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Abstract This study investigates a novel microfluidic mixing technique that uses the resonant oscillation of coalescent droplets. During the vertical contact-separation process, solutes are initially separated as a result of the combined effects of diffusion and gravity. We show that the application of alternating current (AC) voltage to microelectrodes below the droplets causes a resonant oscillation, which enhances the even distribution of the solute. The difference in concentration between the top and bottom droplets exhibits frequency dependence and indicates the existence of a particular AC frequency that results in a homogeneous concentration. This frequency corresponds to the resonance frequency of the droplet oscillation that is determined using particle tracking velocimetry. To understand the mixing process, a phenomenological model based on the equilibrium between surface tension, viscosity, and electrostatic force was developed. This model accurately predicted the resonance frequency of droplet flow and was consistent with the experimental results. These results suggest that the resonant oscillation of droplets driven by AC voltage significantly enhances the diffusion of solutes, which is an effective approach to microfluid mixing.

Abstract Implementing a signal-switching mechanism for the selective use of integrated sensors and actuators plays a crucial role in streamlining the functionality of miniaturized devices. Here, a liquid metal droplet (LMD)-based signal-switching mechanism is introduced to achieve such functionality. Pressure modulation with a 100-μm spatial resolution enabled precise control of the position of the LMDs within a channel. After integrating the channel with asymmetrically structured electrodes, the effect of the shuttle-like movement of LMD on the temporal changes in the overall capacitance was investigated. Consequently, analysis of the capacitive peaks revealed the directional movement of the LMDs, enabling estimation of the position of the LMDs without direct observation. In addition, we achieved successful signal extraction from the capacitive sensor that was linked to the activated electrodes, thereby enabling selective data retrieval. The proposed signal-switching mechanism method achieved a detection accuracy of ~0.1 pF. The sensor’s ability to simultaneously detect the LMD position and generate a signal underscores its significant potential for multiplexing in multisensing systems, particularly in concealed environments, such as in vivo settings.

Abstract In vertical contact control (VCC), a microdroplet array selectively contacts with an opposite microdroplet array. Generally, VCC is useful for the dispenser mechanism based on solute diffusion between microdroplet pairs. However, sedimentation due to gravity can cause an inhomogeneous distribution of solutes in microdroplets. Therefore, it is necessary to enhance solute diffusion to achieve the accurate dispensing of a large quantity of solute in the direction opposite to that of gravity. Herein, we applied a rotational magnetic field to the microrotors in microdroplets to enhance the solute diffusion in microdroplets. Driven by microrotors, the rotational flow can generate a homogeneous distribution of solutes in microdroplets. We analyzed the diffusion dynamics of solutes using a phenomenological model, and the results showed that the rotation of microrotors can increase the diffusion constant of solutes.

Abstract Transportation of magnetized particles (MPs) against gravity is possible by applying a magnetic field to the particles. This transport phenomenon of MPs in microdroplets can be quantitatively assessed by determining the contribution of individual forces acting on the MPs. We studied the selective transportation of MPs in microdroplets. MPs in microdroplets were transported in the opposite direction to gravity when we applied an external magnetic field larger than a threshold value. We modulated the intensity of the external magnetic field and selectively manipulated the MPs. As a result, MPs were separated into different microdroplets based on their magnetic properties. Our quantitative investigation of transport dynamics shows that the threshold magnetic field depends only on the magnetic susceptibility and the density of MPs. This is a universal criterion for the selective transport of magnetized targets such as magnetized cells in microdroplets.

Abstract The spatial resolution of typical sensor probes is sufficient for measuring the average electric properties of microelectrical devices, but they are unable to measure the distribution with a spatial precision. Liquid crystal droplets (LCDs) are promising candidate for visualizing the distribution. When voltage is applied, the LCDs show rotational and translational behaviors which depend on the location of LCDs within the devices. We demonstrate that by comparing the experimental and numerical results, the electric field and electrostatic energy distribution are visualized by rotating and transporting LCDs, with a spatial resolution of 10 µm and a detection accuracy of 5 µV/µm. In addition, we produced an array of LCDs by designing periodic modulation of the electrostatic energy density in the model device. These findings show that the LCDs serve as a periodic modulator of the refractive index as well as a sensor for the observation of electric properties of microelectrical devices.

Sinusoidal electric fields are applied to liquid crystals (LCs) across various frequencies ranging from 0.1 Hz to 10 kHz to investigate the oscillatory behavior of LC directors. In a 1.5 μ m thick 5CB cell, a significant decline in refractive index change occurs in the frequency range greater than 10 Hz, in which the LC director cannot sufficiently follow the rapid fluctuations in field intensity. To evaluate the response speed of the LCs under sinusoidal electric fields, the cutoff frequency is introduced as a response indicator. Enhancement of the cutoff frequency can be achieved through conventional fast response techniques such as polymer-stabilized LCs.

