Novel Latent Heat Storage Systems Based on Liquid Metal Matrices with Suspended Phase Change Material Microparticles.

Phase change materials (PCMs) are considered useful tools for efficient thermal management and thermal energy utilization in various application fields. In this study, a colloidal PCM-in-liquid metal (LM) system is demonstrated as a novel platform composite with excellent latent heat storage capability, high thermal and electrical conductivities, and unique viscoelastic properties. In the proposed formulation, eutectic Ga-In is utilized as a high-thermal-conductivity and high-fluidity liquid matrix in which paraffinic PCM microparticles with various solid-liquid phase transition temperatures are suspended as fillers. Good compatibility between the fillers and matrix is achieved by the nanosized inorganic oxides (titania) adsorbed at the filler-matrix interface; thus, the composite is produced via simple vortex mixing without tedious pre- or post-processing. The composite shows unique trade-off effects among various properties, i.e., elastic modulus, yield stress, density, thermal conductivity, and melting or crystallization enthalpy, which can be easily controlled by varying the contents of the suspended fillers. A Joule heating device incorporating the composite exhibits improved electrothermal performance owing to the synergy between the high electrical conductivity and latent heat storage capability of the composite. The proposed platform may be exploited for the rational design and facile manufacture of high-performance form-factor-free latent heat storage systems for various potential applications such as battery thermal management and flexible heaters.

Electrostatic Stabilization of Nano Liquid Metals in Doped Nonpolar Liquids.

Liquid metals and alloys are attracting renewed attention owing to their potential for application in various advanced technologies. Eutectic gallium-indium (EGaIn) has been focused on in particular because of its integrated advantages of high conductivity, low melting point, and low toxicity. In this study, the colloidal behavior of nano-dispersed EGaIn in nonpolar oils is investigated. Although the nonpolar oil continuous phase is commonly considered to be free of electric charges, electrostatic repulsion appears to be crucial in the colloidal stabilization of the nano-dispersed EGaIn phases, the modulation of which is possible by doping the oil phases with different types of oil-soluble surfactants. The qualitative correlation between the observed colloidal stabilities and the "zero field" particle mobilities inferred from the field-dependent electrophoretic mobilities indicates that the electric charging of EGaIn particles in surfactant-doped nonpolar oils is a static phenomenon that is maintained in equilibrium, rather than a solely field-induced process. A systematic investigation of the charging properties of these unique biphasic particles, consisting of the liquid Ga-In bulk and the solid Ga2 O3 surface that formed spontaneously, reveals the complicated system-dependent nature of the charging mechanisms mediated by ionic and nonionic surfactants in nonpolar media.

Switchable liquid shutter operated by electrowetting for security of mobile electronics.

This paper presents a new type of switchable liquid shutter for the security and design of mobile electronic devices. The operation test of the liquid shutter is conducted using a prototype sample prepared by standard microfabrication processes. The liquid shutter consists of an opaque liquid for absorbing light and a transparent oil for transmitting light on two parallel plates with patterned indium titanium oxide electrodes. The liquid shutter can be opened and closed by sequentially applying an electrical voltage to the patterned electrodes owing to an electrowetting principle. The switching time of the liquid shutter is measured using a high-speed camera and is found to take about 550 ms to open the shutter and 240 ms to close the shutter at 70 Vrms (1 kHz). To validate the applicability of the liquid shutter, the operation of liquid shutters with different colored liquids mounted on a smartphone is successfully demonstrated. The proposed liquid shutter not only allows a simple design to be easily miniaturized and integrated with electronic devices but also provides a robust and fast switching operation.

Optothermally pulsating microbubble-mediated micro-energy harvesting in underwater medium.

Despite the considerable research interest due to practical importance of pervasive wireless sensing systems in a wide range of engineering fields, power management remains an arduous task for further development of pervasive wireless sensing systems due to inherent needs for self-reliant functionality and portability during their operations. To this end, we here propose a new type of energy harvesting strategy in which an optothermally pulsating microbubble is submerged in an underwater medium. The pulsating microbubble gives rise to the periodic vibration of piezocantilevers in contact, which resultantly can produce electrical outputs. On the basis of this simple idea, mechanical power can be extracted from light energy through optothermally pulsating microbubbles in an aqueous medium and subsequently the mechanical power can be converted to electrical power for wireless devices. To elucidate physical factors affecting the performance of the proposed strategy, we thoroughly explore the effect of the intensity and frequency of the laser beam on the pulsation amplitude of optothermally pulsating bubbles and subsequent electrical outputs (e.g., electrical voltage and power). The dependence of electrical output on wetting property of piezocantilevers and electrical resistance is also established. The present work would provide a new framework for fundamental design of bubble-based microactuators as energy harvesters and microsensors in the near future.

Electromagnetic three dimensional liquid metal manipulation.

In this paper, we report three-dimensional (3-D) liquid metal manipulation using electromagnets, which can be applied to electrical switching applications. The liquid metal droplet was coated with iron (Fe) particles by chemical reaction with hydrochloric acid (HCl), and thus it became responsive to the magnetic field, becoming a magnetic liquid metal marble. Using electromagnets, the magnetic field was turned on and off on-demand. We investigated an average velocity and the maximum working distance of the horizontal and vertical electromagnetic field-driven manipulation of the magnetic liquid metal marble. Linear (1-D) and plane (2-D) manipulation of the marble was successfully demonstrated and 3-D manipulation was verified for electrical switching.

