Multiplexed electrospraying of water in cone-jet mode using a UV-embossed pyramidal micronozzle film.

The electrospraying of water in the cone-jet mode is difficult in practical applications owing to its low throughput and the electrical discharge caused by the high surface tension of water. A film with multiple dielectric micronozzles is essential for multiplexed electrospraying of water in cone-jet mode without electrical discharge. Thus, a pyramidal micronozzle film with five nozzles was fabricated using the UV-embossing process. The pyramidal micronozzle film consisted of pyramidal micronozzles, a micropillar array, and an in-plane extractor, which were proposed to minimize wetting and concentrate the electric field to the water meniscus at the tip of the pyramidal micronozzle. The electrospraying of water using a single pyramidal micronozzle was visualized by a high-speed camera at a flow rate of 0.15-0.50 ml/h with voltages of 0.0-2.3 kV, -1.6 kV, and -4.0 kV at the water, guide ring, and collector, respectively. Three distinct modes, the dripping, spindle, and cone-jet modes, were observed and distinguished according to the motion of the water meniscus at the nozzle tip. The steady Taylor cone and jet were observed in a voltage range of 1.3-2.0 kV in water, particularly in cone-jet mode. Multiplexed electrospraying of water in cone-jet mode at a flow rate of 1.5 ml/h was performed using a pyramidal micronozzle film, demonstrating the potential for a high-throughput electrospraying system.

Interfacial properties of liquid metal immersed in various liquids.

HYPOTHESIS: When gallium-based liquid metal (LM) droplets are injected through different solvent media, the oxygen solubility of the environment influences the droplet eccentricity. The formation of an oxide membrane in solvents can determine whether a bulk-scale droplet behaves in a liquidlike or solidlike manner. In the case of LM emulsions, the solvent's oxygen solubility leads to varying degrees of organic solvent adsorption. The adsorption of solvent molecules changes the surface energy of the oxide layer. EXPERIMENTS: The pinch-off frames of LM droplets immersed in liquids with differing oxygen solubility were captured using a high-speed camera. Through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), the surface composition of micro and nanoscale LM emulsions in different solvents was investigated. The van Oss-Good model was implemented to determine the polar and nonpolar surface energy components of LM layers with adsorbates. FINDINGS: Pear-shaped LM droplets displaying solidlike behavior are created when the mole fraction of dissolved oxygen in the ambient solution is above approximately 2.43×10-4. For LM emulsions sonicated in organic solvents, Carbon/Oxygen (C/O) and Carbon/Gallium (C/Ga) atomic percent ratios display an increasing trend with increasing oxygen solubility. The nonpolar component of surface energy shows a logarithmic relationship with the oxygen solubility of the solvent used to treat the LM layer. The polar component of surface energy is more susceptible to the chemical properties of the solvent.

Crystal capillary origami capsule with self-assembled nanostructures.

The self-assembling mechanism of elasto-capillaries opens new applications in micro and nanotechnology by providing 3D assembly structures with 2D planar unit cells, so-called capillary origami. To date, the final structure has been designed based on the predetermined shape and size of the unit cell. Here, we show that plate-like salt crystallites grow and cover the emulsion interface, which is driven by Laplace pressure. Eventually, it creates a spherical capsule with self-assembled nanostructures. The capsule and the crystallite are investigated by scanning electron microscopy and X-ray diffraction analysis. To explain the mechanism, we develop a theoretical model to estimate the capsule size, the shell thickness, and the conditions necessary to form the shell based on a thin-walled pressure vessel. The proposed crystal capillary origami can fabricate a three-dimensional self-assembled salt capsule without any complicated procedures. We believe that it can offer a new physicochemical avenue for the spontaneous and facile fabrication of water-soluble carrier particles.

Microfluidic flow switching via localized acoustic streaming controlled by surface acoustic waves.

