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Liquid metals such as gallium alloys have a unique potential to enable fully reconfigurable RF electronics. One of the major drawbacks for liquid-metal electronics is their interaction with solid-metal contacts, which results in unwanted changes to the electrical performance and delamination of solid-metal contacts due to atomic diffusion of gallium at the liquid–solid interface. In this article, we present a solution to this problem, namely, Laplace barriers, which demonstrate reversible liquid-metal-to-liquid-metal RF connections that provide pressure-directed control of the fluids. While the effect of these channel designs provide superior control over the fluid’s position, their effect on RF transmission has not been explored, and the inherent trade space between the microfluidic design and RF performance has never been detailed. RF tunability is demonstrated by fabricating, testing, and modeling a reconfigurable RF microstrip transmission line with integrated the Laplace barriers which operates between 0.5 and 5 GHz. The effect of the Laplace barrier length on both fluidic control and RF performance is reported and compared with the extreme example of a tapered line, which is analogous to a single elongated gap between two Laplace barriers. This approach opens the potential for future all-liquid reconfigurable RF electronic circuits where physical connections between solid and liquid metals are minimized or possibly eliminated.

A tunable parallel-plate capacitor is demonstrated using continuous electrowetting actuation of liquid metal. A 1-mm gap is made in one conductive plate of the capacitor, and the manipulation of conductive materials changes the effective area of the plate. The addition of an electrolyte to fill the gap yields a 5.43-pF difference, while further fine-tuning using the actuation of liquid metal yields a 0.58-pF change in capacitance. The tunable capacitor can be varied at 1 GHz from 2.90 pF to 10.34 pF, resulting in a 1:3.57 change in capacitance.

Room-temperature liquid metals such as eutectic gallium-indium (EGaIn) alloys have the potential to realize physically reconfigurable microwave components and circuits. Integrating microfluidic precision control of these liquid conductors with standard microwave board circuits enables a new paradigm of tunability for microwave components. One area that would greatly benefit from passive component agility is impedance-matching networks, where fine tuning the re-actance is essential for reconfigurability. We demonstrate a microfluidic-integrated microwave inductor device that employs a high-impedance microstrip trace with three switchable taps to provide inductances of 1.2 nH, 1.9 nH and 2.3 nH at 5 GHz. Precise volumes of liquid metal are dispensed and confined to bridge existing gaps between copper microstrip traces to reconfigure the total length of the narrow trace, providing an effective method to tune the component’s inductance.

Pixelated liquid-metal (LM) arrays have shown promise in realizing versatile, reconflgurable RF components. However, unused LM pixels introduce parasitic effects. Here we demonstrate comparative designs of a pixelated nodal LM array combining electrocapillary and continuous electrowetting actuation methods. LM is actuated both two dimensionally and three dimensionally to switch between “ON” and “OFF” states. Unused LM elements are hidden at a bottom reservoir during the OFF state, eliminating their parasitic effect.

This paper presents an alternative method to material characterization by using a low-cost rapid prototyping technique to fabricate microstrip ring resonators. After the design of the ring resonator, the layout is drawn and cut using a craft cutter tool. The copper tape design is placed on top of a single-sided copper-clad Rogers RT/duroid® 5880 substrate and the S-parameters are measured using 8.5 GHz vector network analyzer. The extracted dielectric constant compares well with the datasheet value of $\mathbf{2.20}\pm\mathbf{0.02}$ , which is determined using a split-cylinder resonator at 10 GHz. We see this technique as a way of obtaining first order material properties in less time compared to clean room processing and for substrates that do not benefit from thin film processing.

A liquid-metal switch actuated by continuous electrowetting is designed and simulated using varied concentrations of sodium hydroxide electrolyte as the immersive media. The observed strong agreement between simulated and measured switch characteristics suggests that the behavior of similar liquid-metal-based systems may be predicted through simulation, eliminating the need for inefficient trial-and-error physical prototyping.

Gallium alloys are liquids at room temperature, and are suitable as conductors in electronic circuits. Furthermore, gallium-based liquid metals immersed in a water-based electrolyte such as sodium hydroxide (NaOH) can be electrically actuated, enabling reconfigurable electronics such as RF switches, tunable filters, and tunable antennas. However, NaOH in liquid-metal reconfigurable electronics also causes RF losses that should be minimized by careful design and simulation. To accurately simulate the effects of NaOH at microwave frequencies, the complex permittivity of NaOH is required over the operating frequency range. Here, the complex dielectric permittivity of aqueous NaOH solutions is determined from 0.2 to 20 GHz by dielectric spectroscopy. NaOH solutions with concentrations of 0.01 moles/liter (M), 0.1 M, 0.25 M, 0.5 M, 0.75 M, 1.0 M, 1.25 M, and 1.5 M are investigated at 20 °C. The complex permittivity spectra are fitted by a Cole-Cole relaxation time distribution. In addition, the fitting parameters, including static permittivity $\varepsilon _{s} $ and relaxation time $\tau $ are reported, along with the distribution parameter $\alpha $ . The measured permittivity of NaOH is used to simulate two liquid-metal RF components using NaOH. The measured RF performance are in good agreement with the simulated results that include the effects of NaOH.

