Cascade reaction networks within audible sound induced transient domains in a solution.

Spatiotemporal control of chemical cascade reactions within compartmentalized domains is one of the difficult challenges to achieve. To implement such control, scientists have been working on the development of various artificial compartmentalized systems such as liposomes, vesicles, polymersomes, etc. Although a considerable amount of progress has been made in this direction, one still needs to develop alternative strategies for controlling cascade reaction networks within spatiotemporally controlled domains in a solution, which remains a non-trivial issue. Herein, we present the utilization of audible sound induced liquid vibrations for the generation of transient domains in an aqueous medium, which can be used for the control of cascade chemical reactions in a spatiotemporal fashion. This approach gives us access to highly reproducible spatiotemporal chemical gradients and patterns, in situ growth and aggregation of gold nanoparticles at predetermined locations or domains formed in a solution. Our strategy also gives us access to nanoparticle patterned hydrogels and their applications for region specific cell growth.

Compact reaction-module on a pad for scalable flow-production of organophosphates as drug scaffolds.

Continuous pharmaceutical manufacturing receives intense attention as an alternative way to meet flexible market needs with the assurance of higher safety and quality control. Here, we report a compact reaction-module on a pad (CRP, 170 × 170 × 1.2 mm) for scale-up production of drug precursors in a continuous-flow. The CRP system was devised by stacking 9 films of the patterned polyimide to integrate micro-flow circuits, combining the functions of the even distribution of feeds, being completely mixed in less than a few milliseconds. A methodology of using a highly reactive species for the single-step synthesis of α-phosphonyloxy ketone, a drug scaffold, required the synthesis time of a few seconds in microfluidics. The fast reaction in the single CRP was capable of producing 19.2 g h-1 drug precursor, which indicates a solid step toward kilogram-scale pharmaceutical manufacturing in small footage.

One-directional flow of ionic solutions along fine electrodes under an alternating current electric field.

Electric fields are widely used for controlling liquids in various research fields. To control a liquid, an alternating current (AC) electric field can offer unique advantages over a direct current (DC) electric field, such as fast and programmable flows and reduced side effects, namely the generation of gas bubbles. Here, we demonstrate one-directional flow along carbon nanotube nanowires under an AC electric field, with no additional equipment or frequency matching. This phenomenon has the following characteristics: First, the flow rates of the transported liquid were changed by altering the frequency showing Gaussian behaviour. Second, a particular frequency generated maximum liquid flow. Third, flow rates with an AC electric field (approximately nanolitre per minute) were much faster than those of a DC electric field (approximately picolitre per minute). Fourth, the flow rates could be controlled by changing the applied voltage, frequency, ion concentration of the solution and offset voltage. Our finding of microfluidic control using an AC electric field could provide a new method for controlling liquids in various research fields.

Specific ion effects on electrocapillarity in aqueous electrolytes confined within nanochannels.

A nanoslit is a long, extremely narrow (nanometers apart) opening between two parallel plates. An overlapped electric double layer is formed when an electrolyte is present inside the slit and there exist distributions of the osmotic pressure and the Maxwell stress across the nanoslit, which lead to the electrocapillarity effect. This feature can be incorporated with the specific ion effects by considering the nonelectrostatic interactions between ions and confining walls, as they significantly influence the potential, electric field, and ion distributions across the nanoslit. In the present work, the electromechanical approach is integrated with the concept of specific ion effects to analyze the behavior of an electrolyte confined in a one-dimensional nanochannel. For a nanochannel, the average outward normal stress exerted on the cross section of a channel (P_{zz}[over ¯]) can be regarded as a measure of electrocapillarity and it is the driving force of the flow. This electrocapillarity measure is analyzed by using the solution of the modified Poisson-Boltzmann equation as a function of the bulk concentration of the electrolyte, the boundary potential, and most importantly, the ion-specific interfacial interactions. The significance of the present work can be manifested by the increasing usage of extremely narrow channels in nanoscaled systems, which will require proper consideration of specific ion effects in determining the behavior of the confined electrolyte.

