Daeyoung Kim

Reconfigurable antennas capable of post-fabrication modification are highly desirable for adaptive RF systems. In this work, we demonstrate a redrawable and physically reconfigurable liquid-metal microstrip patch antenna on a paper substrate. Gallium-based liquid metal was patterned onto laser-printed paper using a simple hand-drawing process. Because the liquid metal can be removed using ethanol, the antenna geometry was selectively modified through erasing and redrawing. Removal of the inner annular region shifted the resonant frequency from 3.97 GHz to 3.78 GHz, while formation of a circular patch shifted it to 4.28 GHz. Redrawing restored the original resonant characteristics. Physical reconfigurability was further demonstrated by cutting and reconnecting the antenna structure, where the resonant frequency shifted to 2.56 GHz after cutting and recovered to 3.93 GHz after reconnection. The proposed approach provides a simple and low-cost strategy for flexible and reconfigurable RF devices.

A gallium‐based liquid metal patterning for paper‐based soft electronics is reported. A method is proposed that is rapid and straightforward liquid metal patterning on a printing paper using selective wetting properties obtained by lyophilic and lyophobic coating. The paper is coated with a lyophilic waterproof agent followed by a lyophobic printing toner with a desired pattern in an inverse pattern. Then a sponge roller soaked with liquid metal is rolled and filled into the exposed waterproof agent coating to create the desired pattern. It is believed that this is one of the most convenient yet quite versatile liquid metal patterning methods. The minimum line pattern width, spacing, and patternable angle are 50 μm, 200 μm, and 3°, respectively. To demonstrate versatility, a university symbol and a map shape are coated with liquid metal. The bending, folding, cutting, pasting, and burning of the light‐emitting diode circuit on paper is also demonstrated. Additionally, the deposition of 3 μm Parylene on top of the liquid metal pattern works as a passivation layer, and no damage is noticed when the liquid metal pattern on the paper is repeatedly folded and bent. Finally, a pressure sensor using the proposed method by measuring resistance is demonstrated.

RADAR(Radio Detection and Ranging) is an important system for surveillance and reconnaissance by detecting a reflected signal which obtains the range from the radar to the target, and the velocity of the target. The magnitude of the reflected signal varies due to the radar cross section of the target, characteristic of the transmission and reception antenna, distance between the radar and the target, and power and wavelength of the transmitted signal. Thus, the RCS is the important characteristic of the target to determine if the target can be observed by the RADAR system. It is based on the material and shape of the target. We have measured the reflection signal of a simple square-shaped (20 × 20 cm) target made of a new material, a gallium-based liquid metal alloy and compared that of well-known metals including copper, aluminum. The magnitude of reflected signal of the aluminum target was the largest and it was 2.4 times larger than that of the liquid metal target. We also investigated the effect of the shape by measuring reflectance of the F-22 3D model(~1/95 ratio) target covered with/without copper, aluminium, and liquid metal. The largest magnitude of the reflected signal measured from side-view with the copper-covered F-22 model was 2.6 times greater than that of liquid metal. The reflectance study of the liquid metal would be helpful for liquid metal-based frequency selective surface or metamaterials.

In this paper, we report bouncing and separating dynamic behaviors of a liquid metal droplet with/without the oxide layer in response to the applied acoustic wave. The oxidized liquid metal droplet is readily bounced off from the surface when it is excited by acoustic wave, while the HCl treated liquid metal droplet is fragmented into several small droplets. The bouncing height of the oxidized liquid metal is proportional to the applied acoustic wave amplitude. The number of the fragmented liquid metal droplets for the HCl-treated liquid metal according to time and acoustic wave amplitude was investigated. We also demonstrated the acoustic wave-based inkjet application to generate liquid metal droplets based on the pinch-off and the Rayleigh instability by changing amplitude of the acoustic wave. The probability for the generation of various droplet sizes with different acoustic wave amplitude was also studied.

We report a liquid metal microstrip antenna on a printing paper whose resonant frequency was tunable by folding the paper. The gallium-based alloy was used as liquid metal, which is instantly oxidized in ambient air. The oxidized liquid metal is easily wettable on printing paper surfaces, while it is non-wettable on laser-printed toner surfaces. An antenna with a 3 mm wide and 39 mm long copper-liquid metal overlapped-feedline linked to the liquid metal microstrip rectangle-shaped patch (40 × 31 mm²) was designed. The antenna pattern was printed on a printing paper using a laser printer. A liquid metal soaked roller was rolled on the antenna pattern printed paper, resulting in a liquid metal pattern where there was no toner. The resonant frequency of the liquid metal on the paper microstrip patch antenna was 2.88 GHz with −18.5 dB S11. The measured characteristics of the antenna were in good agreement with the simulation results. When the antenna was folded, the resonant frequency was changed to 2.68 GHz (x-axis folding) or 2.72 GHz (y-axis folding). Liquid metal on paper fabrication is simple but effective for creating custom-designed antennas that are easily re-designed and fabricated in the field.

