High-performance flow-focusing geometry for spontaneous generation of monodispersed droplets

A high-performance flow-focusing geometry for spontaneous generation of monodispersed droplets is demonstrated. In this geometry, a two-phase flow is forced through a circular orifice integrated inside a silicon-based microchannel. The orifice with its cusp-like edge exerts a ring of maximized stress around the flow and ensures controlled breakup of droplets for a wide range of flow rates, forming highly periodic and reproducible dispersions. The droplet generation can be remarkably rapid, exceeding 10(4) s(-1) for water-in-oil droplets and reaching 10(3) s(-1) for oil-in-water droplets, being largely controlled by flow rate of the continuous phase. The droplet diameter and generation frequency are compared against a quasi-equilibrium model based on the critical Capillary number. The droplets are obtained despite the low Capillary number, below the critical value identified by the ratio of viscosities between the two phases and simple shear-flow.     

Emulsion Templating of Poly(lactic acid) Particles: Droplet Formation Behavior

Monodisperse poly(dl-lactic acid) (PLA) particles of diameters between 11 and 121 μm were fabricated in flow focusing glass microcapillary devices by evaporation of dichloromethane (DCM) from emulsion droplets at room temperature. The dispersed phase was 5% (w/w) PLA in DCM containing 0.1-2 mM Nile Red and the continuous phase was 5% (w/w) poly(vinyl alcohol) in reverse osmosis water. Particle diameter was 2.7 times smaller than the diameter of the emulsion droplet template, indicating very low particle porosity. Monodisperse droplets have only been produced under dripping regime using a wide range of dispersed phase flow rates (0.002-7.2 cm(3)·h(-1)), continuous phase flow rates (0.3-30 cm(3)·h(-1)), and orifice diameters (50-237 μm). In the dripping regime, the ratio of droplet diameter to orifice diameter was inversely proportional to the 0.39 power of the ratio of the continuous phase flow rate to dispersed phase flow rate. Highly uniform droplets with a coefficient of variation (CV) below 2% and a ratio of the droplet diameter to orifice diameter of 0.5-1 were obtained at flow rate ratios of 4-25. Under jetting regime, polydisperse droplets (CV > 6%) were formed by detachment from relatively long jets (between 4 and 10 times longer than droplet diameter) and a ratio of the droplet size to orifice size of 2-5.     

Deformation and breakup of micro- and nanoparticle stabilized droplets in microfluidic extensional flows

Using a microfluidic flow-focusing device, monodisperse water droplets in oil were generated and their interface populated by either 1 μm or 500 nm amine modified silica particles suspended in the water phase. The deformation and breakup of these Pickering droplets were studied in both pure extensional flow and combined extensional and shear flow at various capillary numbers using a microfluidic hyperbolic contraction. The shear resulted from droplet confinement and increased with droplet size and position along the hyperbolic contraction. Droplet deformation was found to increase with increasing confinement and capillary number. At low confinements and low capillary numbers, the droplet deformation followed the predictions of theory. For fully confined droplets, where the interface was populated by 1 μm silica particles, the droplet deformation increased precipitously and two tails were observed to form at the rear of the droplet. These tails were similar to those seen for surfactant covered droplets. At a critical capillary number, daughter droplets were observed to stream from these tails. Due to the elasticity of the particle-laden interface, these drops did not return to a spherical shape, but were observed to buckle. Although increases in droplet deformation were observed, no tail streaming occurred for the 500 nm silica particle covered droplets over the range of capillary numbers studied.     

Electro-optic effect,propagation loss,and switching speed in polymers containing nano-sized droplets of liquid crystal

Polymers containing droplets of liquid crystal smaller than 100 nm, which have good transparency and easily form films, were prepared under various conditions to evaluate their potential as electro-optic materials for waveguide-type devices. By varying the liquid crystal concentration and the strength of the UV irradiation causing photo-induced phase separation of the droplets, we were able to control the droplet size and density. We have clarified how the birefringence generated in an applied electric field, switching speed, and optical loss of light propagating in the film depend on droplet size and density. Polymer materials having a large electro-optic effect (δn = 0.001 at 8 V μm^-1), low propagation loss (~2.5 dB cm^-1), and fast response time (~10 μs) have been developed.     