Seismic phononic crystals by elastodynamic Navier equation.

The phenomenon, known as a complete band gap in photonic crystals consisting of periodically arranged manmade nanostructures, caused a huge sensation in photonics. Inspired by the physical methodology, we extend it to large-scale wave propagation for seismic waves. In particular, we exploit the elastodynamic Navier equation in the medium for seismic phononic crystals to induce complete band gaps of body (P) and shear (S) waves. We also show a technique that uses weak formulation to analyze band structures. Estimation of evanescent modes by complex-valued wave vectors yields propagation length, then we redesign bulky phononic crystals to be as thin as possible. We also investigate how material properties affect the relation between propagation length and effective particle velocity. This study will contribute to seismic-resistant techniques in seismology.

A continuous droplet electroporation system for high throughput processing.

A continuous droplet electroporation (EP) system capable of handling a billion cells has been proposed and demonstrated using a proof-of-concept prototype design. Numerical simulations were conducted to design the new system and to compare the continuous droplet EP system with the previous single droplet EP system. Through parametric studies on the applied voltage and flow rate, a much higher cyan fluorescent protein transgene expression efficiency (38.8 ± 8.9%) was accomplished over that of the previous single droplet EP system. A parallel continuous droplet EP system is also demonstrated by introducing additional electrode pairs into the continuous droplet EP system to achieve ultrahigh throughput. Finally, the significance and meaning of the present work and future development direction have been discussed.

Effect of ion migration in electro-generated chemiluminescence depending on the luminophore types and operating conditions.

Electro-generated chemiluminescence (ECL) has attracted increasing attention as a new platform for light-emitting devices; in particular, the use of mechanically stretchable ECL gels opens up the opportunity to achieve deformable displays. The movements of radical ions under an external electric field include short-range diffusion near the electrodes and long-distance migration between the electrodes. So far, only the diffusion of radical ions has been considered as the operating principle behind ECL. In this study, electrochemical and optical analysis was performed systematically to investigate the role of ion migration in ECL devices. This study reveals that long-distance migration of radical ions can be a key variable in ECL at low frequencies and that this effect depends on the type of ion species and the operating conditions (e.g. voltage and frequency). We also report that the emissions from the two electrodes are not identical, and the emission behaviors are different in the optimal operating conditions for the red, green, and blue ECL emissions.

Geometric effects on electrocapillarity in nanochannels with an overlapped electric double layer.

Unsteady filling of electrolyte solution inside a nanochannel by the electrocapillarity effect is studied. The filling rate is predicted as a function of the bulk concentration of the electrolyte, the surface potential (or surface charge density), and the cross sectional shape of the channel. For a nanochannel, the average outward normal stress exerted on the cross section of a channel (P[over ¯]_{zz}^{}) can be regarded as a measure of electrocapillarity and it is the driving force of the flow. This electrocapillarity measure is first analyzed by using the solution of the Poisson-Boltzmann equation. From the analysis, it is found that the results for many different cross sectional shapes can be unified with good accuracy if the hydraulic radius is adopted as the characteristic length scale of the problem. Especially in the case of constant surface potential, for both limits of κh→0 and κh→∞, it can be shown theoretically that the electrocapillarity is independent of the cross sectional shape if the hydraulic radius is the same. In order to analyze the geometric effects more systematically, we consider the regular N-polygons with the same hydraulic radius and the rectangles of different aspect ratios. Washburn's approach is then adopted to predict the filling rate of electrolyte solution inside a nanochannel. It is found that the average filling velocity decreases as N increases in the case of regular N-polygons with the same hydraulic radius. This is because the regular N-polygons of the same hydraulic radius share the same inscribing circle.

Electrically Controllable Microparticle Synthesis and Digital Microfluidic Manipulation by Electric-Field-Induced Droplet Dispensing into Immiscible Fluids.

