Electrokinetic Actuation of Liquid Metal for Reconfigurable Radio Frequency Devices

Abstract

Liquid metals are an attractive material choice for designers wishing to combine the advantages of metals, such as high electrical conductivity, thermal conductivity, and reflectivity, with the inherently dynamic nature of fluids. Liquid metals have been utilized for a wide variety of applications, but their high electrical conductivity, surface smoothness, and linear response makes them especially attractive as tuning elements within reconfigurable radio frequency (RF) devices. The recent introduction of non-toxic liquid metal alloys onto the commercial market has further fueled interest in this versatile material. Early experiments with liquid metal as an RF tuning element have yielded promising results, but have largely depended on externally applied pressure to actuate the liquid metal. For commercial implementation this would necessitate the use of clunky and inefficient micro-pumps, which can require both high voltages and high power consumption. This reliance on hydraulic pumping has been a significant barrier to the incorporation of liquid metal as an RF tuning element in applications outside of a laboratory setting. Here, several electrical actuation techniques are demonstrated that allow for the rapid and repeatable actuation of non-toxic gallium alloys as tuning elements within reconfigurable RF devices. These techniques leverage the naturally high surface tension of liquid metals, as well as the unique electrochemistry of gallium-based alloys, to exercise wide-ranging and high fidelity control over both the metal’s shape and position. Furthermore, this control is exercised with voltage and power levels that are each better than an order of magnitude below that achievable with conventional micro-pumps. This control does not require the constant application of actuation signals in order to maintain an actuated state, and can even be ‘self-actuated’, with the liquid metal supplying its own kinetic energy via the electrochemical conversion of its native oxide layer. Several proof-of-concept devices are designed and tested to demonstrate the effectiveness of these electrical actuation techniques. A pair of tunable slot antennas are presented that achieve frequency reconfigurability through different implementations of liquid metal tuning elements – the first uses liquid metal as a dynamic short-circuit boundary condition for the magnetic current within the resonant aperture, and the second as a variable-length transmission stub that adds and removes reactance from the antenna. The two antennas are tunable across effective bandwidths of 19% and 15%, respectively. In addition, a tunable bandpass filter is demonstrated in which a central liquid-metal resonant element is ‘stretched’ to lower the passband of the filter by 10% without impacting the insertion loss. Finally, it is demonstrated how liquid metal can be formed into arbitrary shapes at high speeds (approximately 2.5 cm/s) without the need for an external power supply.