In this paper, we report a volume expansion phenomenon of a liquid metal droplet naturally oxidized in an ambient environment by applying an acoustic wave. An oxidized gallium-based liquid metal droplet was placed on a paper towel, and a piezo-actuator was attached underneath it. When a liquid metal droplet was excited by acoustic wave, the volume of liquid metal was expanded due to the inflow of air throughout the oxide crack. The liquid metal without the oxide layer cannot be expanded with an applied acoustic wave. To confirm the effect of the expansion of the oxidized liquid metal droplet, we measured an expansion ratio, which was calculated by comparing the expanded size in the x (horizontal), y (vertical) axis to the initial size of the liquid metal droplet, using a high-speed camera. For various volumes of the droplet, when we applied various voltages in the range of 5~8 Vrms with 18.5~24.5 kHz using the piezo-actuator, we obtained a maximum expansion ratio of 2.4 in the x axis and 3.8 in the y axis, respectively. In addition, we investigated that the time to reach the maximum expansion in proportion to the volume size of liquid metal differed by five times from 4 s to 20 s, and that the time to maintain the maximum expansion differed from 23 s to 2.5 s, which was inversely proportional to the volume size. We also investigated the expansion ratios depending on the exposure time to the atmosphere. Finally, a circuit containing LED, which can be turned on by expanded liquid metal droplet, was demonstrated.

A gallium-based liquid metal got high attention recently, due to the excellent material properties that are useful in various research areas. We report here on electric field-induced liquid metal droplet generation and falling direction manipulation. The well-analyzed electro-hydrodynamic method is a selectable way to control the liquid metal, as the liquid metal is conductive. The electric field-induced liquid metal manipulation can be affected by the flow rate (0.05~0.2 mL/min), voltage (0~7 kV), and distance (15 and 30 mm) between electrodes, which changes the volume of the electric field-induced generated liquid metal droplet and the number of the generated droplets. When the electric field intensity increases or the flow rate increases, the generated droplet volume decreases, and the number of droplets increases. With the highest voltage of 7 kV with 15 mm between electrodes at the 0.2 mL/min flow rate, the lowest volume and the largest number of the generated droplets for 10 s were ~10 nL and 541, respectively. Additionally, we controlled the direction of the generated droplet by changing the electric field. The direction of the liquid metal droplet was controlled with the maximum angle of ~12°. Moreover, we exhibited a short circuit demonstration by controlling the volume or falling direction of the generated liquid metal droplet with an applied electric field.

We report magnetic field-driven dynamic morphological change of a liquid metal droplet: deformation and recovery; separation and merge. Low melting, non-toxic gallium-based liquid metal has been widely investigated due to its material properties and thus utilized to various applications. Morphological change of the liquid metal in required to tune characteristics of application. In order to enable the liquid metal to be non-wettable and magnetically controllable, we coated the surface with ferromagnetic iron (Fe) particles using hydrochloric acid (HCl) solution resulting in a magnetic liquid metal marble. With and without magnetic field, Fe particles-coated magnetic liquid metal marble was deformed and recovered readily. Against ~8 μL magnetic liquid metal marble, depending on the coated Fe particles mass, the shape deformation under magnetic field was changed: conventional (Fe < 4mg) and arbitrary (Fe ≥q 4mg) wetting shape. Accordingly, the deformability and recovery time of the magnetic liquid metal marble were varied. With an applied magnetic field, one marble was readily separated into 2, 3, and 4 marbles. Due to re-oxidation of the magnetic liquid metal marble surface, we treated the marble with HCl solution and vapor. Split and merge occurred depending on treatment time of HCl solution or vapor. Finally, we demonstrated magnetic field-based dynamic behavior which is reversible. [2020-0218]

We report reversible dynamic behavior of a magnetic liquid metal marble by changing magnetic field polarity. We coated iron (Fe) particles on the surface of the oxidized liquid metal by HCl-based chemical reaction to make it to be magnetized and to be non-wettable. We utilized electromagnets and changed current direction on-demand to change the magnetic polarity of the electromagnet. In 1-dimension (1-D), with an applied magnetic field of reverse polarity from the bottom, the conventional wetting shape of the magnetic liquid metal marble was observed, whereas the split was occurred with the applied magnetic field of same polarity due to attractive or repulsive force. With an applied magnetic field of reverse polarity, the split marbles was merged. By removing the magnetic field, the deformed magnetic liquid metal marble was recovered. The applied current dependent deformability and recoverability, and split and merge time were investigated. In addition, in 2-dimension (2-D), we observed various deformations and split behavior of the magnetic liquid metal marble with various magnetic field polarity combinations by utilizing electromagnets, which is reversible.