The dispensing of tiny droplets is a basic and crucial process in a myriad of applications, such as DNA/protein microarray, cell cultures, chemical synthesis of microparticles, and digital microfluidics. This work systematically demonstrates droplet dispensing into immiscible fluids through electric charge concentration (ECC) method. It exhibits three main modes (i.e., attaching, uniform, and bursting modes) as a function of flow rates, applied voltages, and gap distances between the nozzle and the oil surface. Through a conventional nozzle with diameter of a few millimeters, charged droplets with volumes ranging from a few µL to a few tens of nL can be uniformly dispensed into the oil chamber without reduction in nozzle size. Based on the features of the proposed method (e.g., formation of droplets with controllable polarity and amount of electric charge in water and oil system), a simple and straightforward method is developed for microparticle synthesis, including preparation of colloidosomes and fabrication of Janus microparticles with anisotropic internal structures. Finally, a combined system consisting of ECC-induced droplet dispensing and electrophoresis of charged droplet (ECD)-driven manipulation systems is constructed. This integrated platform will provide increased utility and flexibility in microfluidic applications because a charged droplet can be delivered toward the intended position by programmable electric control.

Colorimetric detection of pathogenic bacteria using platinum-coated magnetic nanoparticle clusters and magnetophoretic chromatography.

A colorimetric method that uses platinum-coated magnetic nanoparticle clusters (Pt/MNCs) and magnetophoretic chromatography is developed to detect pathogenic bacteria. Half-fragments of monoclonal Escherichia coli O157:H7 (EC) antibodies were functionalized to Pt/MNCs and used to capture E. coli bacteria in milk. After magnetic separation of free Pt/MNCs and Pt/MNC-EC complexes from the milk, a precision pipette was used to imbibe the E. coli-containing solution, then a viscous polyethylene glycol solution. Due to difference in viscosities, the solutions separate into two liquid layers inside the pipette tip. The Pt/MNC-EC complexes were separated from the free Pt/MNCs by applying an external magnetic field, then added to a tetramethylbenzidine (TMB) solution. Catalytic oxidation of TMB by Pt produced color changes of the solution, which enabled identification of the presence of 10 cfu mL(-1) E. coli bacteria with the naked eye. The total assay time including separation, binding and detection was 30 min.

Digital Microfluidic Approach for Efficient Electroporation with High Productivity: Transgene Expression of Microalgae without Cell Wall Removal.

A unique digital microfluidic electroporation (EP) system successfully demonstrates higher transgene expression than that of conventional techniques, in addition to reliable productivity and feasible integrated processes. By systematic investigations into the effects of the droplet EP conditions for a wild-type microalgae, 1 order of magnitude higher transgene expression is accomplished without cell wall removal over the conventional bulk EP system. In addition, the newly proposed droplet EP method by a droplet contact charging phenomena shows a great potential for the integration of EP processes and on-chip cell culture providing easy controllability of each process. Finally, the implications of the accomplishments and future directions for development of the proposed technology are discussed.

Characterization of electrode alignment for optimal droplet charging and actuation in droplet-based microfluidic system.

The actuation method using electric force as a driving force is utilized widely in droplet-based microfluidic systems. In this work, the effects of charging electrode alignment on direct charging of a droplet on electrified electrodes and a subsequent electrophoretic control of the droplet are investigated. The charging characteristics of a droplet according to different electrode alignments are quantitatively examined through experiments and systematic numerical simulations with varying distances and angles between the two electrodes. The droplet charge acquired from the electrified electrode is directly proportional to the distance and barely affected by the angle between the two electrodes. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of angle provides a great degree of freedom in designing such devices. Not only the droplet charge acquired from the electrode but also the force exerted on the droplet is analyzed. Finally, the implications and design guidance for microfluidic systems are discussed with an electrophoresis of a charged droplet method-based digital microfluidic device.

Osmotic pressure of ionic liquids in an electric double layer: Prediction based on a continuum model.