Abstract We proposed the detection mechanism of the micro-levels of temperature gradient in a micro-electromechanical system using the unidirectional rotation of cholesteric-liquid crystal (Ch-LC) droplets. Ch-LC droplets in the presence of an isotropic phase subjected to a heat flux rotate with a speed proportional to the magnitude of the temperature gradient. We further quantified the temperature gradient-to-torque conversion efficiency to apply the thermomechanical cross-correlation to the detection of temperature gradient. Then, we observed the rotational behavior of Ch-LC droplets after introducing them onto model devices containing patterned Au thin-film electrodes. Direct electric current applied to these Au electrodes results in unidirectional rotation of the Ch-LC droplets in response to heat flux generated from the Au electrodes. By evaluating the possible temperature gradient detection resolution using Ch-LC droplet rotation, we show that Ch-LC droplets can achieve both high spatial resolution (~ 10 µm) and high detection accuracy (~ 0.1 mK/µm).

Abstract Multiple sensors and actuators integrated in a small space, especially an elongated thin structure, require equivalent number of signal lines between microdevices, but there is limited space for signal wires. Thus, we propose a mechanism using a single microchannel where a liquid metal droplet moves and shuttles. A shuttling droplet switches multiple terminals of signal lines along a microchannel based on a traditional switching mechanism using a liquid metal droplet. Electrically conductive gallium alloy liquid metals (Galinstan) can flow in a microchannel due to their fluidity. The terminals consist of opposing electrode pairs in a microchannel. A change in a variable impedance connected to a terminal as a pseudo sensor can be read when a droplet flows in and connects electrode pairs. This paper presents switching and addressing objective terminals of chromium electrodes by a shuttling conductive droplet (500 µm in diameter and 10 mm long) in a microchannel (500 µm in diameter and 100 mm long). A demonstrated simple mechanism enables communication between multiple microdevices along a microchannel. We anticipate wide application of proposed mechanism toward a multiplexer, especially in microfluidic devices because of the advantages of utilizing microchannels as common microstructures for both microdevices and signal lines.

Abstract Gallium alloy liquid metals (Galinstan) possessing fluidity, electric conductivity, and low toxicity are attractive for use in flexible devices and microfluidic devices. However, the oxide skin of Galinstan in the atmosphere adheres to the microchannel surface, preventing the transportation of Galinstan in the channel. To tackle the problem of the adhesion of Galinstan to microchannel, we introduced liquid with Galinstan into a channel with a diameter of 1000 μm. Then, we found that the cylindrical shape of the channel enabled smooth transportation of Galinstan independently of both the liquid and the channel material. The liquid introduced with Galinstan not only prevents adhesion but also improves the spatial controllability of Galinstan in the channel. We can control the position of Galinstan with 100 μm resolution using highly viscous (> 10 cSt) liquid. In addition, we combined the microchannel with patterned electrodes, fabricating a serially arranged capacitive device. The local capacitance detected by the patterned electrodes changed by more than 6% via the smooth transportation of Galinstan. The analysis results based on an equivalent circuit quantitatively agree with our experimental results. We can modulate the serially arranged capacitors using the smooth transportation of Galinstan in the channel.

We investigated the contact-separation behavior of micro fluids, which is an efficient candidate for dispensing mechanisms that can replace pipetting in biochemical assays. During the Vertical contact-separation process (VCSP) of droplets, gravity causes a volume difference, ΔV. To solve this problem, we designed the radius difference of the solid-liquid interface, δR, and proposed to manipulate droplets against gravity. Systematic observations showed that the ΔV monotonically decreased with an increase in the δR, and the droplet volume was maintained (ΔV = 0) at a critical δR+. This behavior quantitatively correlates with a theoretical model based on the force balance between gravity and surface tension under asymmetric boundary conditions. Thus, after the VCSP, the ΔV was maintained at zero and the arbitrary volume of droplets was controlled using the δR. The results showed that the proposed mechanism can suppress the ΔV and quantitatively control the droplet volume. This study contributes to the three-dimensional and accurate manipulation of droplets. [ABSTRACT FROM AUTHOR]

We fabricate smectic-A liquid-crystalline emulsions (SmA-LCEs) in which monodisperse 8CB-swollen micelles are dispersed in water. Synchrotron X-ray measurements reveal that the correlation length of the layer order in LCEs cannot diverge at the nematic (N)-SmA transition, and the saturated length linearly increases with the increase in the diameter of LC-swollen micelles. Moreover, we investigate the permeation of 7CB N molecules into 8CB SmA-LCEs by measuring the enhancement of the depolarized light scattering intensity due to the shift of the SmA-N phase transition, and confirm that the layer order suppresses the mobility of LC molecules in LCEs similar to the bulk state of SmA LCs. The mobility drastically accelerates near the SmA-N transition temperature due to the continuous extinction of the smectic layer order. The slow permeation process of 7CB is also confirmed via synchrotron X-ray diffraction examining the shrinkage of the smectic layer repeat distance, caused by intercalation of 7CB. [ABSTRACT FROM AUTHOR]