Electrophoretic mobility of oil droplets in electrolyte and surfactant solutions

Electrophoretic mobility of oil droplets of micron sizes in PBS and ionic surfactant solutions was measured in this paper. The experimental results show that, in addition to the applied electric field, the speed and the direction of electrophoretic motion of oil droplets depend on the surfactant concentration and on if the droplet is in negatively charged SDS solutions or in positively charged hexadecyltrimethylammonium bromide (CTAB) solutions. The absolute value of the electrophoretic mobility increases with increased surfactant concentration before the surfactant concentration reaches to the CMC. It was also found that there are two vortices around the oil droplet under the applied electric field. The size of the vortices changes with the surfactant and with the electric field. The vortices around the droplet directly affect the drag of the flow field to the droplet motion and should be considered in the studies of electrophoretic mobility of oil droplets. The existence of the vortices will also influence the determination and the interpretation of the zeta potential of the oil droplets based on the measured mobility data.     

High-throughput sorting of nanoliter droplets enabled by a sequentially addressable dielectrophoretic array

Droplet microfluidics has emerged as a powerful tool for a diverse range of biomedical and industrial applications such as single-cell analysis, directed evolution, and metabolic engineering. In these applications, droplet sorting has been effective for isolating small droplets encapsulating molecules, cells, or crystals of interest. Recently, there is an increased interest in extending the applicability of droplet sorting to larger droplets to utilize their size advantage. However, sorting throughputs of large droplets have been limited, hampering their wide adoption. Here, we report our demonstration of high-throughput fluorescence-activated droplet sorting of 1 nL droplets using an upgraded version of the sequentially addressable dielectrophoretic array (SADA), which we reported previously. The SADA is an array of electrodes that are individually and sequentially activated/deactivated according to the speed and position of a droplet passing nearby the array. We upgraded the SADA by increasing the number of driving electrodes constituting the SADA and incorporating a slanted microchannel. By using a ten-electrode SADA with the slanted microchannel, we achieved fluorescence-activated droplet sorting of 1 nL droplets at a record high throughput of 1752 droplets/s, twice as high as the previously reported maximum sorting throughput of 1 nL droplets.     

Sizes of large He droplets

Helium droplets spanning a wide size range, N(He) = 10(3)-10(10), were formed in a continuous-nozzle beam expansion at different nozzle temperatures and a constant stagnation pressure of 20 bars. The average sizes of the droplets have been obtained by attenuation of the droplet beam through collisions with argon and helium gases at room temperature. The results obtained are in good agreement with previous measurements in the size range N(He) = 10(5)-10(7). Moreover, the measurements give the average sizes in the previously uncharacterized range of very large droplets of 10(7)-10(10) atoms. The droplet sizes and beam flux increase rapidly at nozzle temperatures below 6 K, which is ascribed to the formation of droplets within the nozzle interior. The mass spectra of the droplet beam upon electron impact ionization have also been obtained. The spectra show a large increase in the intensity of the He(4) (+) signal upon increase of the droplet size, an effect which can be used as a secondary size standard in the droplet size range N(He) = 10(4)-10(9) atoms.     

Formation of droplets and bubbles in a microfluidic T-junction-scaling and mechanism of break-up

This article describes the process of formation of droplets and bubbles in microfluidic T-junction geometries. At low capillary numbers break-up is not dominated by shear stresses: experimental results support the assertion that the dominant contribution to the dynamics of break-up arises from the pressure drop across the emerging droplet or bubble. This pressure drop results from the high resistance to flow of the continuous (carrier) fluid in the thin films that separate the droplet from the walls of the microchannel when the droplet fills almost the entire cross-section of the channel. A simple scaling relation, based on this assertion, predicts the size of droplets and bubbles produced in the T-junctions over a range of rates of flow of the two immiscible phases, the viscosity of the continuous phase, the interfacial tension, and the geometrical dimensions of the device.     