Liquid-metal pixels are used to demonstrate a frequency-tunable dipole antenna that can be reconfigured for horizontal or vertical polarisation. Each dipole consists of 14 pixels, each of which can be electrically actuated with the liquid metal Galinstan to achieve different antenna states.

An electrically actuated reconfigurable liquid-metal dipole antenna is presented. Using electrocapillary actuation, a 5 VDC signal can actuate the liquid metal Galinstan into five discrete states with varying polarizations and null directions. Local surface-energy wells built into the polyimide fixture encasing the fluids enable metastable locking of the Galinstan, eliminating the need for a continuous dc bias voltage to maintain each state.

A liquid-metal RF shunt switch is presented that combines a dielectric spacer over the RF path with electrolyte-filled capillary troughs to facilitate low-power electrical actuation. The switch demonstrates between 20- and 30-dB isolation in the dc to 5-GHz range, and greater than 10-dB isolation from 5 to 11 GHz. This switch has between 0.2- and 1.2-dB insertion loss from dc to 5 GHz.

Liquid metals such as gallium alloys have a unique potential to enable fully reconfigurable RF electronics. One of the major concerns for liquid-metal electronics is their interaction with solid-metal contacts, which results in unwanted changes to electrical performance and delamination of solid-metal contacts due to atomic diffusion of gallium at the liquid/solid interface. In this paper, we present a solution to this problem through way of liquid-metal/liquid-metal RF connections by implementing La-place barriers, which control fluid flow and position via pressure-sensitive thresholds to facilitate physical movement of the fluids within the channels. We demonstrate RF switching within the channel systems by fabricating, testing, and modeling a reconfigurable RF microstrip transmission line with integrated Laplace barriers which operates between 0.5–5 GHz. This approach opens the potential for future all-liquid reconfigurable RF electronic circuits where physical connections between solid and liquid metals are minimized or possibly eliminated altogether.

Patterned conformal conductive structures are used to realize flexible electronics for applications such as electronic skin, communication devices, and sensors. Thus, there is a demand for low-cost rapid fabrication techniques for flexible and stretchable conductors. Spray-coating of liquid metals is a prototyping method that is compatible with elastic substrates. In this work, UV-curable and polyimide masks were used to pattern sprayed liquid metal (LM). The effect of the spraying parameters on the thickness and conductivity of the LM was characterized. A minimum LM linewidth of 48 &micro, m was achieved, along with a minimum gap width of 34 &micro, m. A LM patch antenna and transmission line, which can potentially be used for communication systems, were demonstrated using this fabrication process.

Rapid, Economical Circuit Prototyping (RECi-P) is a method for the rapid prototyping of conductive traces that can be used for electronic circuits. This method utilizes an inexpensive craft cutter that can cut copper tape with sub-millimeter accuracy. The layout of cut copper tape pieces is preserved, making it easier to accurately create complicated patterns. Using RECi-P, designs can be fabricated and tested in under 30 minutes, for approximately the same cost as prototypes made by manually cutting copper tape. Using a 5-GHz coupled-line bandpass filter as a demonstration device, three fabrication methods were compared: RECi-P, photolithography and wet etching, and mechanical milling. The performance of the filter fabricated using RECi-P is similar to that of the other methods, showing that RECi-P is a rapid, economical method for the prototyping of devices and circuits.

A polarization-reconfigurable antipodal dipole antenna is presented. The antenna switches between two orthogonal linear polarizations using low-power electrical actuation of liquid metal. Previous electrically actuated liquid-metal antennas have required continuous applied voltages to maintain an operating state, and had reduced gain from exposure to the lossy electrolyte needed for actuation. This antenna uses metastable locking so that applied voltages are only needed to change operating states, not to maintain them. In addition, the compact design of this antenna reduces the amount of electrolyte, and the antenna performance with and without the electrolyte solution is quantified. The dipole has a maximum gain between 1.8 and 2.3 dBi in the presence of the electrolyte and between 2.4 and 3.0 dBi when the solution is removed. The resonant frequencies of the orthogonal polarizations are less than 5.3% apart, and all states have greater than 15-dB return loss at the 3-GHz operating frequency.