We propose an acoustic flow switching device that utilizes high-frequency surface acoustic waves (SAWs) produced by a slanted-finger interdigitated transducer. As the acoustic field induced by the SAWs was attenuated in the fluid, it produced an acoustic streaming flow in the form of a pair of symmetrical microvortices, which induced flow switching between two fluid streams in a controlled manner. The microfluidic device was composed of a piezoelectric substrate attached to a polydimethylsiloxane (PDMS) microchannel having an H-shaped junction that connected two fluid streams in the middle. The two immiscible fluids, separated by the PDMS wall, flowed in parallel, briefly came in contact at the junction, and separated again into the downstream microchannels. The acoustic streaming flow induced by the SAWs rotated the fluid streams within the microchannel cross-section, thereby altering the respective positions of the two fluids and directing them into the opposite flow paths. The characteristics of the flow switching mechanism were investigated by tuning the input voltage and the flowrates. On-demand acoustic flow switching was successfully achieved without additional moving parts inside the microchannel. This technique may be useful for fundamental studies that integrate complex experimental platforms into a single chip.

On-demand acoustic droplet splitting and steering in a disposable microfluidic chip.

On-chip droplet splitting is one of the fundamental droplet-based microfluidic unit operations to control droplet volume after production and increase operational capability, flexibility, and throughput. Various droplet splitting methods have been proposed, and among them the acoustic droplet splitting method is promising because of its label-free operation without any physical or thermal damage to droplets. Previous acoustic droplet splitting methods faced several limitations: first, they employed a cross-type acoustofluidic device that precluded multichannel droplet splitting; second, they required irreversible bonding between a piezoelectric substrate and a microfluidic chip, such that the fluidic chip was not replaceable. Here, we present a parallel-type acoustofluidic device with a disposable microfluidic chip to address the limitations of previous acoustic droplet splitting devices. In the proposed device, an acoustic field is applied in the direction opposite to the flow direction to achieve multichannel droplet splitting and steering. A disposable polydimethylsiloxane microfluidic chip is employed in the developed device, thereby removing the need for permanent bonding and improving the flexibility of the droplet microfluidic device. We experimentally demonstrated on-demand acoustic droplet bi-splitting and steering with precise control over the droplet splitting ratio, and we investigated the underlying physical mechanisms of droplet splitting and steering based on Laplace pressure and ray acoustics analyses, respectively. We also demonstrated droplet tri-splitting to prove the feasibility of multichannel droplet splitting. The proposed on-demand acoustic droplet splitting device enables on-chip droplet volume control in various droplet-based microfluidic applications.

A Pumpless Acoustofluidic Platform for Size-Selective Concentration and Separation of Microparticles.

We have designed a pumpless acoustofluidic device for the concentration and separation of different sized particles inside a single-layered straight polydimethylsiloxane (PDMS) microfluidic channel. The proposed device comprises two parallel interdigitated transducers (IDTs) positioned underneath the PDMS microchannel. The IDTs produce high-frequency surface acoustic waves that generate semipermeable virtual acoustic radiation force field walls that selectively trap and concentrate larger particles at different locations inside the microchannel and allow the smaller particles to pass through the acoustic filter. The performance of the acoustofluidic device was first characterized by injecting into the microchannel a uniform flow of suspended 9.9 µm diameter particles with various initial concentrations (as low as 10 particles/mL) using a syringe pump. The particles were trapped with ∼100% efficiency by a single IDT actuated at 73 MHz. The acoustofluidic platform was used to demonstrate the pumpless separation of 12.0, 4.8, and 2.1 µm microparticles by trapping the 12 and 4.8 µm particles using the two IDTs actuated at 73 and 140 MHz, respectively. However, most of the 2.1 µm particles flowed over the IDTs unaffected. The acoustofluidic device was capable of rapidly processing a large volume of sample fluid pumped through the microchannel using an external syringe pump. A small volume of the sample fluid was processed through the device using a capillary flow and a hydrodynamic pressure difference that did not require an external pumping device.

Acoustic Wave-Driven Functionalized Particles for Aptamer-Based Target Biomolecule Separation.

