as a depletion indicator. By using this method, almost all the confocal microscopes can be upgraded to a STED system without losing its original functions. After the pulse synchronization,our STED system achieved sub-100-nm resolution for fluorescent nanospheres and single-cell imaging.     

Manufacture of large uniform droplets using rotating membrane emulsification

A new rotating membrane emulsification system using a stainless steel membrane with 100 microm laser drilled pores was used to produce oil/water emulsions consisting of 2 wt% Tween 20 as emulsifier, paraffin wax as dispersed oil phase and 0.01-0.25 wt% Carbomer (Carbopol ETD 2050) as stabilizer. The membrane tube, 1 cm in diameter, was rotated inside a stationary glass cylinder, diameter of 3 cm, at a constant speed in the range 50-1500 rpm. The oil phase was introduced inside the membrane tube and permeated through the porous wall moving radially into the continuous phase in the form of individual droplets. Increasing the membrane rotational speed increased the wall shear stress which resulted in a smaller average droplet diameter being produced. For a constant rotational speed, the average droplet diameter increased as the stabilizer content in the continuous phase was lowered. The optimal conditions for producing uniform emulsion droplets were a Carbomer content of 0.1-0.25 wt% and a membrane rotational speed of 350 rpm, under which the average droplet diameter was 105-107 microm and very narrow coefficients of variation of 4.8-4.9%. A model describing the operation is presented and it is concluded that the methodology holds potential as a manufacturing protocol for both coarse and fine droplets and capsules.     

Control of aqueous droplets using magnetic and electrostatic forces

Basic control operations were successfully performed on an aqueous droplet using both magnetic and electrostatic forces. In our droplet-based microfluidics, magnetic beads were incorporated in an aqueous droplet as a force mediator. This report describes droplet anchoring and separation of the beads from the droplet using a combination of magnetic and electrostatic forces. When an aqueous droplet is placed in an oil-filled reservoir, the droplet sinks to the bottom, under which an electrode had been placed. The droplet was adsorbed (or anchored) to the bottom surface on the electrode when a DC voltage was applied to the electrode. The magnetic beads were removed with magnetic force after the droplet had been anchored. Surfactant addition into droplet solution was very effective for the elimination of electric charge, which resulted in the stable adsorption of a droplet to hydrophobic substrate under an applied voltage of DC 0.5-3 kV. In a sequential process, small volume of aqueous liquid was successfully transferred using both magnetic and electrostatic forces.     

Design of anti-icing coatings using supercooled droplets as nano-to-microscale probes

A multiscale simulation-based approach is presented for predicting anti-icing properties of nanocomposite coatings. Development of robust anti-icing coatings is a challenging task. An anti-icing coating that can prevent in-flight icing is of particular interest to the aircraft industry. A multiscale simulations based approach is developed to provide insights into the complex effect of coating material and surface topology on the prevention of in-flight icing. Chemical properties of different coatings and kinetics of icing or inhibition of ice nucleation are calculated from nanoscale atomistic simulations. In addition, in-flight icing environments including impingement and rolling of supercooled microdroplet and nucleation of ice under wind shear have been implemented using fluid dynamics methodologies. A model for icing in nano-to-microscale for surfaces with known chemical composition and surface topology is used for developing predictive capabilities regarding anti-icing performance of potential coatings. In this work, fluorinated polyhedral oligomericsilsesquioxanes molecules have been used to increase nanoscale roughness when embedded in a polycarbonate polymeric matrix. The findings suggest that a successful anti-icing coating will require precise control over nanoscale and microscale roughness. The multiscale methodology presented therefore can potentially help in identifying coupled effects of material, surface topology, and icing environment for promising coatings before performing icing tunnel experiments.     

Nucleation rate measurement of colloidal crystallization using microfluidic emulsion droplets

An emulsion crystallization method has been demonstrated to measure the nucleation rate of a thermoresponsive colloidal poly-N-isopropylacrylamide (PNIPAM) system. The colloidal PNIPAM suspension was injected into a microfluidic flow-focusing device to generate monodispersed droplets in oil. The temperature was controlled to fine tune the volume fraction of the PNIPAM particles, and the microfluidic flow rate was varied to change the droplet sizes, thus altering the nucleation volume. Using independent droplets, we can isolate the nucleation events to eliminate the interactions among crystallites that existed in bulk or large droplet systems. Therefore, we were able to carry out accurate nucleation rate measurements of colloidal crystals. This emulsion crystallization method is promising for bridging the gap among theories, simulations, and experiments for nucleation kinetics studies.     

Local heating of discrete droplets using magnetic porous silicon-based photonic crystals

This paper describes a method for local heating of discrete microliter-scale liquid droplets. The droplets are covered with magnetic porous Si microparticles, and heating is achieved by application of an external alternating electromagnetic field. The magnetic porous Si microparticles consist of two layers. The top layer contains a photonic code and it is hydrophobic, with surface-grafted dodecyl moieties. The bottom layer consists of a hydrophilic silicon oxide host layer that is infused with Fe3O4 nanoparticles. The amphiphilic microparticles spontaneously align at the interface of a water droplet immersed in mineral oil, allowing manipulation of the droplets by application of a magnetic field. Application of an oscillating magnetic field (338 kHz, 18 A rms current in a coil surrounding the experiment) generates heat in the superparamagnetic particles that can raise the temperature of the enclosed water droplet to >80 degrees C within 5 min. A simple microfluidics application is demonstrated: combining complementary DNA strands contained in separate droplets and then thermally inducing dehybridization of the conjugate. The complementary oligonucleotides were conjugated with the cyanine dye fluorophores Cy3 and Cy5 to quantify the melting/rebinding reaction by fluorescence resonance energy transfer (FRET). The magnetic porous Si microparticles were prepared as photonic crystals, containing spectral codes that allowed the identification of the droplets by reflectivity spectroscopy. The technique demonstrates the feasibility of tagging, manipulating, and heating small volumes of liquids without the use of conventional microfluidic channel and heating systems.