A numerical study on the coalescence of emulsion droplets in a constricted capillary tube

Using a new computational model, we have studied the dynamics and coalescence of a pair of two-dimensional droplets in pressure-driven flow through a constricted capillary tube, which is a prototype problem for the analysis of the interaction of emulsion droplets in porous media. We present simulations that quantify the effects of various system parameters on the droplet stability. These include the capillary number, the interfacial tension, the suspended-to-suspending-phase viscosity ratio, the valence and concentration of added electrolytes, the droplet-to-pore-size ratio, the pore-body-to-throat-size ratio, and the type of pore geometry. Our simulations show that the capillary number Ca plays an important role in determining whether the drops coalesce. At low Ca, drops deform only slightly and coalescence occurs at the entrance of the pore throat, whereas significant deformation enables the drops move through the pore without coalescence at high Ca. Coalescence is favored at intermediate values of the viscosity ratio. The destabilizing effect of added electrolytes is found to be insignificant for 10-mum drops, but significant for micron-size drops. Among the geometric-related parameters, the drop-to-pore-size ratio is the most significant.     

Influence of substrate heating on the evaporation dynamics of pinned water droplets

The dynamics of evaporating water droplets deposited on a heated substrate is investigated numerically. Droplets are pinned with a contact line radius of R = 1 mm. Evaporative mass flow and convection occurring inside the droplets are studied for different heating substrate sizes L S and heating temperatures T S. A simplified model neglecting hydrodynamics in air and evaporative cooling and assuming droplets to be spherical caps is simulated with a finite element method. A toruslike convective cell appears inside the droplets as evaporation takes place. For L S/ R > 1, the contact line is warmer than the apex of the droplets, and convection generates a downstream flow in the vicinity of the symmetry axis of the droplets. For L S/ R < 1, it is the apex that is warmer. Convection then generates an upstream flow. The overall evaporation time is described. It slows when L S/ R > 1.     

Electrohydrodynamic deposition of polymeric droplets under low-frequency pulsation

Circularly shaped polymeric droplets with diameter of about 20 μm have been intermittently ejected and deposited in an orderly manner on a collector from a syringe needle by means of near-field, electrohydrodynamic reactions using pulsating voltages at around 2.25 kV. The needle has an inner diameter of 100 μm and was placed 1 mm above a silicon conductor substrate to have location control for droplet depositions. Under low-frequency operation of less than 100 Hz, the deposition frequency of droplets, f(dep), has been observed to be equal to the frequency of the applied driving voltage divided by an integer, N, as small as 1. Furthermore, the diameter of the deposited droplets has been found to be linearly dependent on (Q/f(dep))(1/3), where Q is the polymer solution supply rate at around 30 nL/s. These experimentally observed droplet ejection rules under low-frequency pulsation provide useful design guidelines for controllable deposition of polymer droplets in various potential applications, including electrohydrodynamic printing.     

Freezing of water and aqueous NaCl droplets coated by organic monolayers as a function of surfactant properties and water activity