We developed a hybrid microfluidic device that utilized acoustic waves to drive functionalized microparticles inside a continuous flow microchannel and to separate particle-conjugated target proteins from a complex fluid. The acoustofluidic device is composed of an interdigitated transducer that produces high-frequency surface acoustic waves (SAW) and a polydimethylsiloxane (PDMS) microfluidic channel. The SAW interacted with the sample fluid inside the microchannel and deflected particles from their original streamlines to achieve separation. Streptavidin-functionalized polystyrene (PS) microparticles were used to capture aptamer (single-stranded DNA) labeled at one end with a biotin molecule. The free end of the customized aptamer15 (apt15), which was attached to the microparticles via streptavidin-biotin linkage to form the PS-apt15 conjugate, was used to capture the model target protein, thrombin (th), by binding at exosite I to form the PS-apt15-th complex. We demonstrated that the PS-apt15 conjugate selectively captured thrombin molecules in a complex fluid. After the PS-apt15-th complex was formed, the sample fluid was pumped through a PDMS microchannel along with two buffer sheath flows that hydrodynamically focused the sample flow prior to SAW exposure for PS-apt15-th separation from the non-target proteins. We successfully separated thrombin from mCardinal2 and human serum using the proposed acoustofluidic device.

Acoustothermal tweezer for droplet sorting in a disposable microfluidic chip.

Precise control over droplet position within a microchannel is fundamental to droplet microfluidic applications. This article proposes acoustothermal tweezer for the control of droplet position, which is based on thermocapillary droplet migration actuated by acoustothermal heating. The proposed system comprises an acoustothermal heater, which is composed of a slanted finger interdigital transducer patterned on a piezoelectric substrate and a thin PDMS membrane, and a PDMS microchannel. In the proposed system, droplets moving in a droplet microfluidic chip experience spatiotemporally varying thermal stimuli produced by acoustothermal heating and thus migrate laterally. In comparison to previous methods for droplet sorting, the acoustothermal tweezer offers significant advantages: first, the droplet position can be manipulated in two opposite directions, which enables bidirectional droplet sorting to one of three outlets downstream; second, precise control over the droplet position as well as improved droplet lateral displacement on the order of hundreds of micrometers can be achieved in a deterministic manner, thereby enabling multichannel droplet sorting; third, the PDMS microfluidic chip is disposable and thus can be easily replaced since it is attached to the substrate by reversible bonding, which allows the acoustothermal heater to be reused. Given these advantages, the proposed droplet sorting system is a promising droplet microfluidic lab-on-a-chip platform for tunable, on-demand droplet position control.

On-Demand Droplet Capture and Release Using Microwell-Assisted Surface Acoustic Waves.

We demonstrate an acoustofluidic platform that uses surface acoustic waves (SAWs) for the facile capture of droplets inside microwells and their on-demand release. When the ac signal applied to the device is tuned to modulate the location of the SAW, the SAW-based acoustic radiation force retracts or pushes the droplets into or out of one of three microwells fabricated inside a microchannel to selectively capture or release the droplet.

In-droplet microparticle separation using travelling surface acoustic wave.

Droplets in microfluidic systems can contain microscale objects such as cells and microparticles. The control of the positions of microscale objects within a microchannel is crucial for practical applications in not only continuous-flow-based but also droplet-based systems. This paper proposes an active method for the separation of microparticles inside moving droplets which uses travelling surface acoustic waves (TSAWs). We demonstrate the preconcentration and separation of 5 and 10 µm polystyrene microparticles in moving water-in-oil droplets through the application of TSAWs with two different frequencies. The microparticles inside the droplets are affected by the acoustic radiation force induced by the TSAWs to move laterally in the direction of the TSAW propagation and are thereby separated according to their size. In-droplet separation is then demonstrated through droplet splitting at a Y-junction. Compared to our previous studies, this acoustic approach offers the label-free and on-demand separation of different-sized micro-objects in moving droplets. The present method has potential uses such as in-droplet sample purification and enrichment.