An analysis has been performed for the osmotic pressure of ionic liquids in the electric double layer (EDL). By using the electromechanical approach, we first derive a differential equation that is valid for computing the osmotic pressure in the continuum limit of any incompressible fluid in EDL. Then a specific model for ionic liquids proposed by Bazant et al. [M. Z. Bazant, B. D. Storey, and A. A. Kornyshev, Phys. Rev. Lett. 106, 046102 (2011)] is adopted for more detailed computation of the osmotic pressure. Ionic liquids are characterized by the correlation and the steric effects of ions and their effects are analyzed. In the low voltage cases, the correlation effect is dominant and the problem becomes linear. For this low voltage limit, a closed form formula is derived for predicting the osmotic pressure in EDL with no overlapping. It is found that the osmotic pressure decreases as the correlation effect increases. The osmotic pressures at the nanoslit surface and nanoslit centerline are also obtained for the low voltage limit. For the cases of moderately high voltage with high correlation factor, approximate formulas are derived for estimating osmotic pressure values based on the concept of a condensed layer near the electrode. In order to corroborate the results predicted by analytical studies, the full nonlinear model has been solved numerically.

Electrocapillarity of an electrolyte solution in a nanoslit with overlapped electric double layer: continuum approach.

A nanoslit is a long narrow opening between two parallel plates that are nanometers apart from each other. When an electrolyte solution is present inside a nanoslit, an overlapped electrical double layer (EDL) is formed and there exist distributions of the osmotic pressure and the Maxwell stress across the nanoslit. It is well known that the total normal stress (osmotic pressure contribution + Maxwell stress contribution) in the direction normal to the nanoslit surface is uniform and the value is the same as the osmotic pressure at the centerline. On the other hand, it is not well known that the total normal stress in the direction parallel to the slit surface is not uniform. When there is an electrolyte-gas interface inside a nanoslit, this total normal stress in the direction parallel to the slit surface generates the electrocapillarity effect. In the present work, the electromechanical approach is adopted to estimate the electrocapillarity effect in terms of the slit surface potential (or the surface charge density), the gap size, and the bulk ion concentrations. In order to handle the problem analyically, it is assumed that the nanoslit problem is in the continuum range and the interface is initially flat. The deformation of the interface due to the nonuniform total normal stress along the interface is also obtained by using the first order perturbation method. The significance of the present work can be manifested by the fact that external voltage is frequently used in nanoscaled systems and the electrocapillarity effect should be considered in addition to the intrinsic capillarity due to surface tension.

Electrowetting-induced droplet detachment from hydrophobic surfaces.

Detachment of droplets from solid surfaces is a basic and crucial process in practical applications such as heat transfer and digital microfluidics. In this study, electrowetting actuations with square pulse signals are employed to detach droplets from a hydrophobic surface. The threshold voltage for droplet detachment is obtained both experimentally and theoretically to find that it is almost constant for various droplet volumes ranging from 0.4 to 10 µL. It is also found that droplets can be detached more easily when the width of applied pulse is well-matched to the spreading time (i.e., the time to reach the maximum spread diameter). When the droplet is actuated by a double square pulse, the threshold voltage is reduced by ∼20% from that for a single square pulse actuation. Finally, by introducing an interdigitated electrode system, it is demonstrated that droplets can be detached from the solid bottom surface without using a top needle electrode.

Interfacial tension of complex coacervated mussel adhesive protein according to the Hofmeister series.

Complex coacervation is a liquid-liquid phase separation in a colloidal system of two oppositely charged polyelectrolytes or colloids. The interfacial tension of the coacervate phase is the key parameter for micelle formation and interactions with the encapsulating material. However, the relationship between interfacial tensions and various salt solutions is poorly understood in complex coacervation. In the present work, the complex coacervate dynamics of recombinant mussel adhesive protein (MAP) with hyaluronic acid (HA) were determined in the presence of Hofmeister series salt ions. Using measurements of absorbance, hydrodynamic diameter, capillary force, and receding contact angle in the bulk phase, the interfacial tensions of complex coacervated MAP/HA were determined to be 0.236, 0.256, and 0.287 mN/m in 250 mM NaHCOO, NaCl, and NaNO3 solutions, respectively. The sequences of interfacial tensions and contact angles of the complex coacervates in the presence of three sodium salts with different anions were found to follow the Hofmeister ordering. The tendency of interfacial tension between the coacervate and dilute phases in the presence of different types of Hofmeister salt ions could provide a better understanding of Hofmeister effects on complex coacervated materials based on the protein-polysaccharide system. This information can also be utilized for microencapsulation and adsorption by controlling intramolecular interactions. In addition, the injection molding dynamics of mussel byssus formation was potentially explained based on the measured interfacial tension of coacervated MAP.