C. elegans-on-a-chip for in situ and in vivo Ag nanoparticles' uptake and toxicity assay.

Nanomaterials are extensively used in consumer products and medical applications, but little is known about their environmental and biological toxicities. Moreover, the toxicity analysis requires sophisticated instruments and labor-intensive experiments. Here we report a microfluidic chip incorporated with the nematode Caenorhabditis elegans that rapidly displays the changes in body growth and gene expression specifically responsive to the silver nanoparticles (AgNPs). C. elegans were cultured in microfluidic chambers in the presence or absence of AgNPs and were consequently transferred to wedge-shaped channels, which immobilized the animals, allowing the evaluation of parameters such as length, moving distance, and fluorescence from the reporter gene. The AgNPs reduced the length of C. elegans body, which was easily identified in the channel of chip. In addition, the decrease of body width enabled the worm to advance the longer distance compared to the animal without nanoparticles in a wedge-shaped channel. The transgenic marker DNA, mtl-2::gfp was highly expressed upon the uptake of AgNPs, resulting in green fluorescence emission. The comparative investigation using gold nanoparticles and heavy-metal ions indicated that these parameters are specific to AgNPs. These results demonstrate that C. elegans-on-a-chip has a great potential as a rapid and specific nanoparticle detection or nanotoxicity assessment system.

On-demand magnetic manipulation of liquid metal in microfluidic channels for electrical switching applications.

We report magnetic-field-driven on-demand manipulation of liquid metal in microfluidic channels filled with base or acid. The liquid metal was coated with iron (Fe) particles and treated with hydrochloric acid to have strong bonding strength with the Fe particles. The magnetic liquid metal slug inserted in the microchannel is manipulated, merged, and separated. In addition, corresponding to the repositioning of an external magnet, the liquid metal slug can be readily moved in microfluidic channels with different angles (>90°) and cross-linked channels in any direction. We demonstrated the functionality of the liquid metal in the microfluidic channel for electrical switching applications by manipulation of the liquid metal, resulting in the sequential turning on of light emitting diodes (LEDs).

Fast Electrically Driven Capillary Rise Using Overdrive Voltage.

Enhancement of response speed (or reduction of response time) is crucial for the commercialization of devices based on electrowetting (EW), such as liquid lenses and reflective displays, and presents one of the main challenges in EW research studies. We demonstrate here that an overdrive EW actuation gives rise to a faster rise of a liquid column between parallel electrodes, compared to a DC EW actuation. Here, DC actuation is actually a simple applied step function, and overdrive is an applied step followed by reduction to a lower voltage. Transient behaviors and response time (i.e., the time required to reach the equilibrium height) of the rising liquid column are explored under different DC and overdrive EW actuations. When the liquid column rises up to a target height by means of an overdrive EW, the response time is reduced to as low as 1/6 of the response time using DC EW. We develop a theoretical model to simulate the EW-driven capillary rise by combining the kinetic equation of capillary flow (i.e., Lucas-Washburn equation) and the dynamic contact angle model considering contact line friction, contact angle hysteresis, contact angle saturation, and the EW effect. This theoretical model accurately predicts the outcome to within a ± 5% error in regard to the rising behaviors of the liquid column with a low viscosity, under both DC EW and overdrive actuation conditions, except for the early stage (

Streaming flow from ultrasound contrast agents by acoustic waves in a blood vessel model.

To elucidate the effects of streaming flow on ultrasound contrast agent (UCA)-assisted drug delivery, streaming velocity fields from sonicated UCA microbubbles were measured using particle image velocimetry (PIV) in a blood vessel model. At the beginning of ultrasound sonication, the UCA bubbles formed clusters and translated in the direction of the ultrasound field. Bubble cluster formation and translation were faster with 2.25MHz sonication, a frequency close to the resonance frequency of the UCA. Translation of bubble clusters induced streaming jet flow that impinged on the vessel wall, forming symmetric vortices. The maximum streaming velocity was about 60mm/s at 2.25MHz and decreased to 15mm/s at 1.0MHz for the same acoustic pressure amplitude. The effect of the ultrasound frequency on wall shear stress was more noticeable. Maximum wall shear stress decreased from 0.84 to 0.1Pa as the ultrasound frequency decreased from 2.25 to 1.0MHz. The maximum spatial gradient of the wall shear stress also decreased from 1.0 to 0.1Pa/mm. This study showed that streaming flow was induced by bubble cluster formation and translation and was stronger upon sonication by an acoustic wave with a frequency near the UCA resonance frequency. Therefore, the secondary radiant force, which is much stronger at the resonance frequency, should play an important role in UCA-assisted drug delivery.

Drug perfusion enhancement in tissue model by steady streaming induced by oscillating microbubbles.

Drug delivery into neurological tissue is challenging because of the low tissue permeability. Ultrasound incorporating microbubbles has been applied to enhance drug delivery into these tissues, but the effects of a streaming flow by microbubble oscillation on drug perfusion have not been elucidated. In order to clarify the physical effects of steady streaming on drug delivery, an experimental study on dye perfusion into a tissue model was performed using microbubbles excited by acoustic waves. The surface concentration and penetration length of the drug were increased by 12% and 13%, respectively, with streaming flow. The mass of dye perfused into a tissue phantom for 30s was increased by about 20% in the phantom with oscillating bubbles. A computational model that considers fluid structure interaction for streaming flow fields induced by oscillating bubbles was developed, and mass transfer of the drug into the porous tissue model was analyzed. The computed flow fields agreed with the theoretical solutions, and the dye concentration distribution in the tissue agreed well with the experimental data. The computational results showed that steady streaming with a streaming velocity of a few millimeters per second promotes mass transfer into a tissue.