Energy harvesting from aperiodic low-frequency motion using reverse electrowetting

Ionotronics for reverse actuation

In the midst of an ongoing energy crisis, the search for new methods of energy harvesting has never been more important. Here we explore, analyse and discuss principles of ionotronic reverse-actuator devices based on the effect of double-layer charging. The designs that we consider in this paper operate based on a common principle - using external mechanical work, which would otherwise be wasted, to produce changes in the contact area of electrode and electrolyte, translated into the time variation of the double-layer capacitance. Periodic variation of capacitance, when connected to a reference voltage source, produces alternating electric current through a load. This concept is not new and in some forms was realised in the early works of Boland, Krupenkin and several papers of our group. The goal of the present paper is to build a comprehensive analytical platform for a description of operation of such devices in terms of materials, generated power as a function of the frequency of variation of applied force, electrical load, and other factors; the understanding of which allows us to optimise these systems and navigate their construction. The first design, discussed in the paper, is based on flat electrodes. It is the simplest one and, as such, helps elucidate some key factors determining power generation. While being easy to realise experimentally, it generates relatively low power, even when optimised. The second design, based on microporous electrodes is more sophisticated and allows a much larger power harvest. The results are also compared to the recently proposed capacitive rotor device. The developed theory is set to capture the key factors that determine the functioning of the considered reverse actuators. The structures under study are matched to fit into the sole of a shoe and produce power from walking and running. However, they can also be scaled-up to larger operating systems and various external loads.

Robust Impact Effect and Super-Lyophobic Reduced Galinstan on Polymers Applied for Energy Harvester

In this paper, we present a novel reduced Galinstan-based microfluidic energy harvester, which can converse kinetic energy to electricity from an arbitrary vibration source. Firstly, the wetting behaviors of reduced Galinstan are performed, which shows a robust impact effect on polymer substrates. Moreover, the electric circuit model of the reduced Galinstan-based energy harvester is made and discussed by the use of the EDLCs (electrical double layer capacitors). After modeling, the microfluidic energy harvester with coplanar microfluidic channels is designed and fabricated. Finally, the performance of the microfluidic energy harvester is investigated, which can harvest multi-direction vibration energy. The experiment results demonstrate that the novel reduced Galinstan-based microfluidic energy harvester is suitably and uniquely applied in a complex vibration environment.

Spectrally tunable liquid resonator based on electrowetting

We present a tunable on-chip liquid resonator in conjunction with a tapered fiber coupling scheme. The resonator consists of a glycerol droplet submerged within an immiscible liquid bath, which mitigates the effects of environmental fluctuations. The platform is fabricated using standard semiconductor techniques, which enable the future integration of photonic components for an on-chip liquid resonator device. The liquid resonator maintains its high Q-factor on chip (10⁵) due to surface tension forming an atomically smooth liquid-liquid interface. Higher Q-factor resonance modes experienced linewidth broadening due to the random excitation of thermal capillary vibrations. Spectral tuning is demonstrated using the electrowetting effect, increasing the surface's wettability and an expansion in the droplet diameter. A maximum spectral tuning of 1.44 nm ± 5 pm is observed by applying 35 V. The tuning range is twice the free spectral range (FSR) of 0.679 nm measured at a pumping wavelength range of 770-775 nm. A 2D axisymmetric finite-element simulation shows resonance modes in good agreement with experimentally measured spectra and with predicted tuning speeds of 20 nm/s.

Theoretical demonstration of a capacitive rotor for generation of alternating current from mechanical motion

Innovative concepts and materials are enabling energy harvesters for slower motion, particularly for personal wearables or portable small-scale applications, hence contributing to a future sustainable economy. Here we propose a principle for a capacitive rotor device and analyze its operation. This device is based on a rotor containing many capacitors in parallel. The rotation of the rotor causes periodic capacitance changes and, when connected to a reservoir-of-charge capacitor, induces alternating current. The properties of this device depend on the lubricating liquid situated between the capacitor's electrodes, be it a highly polar liquid, organic electrolyte, or ionic liquid - we consider all these scenarios. An advantage of the capacitive rotor is its scalability. Such a lightweight device, weighing tens of grams, can be implemented in a shoe sole, generating a significant power output of the order of Watts. Scaled up, such systems can be used in portable wind or water turbines.

Impact of substrate elasticity on contact angle saturation in electrowetting

The electrostatically assisted wettability enhancement of dielectric solid surfaces, commonly termed as electrowetting-on-dielectric (EWOD), facilitates many microfluidic applications due to simplicity and energy efficiency. The application of a voltage difference between a conductive droplet and an insulated electrode substrate, where the droplet sits, is enough for realizing a considerable contact angle change. The contact angle modification is fast and almost reversible; however it is limited by the well-known saturation phenomenon which sets in at sufficiently high voltages. In this work, we experimentally show and computationally support the effect of elasticity and thickness of the dielectric on the onset of contact angle saturation. We found that the effect of elasticity is important especially for dielectric thickness smaller than 10 μm and becomes negligible for thickness above 20 μm. We attribute our findings on the effect of the dielectric thickness on the electric field, as well as on the induced electric stresses distribution, in the vicinity of the three phase contact line. Electric field and electric stresses distribution are numerically computed and support our findings which are of significant importance for the design of soft materials based microfluidic devices.

Evaluation Methodology of a Smart Clothing Biomechanical Energy Harvesting System for Mountain Rescuers

The article presents a methodology developed for the evaluation of biomechanical energy harvesting systems that permits avoiding long-duration outdoor tests while providing realistic input signals and preserving uniform conditions across repeated tests. It consists of two stages: transducer output signal recording and power conversion and storage system measurements. The proposed approach was applied to assess the usefulness of a commercial electromagnetic transducer for supplying a Global Positioning System (GPS) receiver used as an active component of a smart clothing dedicated for mountain rescuers. Electrical power yield measurements have been presented together with ergonomic tests results. They all involved diverse physical activities performed by mountain rescuers that simulated their true operations, but were conducted in a training room for the sake of standardization. By providing reliable data on the transducer’s performance under realistic use conditions, the proposed evaluation procedure revealed that the true energy yield was much smaller not only with respect to the manufacturer’s assertions, but also substantially lower than what was expected based on an independent review which used unrealistic and non-uniform excitations. On the other hand, ergonomics ratings given by potential end users were very high, which demonstrates that the evaluated transducer can still be useful for supplying active cloth components with a lower power demand. The study also revealed that transducer location and orientation strongly affect its performance, which must be taken into account at the first stage of the evaluation procedure. Moreover, physical activity type and conditions (such as motion speed and ground tilt) influence the output power and should be carefully considered when composing a typical case scenario for the second stage.