This study presents heterogeneous ice nucleation from water and aqueous NaCl droplets coated by 1-nonadecanol and 1-nonadecanoic acid monolayers as a function of water activity (a(w)) from 0.8 to 1 accompanied by measurements of the corresponding pressure-area isotherms and equilibrium spreading pressures. For water and aqueous NaCl solutions of ~0-20 wt % in concentration, 1-nonadecanol exhibits a condensed phase, whereas the phase of 1-nonadecanoic acid changes from an expanded to a condensed state with increasing NaCl content of the aqueous subphase. 1-Nonadecanol-coated aqueous droplets exhibit the highest median freezing temperatures that can be described by a shift in a(w) of the ice melting curve by 0.098 according to the a(w)-based ice nucleation approach. This freezing curve represents a heterogeneous ice nucleation rate coefficient (J(het)) of 0.85 ± 0.30 cm(-2) s(-1). The median freezing temperatures of 1-nonadecanoic acid-coated aqueous droplets decrease less with increasing NaCl content compared to the homogeneous freezing temperatures. This trend in freezing temperature is best described by a linear function in a(w) and not by the a(w)-based ice nucleation approach most likely due to an increased ice nucleation efficiency of 1-nonadecanoic acid governed by the monolayer state. This freezing curve represents J(het) = 0.46 ± 0.16 cm(-2) s(-1). Contact angles (α) for 1-nonadecanol- and 1-nonadecanoic acid-coated aqueous droplets increase as temperature decreases for each droplet composition, but absolute values depend on employed water diffusivity and the interfacial energies of the ice embryo. A parametrization of log[J(het)(Δa(w))] is presented which allows prediction of freezing temperatures and heterogeneous ice nucleation rate coefficients for water and aqueous NaCl droplets coated by 1-nonadecanol without knowledge of the droplet's composition and α.     

High-speed, clinical-scale microfluidic generation of stable phase-change droplets for gas embolotherapy

In this study we report on a microfluidic device and droplet formation regime capable of generating clinical-scale quantities of droplet emulsions suitable in size and functionality for in vivo therapeutics. By increasing the capillary number-based on the flow rate of the continuous outer phase-in our flow-focusing device, we examine three modes of droplet breakup: geometry-controlled, dripping, and jetting. Operation of our device in the dripping regime results in the generation of highly monodisperse liquid perfluoropentane droplets in the appropriate 3-6 μm range at rates exceeding 10(5) droplets per second. Based on experimental results relating droplet diameter and the ratio of the continuous and dispersed phase flow rates, we derive a power series equation, valid in the dripping regime, to predict droplet size, D(d) approximately equal 27(Q(C)/Q(D))(-5/12). The volatile droplets in this study are stable for weeks at room temperature yet undergo rapid liquid-to-gas phase transition, and volume expansion, above a uniform thermal activation threshold. The opportunity exists to potentiate locoregional cancer therapies such as thermal ablation and percutaneous ethanol injection using thermal or acoustic vaporization of these monodisperse phase-change droplets to intentionally occlude the vessels of a cancer.     

Rounded multi-level microchannels with orifices made in one exposure enable aqueous two-phase system droplet microfluidics

Exposure of a negative photoresist-coated glass slide with diffused light from the backside through a mask with disconnected features provides multi-level rounded channels with narrow orifices in one exposure. Using these structures, we construct microfluidic systems capable of creating aqueous two-phase system droplets where one aqueous phase forms droplets and the other aqueous phase forms the surrounding matrix. Unlike water-in-oil droplet systems, aqueous two-phase systems can have very low interfacial tensions that prevent spontaneous droplet formation. The multi-level channels fabricated by backside lithography satisfy two conflicting needs: (i) the requirement to have narrowed channels for efficient valve closure by channel deformation and (ii) the need to have wide channels to reduce the flow velocity, thus reducing the capillary number and enhancing droplet formation.     

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.     