Extraction of Cations from an Ionic Liquid Droplet in a Dielectric Liquid under Electric Field.

Ionic liquids show great promise as excellent solvents or catalysts in energy and biological fields due to their unique chemical and physical properties. In this work, the characteristics of various ionic liquids are investigated with the electrophoresis of a charged droplet (ECD) method. Under normal situation, a charged droplet in a dielectric liquid shows back-and-forth bouncing motion between the positive and negative electrodes continuously. However, for some special ionic liquids, interesting retreating behavior of a charged droplet has been observed. This retreating behavior is due to the loss of positive charges of the droplet, and it suggests that only the positive ions are extracted from the droplet under the applied electric field. Based on this hypothesis of ion extraction, Fourier transform infrared (FTIR) spectroscopy analysis has been performed. The retreating behavior is also discussed from the intermolecular point of view according to the ion species.

Geometric characterization of optimal electrode designs for improved droplet charging and actuation.

Droplet charging characteristics depending on the geometry of charging electrodes have been investigated experimentally and numerically. In the experiments, two contrasting electrode systems are examined: pin-pin versus planar-planar types. To confirm the different charging behaviours on each electrode, an asymmetric system of a pin-planar type has also been examined. From the experimental and numerical results, it has been found that the droplet charge can be significantly increased (more than four times) with pin type electrodes compared with planar ones due to the increase in surface charge density by the intensification of the electric field around the charging electrode. Moreover, as the system scale becomes smaller, the superior charging effect becomes greater. Through comprehensive numerical studies on the effects of the cross-sectional area and length of a charging electrode, we have found the optimal geometric design of an electrode for droplet charging and actuation. The implications for basic understanding of the charging phenomenon and electrode design of microfluidic systems are discussed.

Spontaneous electrical charging of droplets by conventional pipetting.

We report that a droplet dispensed from a micropipette almost always has a considerable electrical charge of a magnitude dependent on the constituents of the droplet, on atmospheric humidity and on the coating material of pipette tip. We show that this natural electrification of a droplet originates from the charge separation between a droplet and pipette tip surface by contact with water due to the ionization of surface chemical groups. Charge on a droplet can make it difficult to detach the droplet from the pipette tip, can decrease its surface tension, can affect the chemical characteristics of solutions due to interactions with charged molecules, and can influence the combination and localization of charged bio-molecules; in all cases, the charge may affect results of experiments in which any of these factors is important. Thus, these findings reveal experimental parameters that should be controlled in experiments that use micropipettes.

Effects of drop size and viscosity on spreading dynamics in DC electrowetting.

This study investigates the effects of drop size and viscosity on spreading dynamics, including response time, maximum velocity, and spreading pattern transition, in response to various DC voltages, based on both experiment and theoretical modeling. It is experimentally found that both switching time (i.e., time to reach maximum wetted radius) and settling time (i.e., time to reach equilibrium radius) are proportional to 1.5th power of the effective base radius. It is also found that the maximum velocity is slightly dependent on drop size but linearly proportional to the electrowetting number. The viscosity effect on drop spreading is investigated by observing spreading patterns with respect to applied voltages, and the critical viscosity at which a spreading pattern changes from under- to overdamped response is obtained. Theoretical models with contact angle hysteresis predict the spreading dynamics of drops with low and high viscosities fairly well. By fitting the theoretical models to experimental results, we obtain the friction coefficient, which is nearly proportional to 0.6th power of viscosity and is rarely influenced by applied voltage and drop size. Finally, we find that drop viscosity has a weak effect on maximum velocity but not a clear one on contact line friction.