James Wissman

Acoustic metamaterials, which manipulate acoustic propagation using architected, discrete structures, have wide ranges of applications but typically have a fixed response upon design and fabrication. We present a novel tunable acoustic structure with a granular jamming-based approach, where the jammed state acts as a solid with a well defined resonant response and the unjammed state acts as a liquid with a broad, weakly resonant response. In between these two extremes, resonance peak tuning is observed. Our approach to particle jamming is uniquely controlled with a dielectric elastomer actuator (DEA), whose applied confining pressure is governed by voltage. Mathematical modeling of the DEA and the confining pressure is used to understand the device behavior. Samples are fabricated and tested acoustically, revealing a novel solid–liquid like transition at an applied voltage of about 6.0 kV and a preceding “rattler” behavior at around 4.25 kV.

Submersible robotics have improved in efficiency and versatility by incorporating features found in aquatic life, ranging from thunniform kinematics to shark skin textures. To fully realize these benefits, sensor systems must be incorporated to aid in object detection and navigation through complex flows. Again, inspiration can be taken from biology, drawing on the lateral line sensor systems and neuromast structures found on fish. To maintain a truly soft-bodied robot, a man-made flow sensor must be developed that is entirely complaint, introducing no rigidity to the artificial “skin.” We present a capacitive cupula inspired by superficial neuromasts. Fabricated via lost wax methods and vacuum injection, our 5 mm tall device exhibits a sensitivity of 0.5 pF/mm (capacitance versus tip deflection) and consists of room temperature liquid metal plates embedded in a soft silicone body. In contrast to existing capacitive examples, our sensor incorporates the transducers into the cupula itself rather than at its base. We present a kinematic theory and energy-based approach to approximate capacitance versus flow, resulting in equations that are verified with a combination of experiments and COMSOL simulations.

We present the design, fabrication, and testing of stretchable pressure sensing membranes. Two sensing techniques are demonstrated: resistive and capacitive. Both designs are incorporated in 400μm-thick films and are fabricated with thin film application of silicone and stencil/mask deposition of conductive materials. The resistive sensor utilizes room temperature liquid metal while the capacitive sensor utilizes multi-walled carbon nanotubes. Tests are performed with 18mm-diameter samples of each. Point load tests and acoustic response in an impedance tube provide feedback on sensor performance. The resistive sensor demonstrates a sensitivity of 0.045Ω/mm, and the sensor’s response has been characterized for in the 30Hz to 10kHz range with varying degrees of sensitivity. The capacitive sensor has a small point-load-deflection sensitivity ranging from 0.018pF/mm to 0.044pF/mm depending on capacitor diameter. Acoustic response are shown for 5Hz to 40 Hz, limited by external electronics. These devices are progress towards developing sensor networks capable of tracking aqueous turbulence.

We present a liquid flow sensor inspired by cupula structures found on a variety of fish. Our 5mm x 5mm x 1.75mm artificial cupula uniquely comprises a pair of differential liquid metal capacitors encased in silicone. Deflection of the structure – manually or by fluid flow – increases capacitance on one side and decreases on the other. To fabricate the complex internal structure, a commercial 3D printer is used to create a mold out of a sacrificial wax-like material. After casting uncured rubber, internal mold structures are melted and dissolved away, leaving channels and voids for liquid metal vacuum injection. The measured sensitivity of ~0.05pF/mm is compared to theoretical capacitance versus deflection values based on kinematics. To test behavior under water flow, a custom flow channel consisting of a 7.5mm x 7.5mm cross-section is employed with rates up to 1L/min. The parabolic capacitive response as a function of flowrate is compared to analytic theory based on kinematics and drag as well as to fluid-structure interaction (FSI) simulations using COMSOL. This device has future applications in the control of bio-inspired soft robotics. [Work sponsored by the Office of Naval Research.]

Eutectic gallium–indium (EGaIn) has attracted significant attention in recent years for its use in soft and stretchable electronics. However, advances in scalable fabrication approaches and effective electromechanical interfaces between liquid metal (LM) traces and microelectronics are still needed to create functional soft and stretchable electronics. In this study, EGaIn–metal interfacing for the effective integration of surface‐mount microelectronics with LM interconnects is investigated. The electrical interconnects are produced by creating copper patterns on a soft‐elastomer substrate, and subsequently exposing the substrate to EGaIn, which selectively wets the Cu traces. To create strong electromechanical connection between EGaIn and microelectronics, the terminals of the LM‐coated traces are “soldered” to the metal pins of the packaged microelectronic circuits using a novel HCl‐vapor treatment. In combination, the fabrication and microelectronics‐interfacing approaches enable creating stretchable circuits composed of LM wiring and packaged microelectronics. It is found that the HCl‐vapor treatment significantly improves electrical conductivity at the LM–pin interface while enhancing the strain limit of the soft circuits and the reproducibility of the interface. The applicability of this approach in creating soft‐matter circuits is demonstrated through two illustrative examples—a circuit with a digital 9‐axis inertial measurement unit and a temperature sensor; and a circuit with a 3‐axis analog accelerometer.

When immersed in an electrolyte, droplets of Ga‐based liquid metal (LM) alloy can be manipulated in ways not possible with conventional electrocapillarity or electrowetting. This study demonstrates how LM electrochemistry can be exploited to coalesce and separate droplets under moderate voltages of ~1–10 V. This novel approach to droplet interaction can be explained with a theory that accounts for oxidation and reduction as well as fluidic instabilities. Based on simulations and experimental analysis, this study finds that droplet separation is governed by a unique limit‐point instability that arises from gradients in bipolar electrochemical reactions that lead to gradients in interfacial tension. The LM coalescence and separation are used to create a field‐programmable electrical switch. As with conventional relays or flip‐flop latch circuits, the system can transition between bistable (separated or coalesced) states, making it useful for memory storage, logic, and shape‐programmable circuitry using entirely liquids instead of solid‐state materials.

We introduce a method for sealing liquid metal (LM) circuits with soft anisotropic conductors that prevent leaking, while simultaneously allowing for electrical contact with skin and surface mounted electronics. These films are composed of polydimethylsiloxane (PDMS) embedded with vertically aligned columns of ferromagnetic Ag–Ni microparticles. The microparticles are magnetically aligned and support electrical conductivity only through the thickness (z-axis) of the elastomer film. Measurements on 10–40% (by wt) composites show moderate volumetric resistivity (as low as ρ = 0.03 Ω/m) through the thickness and no conductivity between adjacent traces. Functionality is demonstrated with several illustrative applications related to tactile sensing and electronics hardware integration.