Selective droplet coalescence using microfluidic systems

We report a microfluidic approach, which allows selective and controlled 1 : 1, 2 : 1 or 3 : 1 droplet fusion. A surfactant-stabilized droplet with an interfacial surfactant coverage, Γ, of >98% will fuse spontaneously with a second droplet when Γ of the latter droplet is <16%. However, when Γ of the second droplet is ~66%, the two droplets will not fuse, unless they have previously been brought into contact for critical time τ. Therefore, controlling the number of droplets in contact for time τ allows precise control over the number of fused droplets. We have demonstrated efficient (proportion of droplets coalesced p(c) = 1.0, n > 1000) and selective 1 : 1, 2 : 1 or 3 : 1 droplet fusion (proportion of correctly fused droplets p(s) > 0.99, n > 1000). Coalescence in this regime is induced by hydrodynamic flow causing interface separation and is efficient at different Ca numbers and using different dispersed phases, continuous phases and surfactants. However, when Γ of the second droplet is ~96% coalescence is no longer observed. Droplet-based microfluidic systems, in which each droplet functions as an independent microreactor, are proving a promising tool for a wide range of ultrahigh-throughput applications in biology and chemistry. The addition of new reagents to pre-formed droplets is critical to many of these applications and we believe the system described here is a simple and flexible method to do so, as well as a new tool to study interfacial stability phenomena.     

Dynamic contact angles and hysteresis under electrowetting-on-dielectric

By designing and implementing a new experimental method, we have measured the dynamic advancing and receding contact angles and the resulting hysteresis of droplets under electrowetting-on-dielectric (EWOD). Measurements were obtained over wide ranges of applied EWOD voltages, or electrowetting numbers (0 ≤ Ew ≤ 0.9), and droplet sliding speeds, or capillary numbers (1.4 × 10(-5) ≤ Ca ≤ 6.9 × 10(-3)). If Ew or Ca is low, dynamic contact angle hysteresis is not affected much by the EWOD voltage or the sliding speed; that is, the hysteresis increases by less than 50% with a 2 order-of-magnitude increase in sliding speed when Ca < 10(-3). If both Ew and Ca are high, however, the hysteresis increases with either the EWOD voltage or the sliding speed. Stick-slip oscillations were observed at Ew > 0.4. Data are interpreted with simplified hydrodynamic (Cox-Voinov) and molecular-kinetic theory (MKT) models; the Cox-Voinov model captures the trend of the data, but it yields unreasonable fitting parameters. MKT fitting parameters associated with the advancing contact line are reasonable, but a lack of symmetry indicates that a more intricate model is required.     

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.     

High-volume production of single and compound emulsions in a microfluidic parallelization arrangement coupled with coaxial annular world-to-chip interfaces

This study describes a microfluidic platform with coaxial annular world-to-chip interfaces for high-throughput production of single and compound emulsion droplets, having controlled sizes and internal compositions. The production module consists of two distinct elements: a planar square chip on which many copies of a microfluidic droplet generator (MFDG) are arranged circularly, and a cubic supporting module with coaxial annular channels for supplying fluids evenly to the inlets of the mounted chip, assembled from blocks with cylinders and holes. Three-dimensional flow was simulated to evaluate the distribution of flow velocity in the coaxial multiple annular channels. By coupling a 1.5 cm × 1.5 cm microfluidic chip with parallelized 144 MFDGs and a supporting module with two annular channels, for example, we could produce simple oil-in-water (O/W) emulsion droplets having a mean diameter of 90.7 μm and a coefficient of variation (CV) of 2.2% at a throughput of 180.0 mL h(-1). Furthermore, we successfully demonstrated high-throughput production of Janus droplets, double emulsions and triple emulsions, by coupling 1.5 cm × 1.5 cm - 4.5 cm × 4.5 cm microfluidic chips with parallelized 32-128 MFDGs of various geometries and supporting modules with 3-4 annular channels.     

Robust liquid crystal droplets

The preparation and properties of cyanobiphenyl liquid crystal droplets encapsulated by the polymerizable lecithin 1,2-bis(10,12-tricosadiynoyl)-sn-glyero-3-phosphocholine (DC_8,9PC) are described. Under a wide variety of preparation conditions the droplets obtain a diameter of approximately 10 mum. These droplets are stable for periods of over one year at room temperature. Furthermore, they are stable upon temperature cycling between the nematic and isotropic phases and between the smectic A to nematic to isotropic phase transitions.