Ferrofluidic actuation of liquid metal, demonstrated for the first time on an RF device, employs the magnetic response of a low-loss hydrocarbon-based ferrofluid for a simple RF switch. The ferrofluid is shown to reduce losses by an average of 4.9 dB from 50 MHz to 20 GHz compared to a lossy aqueous solution commonly used in existing actuation methods for liquid metal. These results offer a promising alternative for liquidmetal reconfigurable RF circuits.

A frequency-tunable microstrip patch antenna with a liquid-metal-actuated loading slot is presented. The electrical length of the loading slot located beneath the resonating patch is altered by filling and evacuating liquid metal from fluidic channels along the ground plane. The slot reactively loads the antenna, altering the resonant frequency as its length is changed. Frequency tuning is achieved from 1.85 to 2.07 GHz, corresponding to an 11.2% tuning bandwidth.

A tunable liquid-metal antenna demonstrating gain reconfigurability is presented. This antenna uses a reconfigurable stub made with the liquid metal Galinstan. Using continuous electrowetting, a 60 Hz signal with an amplitude of 1 Vpp and 75% duty cycle can actuate the Galinstan continuously along a channel, tuning the stub length and antenna gain. Zero external power is required to maintain the position of the slug. The 5 GHz antenna offers more than 10 dB of analog gain tuning, from −5.90 to 4.43 dB.

A student-built unmanned aerial system (UAS) was developed by the University of Hawaii Drone Technologies team for the 2017 Association for Unmanned Vehicle Systems International (AUVSI) Student Unmanned Aerial System (SUAS) competition, which simulates a search-and-rescue (SAR) mission. The UAS comprises a fixed-wing airframe integrated with flight control and communication components, and is capable of autonomous waypoint navigation, in-flight data transfer, aerial image capture with onboard image processing, and aerial payload delivery. The UAS is capable of executing a 30-minute SAR mission in search of a simulated lost hiker. SAR tasks include autonomously navigating to a designated area, conducting a search for alphanumeric targets over a 370,000-m2 search area, and autonomously dropping an 8-oz care package to the lost hiker. Image-processing and computer-vision algorithms can correctly sense and identify the alphanumeric targets with 75% accuracy.

A quantitative study of the effect of introducing gaps of various widths in a rectangular microstrip patch antenna is presented. Simulations were performed using Keysight Advanced Design System®, and the simulations were confirmed by the fabrication and measurement of three antennas. Gaps with widths of $\boldsymbol{\lambda}_{\boldsymbol{eff}}/31$ and smaller have a negligible effect on the return loss and radiation patterns of the patch antennas that were analyzed.

A power-detecting, null-scanning, retrodirective antenna array for CubeSat platforms is presented. The system utilizes several hardware and software enhancements over previous retrodirective array architectures to address the size, weight, and power limitations of the CubeSat structure. Full-duplex retro-directivity is reported at 9.59 and 9.67 GHz for transmit and receive, respectively.

In 1958, the United States launched its first satellite ever, the Explorer 1. This rocket shaped satellite was a little over 2-m long with a diameter of about 16 cm and weighed roughly 14 kg. But as small as it was, the Explorer 1 ended up successfully completing its mission and made a significant impact on science and our overall understanding of the Earth's magnetosphere as the first spacecraft to detect the Van Allen radiation belts.

Liquid metal actuated by pressure-driven air bubbles is guided within channels to dynamically reconfigure the length of the radiating aperture and feed line of a slot antenna. The gallium-based liquid metal is held in position by air bubbles, and liquid-metal motion is induced when a pressure differential is created. In contrast to conventional hydraulic or pneumatic actuation of gallium-based liquid metal, the pressure actuation described here is reversible and repeatable, as the liquid metal is enveloped by a thin layer of NaOH solution, which acts as a slip layer and reduces any metal oxidation. The slot antenna achieves a 26% tunable bandwidth with a contiguous range from 1.42 to 1.84 GHz for which the S 11 is

A reconfigurable five-element Yagi-Uda monopole array is presented that utilizes pressure-driven liquid-metal elements. There is one centrally located driven element and six parasitic elements in the array, all of which utilize liquid metal so that their physical lengths can be varied. This allows the array to tune to different operational frequencies, vary the number of director elements, and change parasitic elements from reflectors to directors, and vice versa. The measured results show beam steering along a single axis and 1.33 dB of gain tuning at 2.4 and 3.87 GHz.

An electrically actuated direct-contact RF shunt switch implemented with the nontoxic liquid metal Galinstan is presented. The Galinstan is actuated with a 5-VDC signal that induces electrocapillary action. Local surface-energy wells facilitate metastable locking of the Galinstan, thus achieving ON/OFF locking without requiring a continuous applied bias voltage. The switch operates from DC to 11.2 GHz with greater than 10 dB isolation and has less than 3 dB insertion loss from DC to 6.7 GHz.

