Electrowetting refers to an electrostatically induced reduction in the contact angle of an electrically conductive liquid droplet on a surface. Most designs ground the droplet by either sandwiching the droplet with a grounding plate on top or by inserting a wire into the droplet. Washizu and others have developed systems capable of generating droplet motion without a top plate while allowing the droplet potential to float. In contrast to these designs, we demonstrate an electrowetting system in which the droplet can be electrically grounded from below using thin conductive lines on top of the dielectric layer. This alternative method of electrically grounding the droplet, which we refer to as groundingfrom-below, enables more robust droplet translation without requiring a top plate or wire. We present a concise electrical-energy analysis that accurately describes the distinction between grounded and nongrounded designs, the improvements in droplet motion, and the simplified control strategy associated with grounding-from-below designs. Electrowetting on a single planar surface offers flexibility for interfacing to liquidhandling instruments, utilizing droplet inertial dynamics to achieve enhanced mixing of two droplets upon coalescence, and increasing droplet translation speeds. In this paper, we present experimental results and a number of design issues associated with the grounding-frombelow approach.

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An acoustic array is proposed as a quench detection method in superconducting magnets. A quench occurs when the current density in the superconductor exceeds a critical value, resulting in a loss of superconductivity and rapid local heating. This event is destructive and must be rapidly detected. It is thought that the quench may act as an acoustic source (Takayasu, 2019), which could be detected and localized by a microphone array inserted into the cryogenic coolant. A main advantage of this method is that acoustics propagate 1000 times faster than the normal zone propagation velocity in HTS conductors, providing for fast detection times. To demonstrate this concept, we first characterized the performance of a piezoelectric MEMS microphone and several potential preamplifiers under cryogenic conditions. An acoustic sense node was then constructed that operates down to 10 K. A cryogenic probe incorporating the MEMS array was used to study a quench event in a segment of a 2 mm wide REBCO tape. Quench experiments were carried out in a 7.6 cm diameter, 111 cm tall cryostat in Helium gas at 20 to 50 K. The MEMS array clearly detects a quench induced failure. Other observed acoustic features of unknown origin will be described.

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