Four types of liquid-metal-based tunable capacitors with a high tuning range are demonstrated. The capacitors are easily fabricated by placing a pair of liquid-metal-filled tubes in contact with one another. Tunability is achieved by varying the length of the liquid metal in one of the tubes using a mechanical pump. Four different structures are demonstrated: parallel-tube, folded-tube, coil, and spiral capacitors. The highest measured tuning ratio is 42:1, and the highest change in capacitance per unit length of the pumped liquid metal is 0.07 pF/mm.

An interleaved, interelement phase-detecting/phase-shifting retrodirective antenna array for interference reduction is presented. This system uses two independent retrodirective subarrays to form a null in one direction while retrodirecting a beam in another direction, while also eliminating the R4 path loss of previous interleaved architectures. Retrodirectivity as well as null steering is reported at 2.4 GHz.

Reconfigurable RF devices can respond dynamically to complex RF environments. Liquid metals have the potential to enable reconfigurable devices that cannot be implemented by more conventional materials and technologies. This paper reviews recent progress by our research group toward the creation of reconfigurable liquid-metal RF devices that use low-power electrical actuation.

A 2-D retrodirective array based on phase detection and heterodyne scanning is presented. The system integrates two single voltage-controlled oscillator phase-shifting networks that are autonomously controlled via a phase-detecting array and control circuit that eliminates the R4 path loss typical of other retrodirective arrays. Retrodirectivity is reported for angles of 0deg, +15deg, and -15deg at a transmitting frequency of 6.5 GHz.

A high-directivity combined self-beam/null-steering array for secure point-to-point binary phase-shift keying communications is introduced. The system provides high directivity and reduced probability of interception using just two antenna elements. Using quadrature phase-shift keying modulators allows for compact single-layer fabrication. The 2.4-GHz prototype is tested at interrogation angles of 0deg, -10deg, and +20deg, and demonstrates high signal-to-interference ratio directivity, completely disabling interception plusmn20deg from the direction of the interrogator. The system should find various applications where secure communications are required

Presented here is a method for actuating a gallium-based liquid-metal alloy without the need for an external power supply. Liquid metal is used as an anode to drive a complementary oxygen reduction reaction, resulting in the spontaneous growth of hydrophilic gallium oxide on the liquid-metal surface, which induces flow of the liquid metal into a channel. The extent and duration of the actuation are controllable throughout the process, and the induced flow is both reversible and repeatable. This self-actuation technique can also be used to trigger other electrokinetic or fluidic mechanisms.

A phase shifter based on reconfigurable filling factors (FF) in a 1D electromagnetic bandgap (EBG) structure is presented. The device is capable of different gradations in phase shift, using liquid metal to fill or evacuate holes in the defected ground plane of a microstrip line. The design consists of three cascaded EBG lattices, each consisting of 4 circles with a FF of 0.17, 0.25, and 0.37 respectively. When actuated, a per-element phase shift of 1.8, 5.7, and 9.4 degrees at 5.6 GHz is introduced. A total of 67.2 degrees of phase shift can be achieved with an average insertion loss of 1.1 dB.

A tunable bandpass filter based on reconfigurable liquid-metal perturbing posts in a substrate integrated waveguide (SIW) cavity is demonstrated. The design consists of four reconfigurable posts in a single cavity which provide five center frequencies from 10.13 GHz to 11.31 GHz. To enable frequency tuning, nontoxic liquid-metal Galinstan is hydraulically actuated to activate and deactivate posts. The five states have an average peak insertion loss of 1.35 dB with fractional bandwidths of 0.6% to 1% and unloaded quality factors of 96 to 154.

A tunable amplifier using reconfigurable liquid-metal double-stub tuners is presented. The impedance tuners are implemented using liquid-metal slugs that are electrically actuated using continuous electrowetting. The amplifier can be configured to multiple frequency states from 3.37 to 6.02 GHz by adjusting the physical lengths of the liquid-metal double-stub tuners.

A 16-element two-dimensional (2-D) retrodirective array using self-oscillating mixers (SOMs) is presented. SOMs allow for easier implementation of larger 2-D arrays by eliminating the complex local-oscillator (LO) feed structure. A 4 /spl times/ 4 element retrodirective array using SOMs is demonstrated at an LO frequency of 7.68 GHz. Each element is phased locked at the LO frequency with an accompanying RF frequency isolation of 17.9 dB between adjacent horizontal elements and 22.2 dB between adjacent vertical elements. A -10-dBm external injection-locking signal is applied to reduce the phase noise of the 16-element array to -68.2 dBc/Hz at 10-kHz offset. Retrodirectivity is observed in the /spl phi/=0/spl deg/, /spl phi/=-45/spl deg/, and /spl phi/=-90 plane for scattering angles of /spl theta/=-15/spl deg/, /spl theta/=0/spl deg/, and /spl theta/=+30/spl deg/.