Electrode and electrolyte configurations for low frequency motion energy harvesting based on reverse electrowetting

Investigation into the optoelectrowetting droplet transport mechanism

To explore the optoelectronic wetting droplet transport mechanism, a transient numerical model of optoelectrowetting (OEW) under the coupling of flow and electric fields is established. The study investigates the impact of externally applied voltage, dielectric constant of the dielectric layer, and interfacial tension between the two phases on the dynamic behavior of droplets during transport. The proposed model employs an improved Young's equation to calculate the instantaneous voltage and contact angle of the droplet on the dielectric layer. Results indicate that, under the influence of OEW, significant variations in the interface contact angle of droplets occur in bright and dark regions, inducing droplet movement. Moreover, the dynamic behavior of droplet transport is closely associated with various parameters, including externally applied voltage, dielectric layer material, and interfacial tension between the two phases, all of which impact the contact angle and, consequently, the transport process. By summarizing the influence patterns of the three key parameters studied, the optimization of droplet transport performance is achieved. The study employs two-dimensional simulation models to emulate the droplet motion under the influence of the electric field, investigating the OEW droplet transport mechanism. The continuous movement of droplets involves three stages: initial wetting, continuous transport, and reaching a steady position. The findings contribute theoretical support for the efficient design of digital microfluidic devices for OEW droplet movement and the selection of key parameters for droplet manipulation.

Elastic Droplet-Based Magnetoelectric Generator for High-performance Energy Collection

Conventional magnetoelectric generators are regarded as effective devices for harvesting concentrated hydraulic power but are ineffective for dispersed hydropower (e.g., raindrops) due to their bulkiness and immobility. Here, we propose a superhydrophobic magnetoelectric generator (MSMEG) based on an elastic magnetic film that can efficiently convert the energy of lightweight water droplets into electricity. The MSMEG consists of five parts: a superhydrophobic magnetic material-based film (SMMF), a coil, a NdFeB magnet, an acrylic housing, and an expandable polystyrene (EPS) base. The SMMF with coil can deform/recover when droplets impact/leave the MSMEG, resulting in a peak current, peak charge density, and peak power density of ∼13.02 mA, ∼1826.5 mC/m2, and ∼1413.0 mW/m2, respectively, with a load resistance of 47 Ω. Related working mechanism is analyzed through Maxwell numerical simulation, which is used for further guidance on increasing the electrical output of the MSMEG. Furthermore, the MSMEG can quickly charge a commercial capacitor with 2.7 V/1 F to 1.18 V within 200 s and power diverse electronic devices (e.g., light emitting diodes (LEDs), fans) with constant excitation by water droplets. We believe that such an MSMEG is expected to provide a promising strategy for efficiently harvesting dispersed raindrop energy.

Robust reverse-electrowetting based energy harvesting on slippery surface

Reversed-electrowetting based droplet electricity generator (REWOD-DEG) shows merits in high power densities, tunable output formats, and wide adaptability to diverse mechanical energies. However, the surface charge trapping and dielectric failure, which are also common challenges for electrowetting system, hinders the development of reliable REWOD-DEGs for long-term running. We innovatively introduce a slippery lubricant-infused porous surface (SLIPS) into REWOD-DEG. Benefits from the significant inhibitory effect for surface charge trapping and ambient contamination, self-healing characteristic given by SLIPS, and robust reversed-electrowetting based energy harvesting were achieved. The SLIPS enhanced REWOD-DEG experienced 100 days of intermittent energy harvesting without deterioration. In addition, the device shows robust performances when exposed to a variety of extreme working conditions, like low temperature, pH, humidity, fouling, and even scratching. This work may address the core application challenges of REWOD based devices, and inspire the development of other robust droplet-based electricity generators.

Tuning Surface Molecular Design of Porous Carbon for Blue Energy Harvesting

Capacitive mixing is a promising blue energy technology due to its membrane-free electricity generation and long electrode life cycle. However, because of limited performance, existing systems do not lend themselves to practical implementation. Although it is a crucial factor directly influencing electrode behavior, surface chemistry has largely been overlooked in capacitive mixing. Here, we show that manipulating surface functionalization alone can tune the responses of electrodes to produce a high voltage rise without altering the pore structure of the electrodes. Our findings reveal that the spontaneous electrode potential of a surface-modified carbon electrode shifts negatively proportional to the surface charge due to the surface groups, which explains why and how manipulating the surface chemistry can improve the power generation capacity. Using electrodes fabricated with identical activated carbon material but with different surface treatments, we have achieved a remarkably high power density of 166 mW/m² delivered to an electrical load under a 0.6 M to 0.01 M salinity gradient, with the total power generated of 225 mW/m². The corresponding volumetric power densities were 0.88 kW/m³ net and 1.17 kW/m³ total. The volumetric power density of our prototype is comparable to or better than those of prevailing membrane technologies, such as pressure retarded osmosis and reverse electrolysis, whose volumetric power density values are 1.1 kW/m³ and 0.16 kW/m³, respectively. In the seawater stage, the net power density reached 432 mW/m² or 2.3 kW/m³. Such performance far exceeds existing membrane-free systems, with the highest reported power density of 65 mW/m² under a 0.5 M to 0.02 M salinity gradient (121 mW/m² in this work). The device demonstrated unparalleled durability, maintaining 90% of the maximum energy capacity after 54,000 charge-discharge cycles.

Influence of the Ground Electrode on the Dynamics of Electrowetting

The ability to manipulate a liquid meniscus using electrowetting has many applications. In any electrowetting design, at least two electrodes are required: one forms the field to change the contact angle and the other functions as a ground electrode. The contribution of the ground electrode (GE) to the dynamics of electrowetting has not yet been thoroughly investigated. In this paper, we discovered that with a bare ground electrode, the contact angle of a sessile drop increases instead of decreases when a direct current (DC) voltage varying from zero to the threshold voltage is applied. This phenomenon is opposite to what occurs when the GE is coated with a dielectric, where the contact-angle change follows the Lippmann-Young equation above the threshold voltage of electrowetting. However, this behaviour is not observed with either a dielectric-coated electrode using direct current (DC) or a bare ground electrode using alternating current (AC) voltage electrowetting. This study explains this phenomenon with finite element simulation and theory. From previous research work, the ground electrode configuration is inconsistent. In some studies, the ground electrode is exposed to water; in other studies, the ground electrode is covered with dielectric. This study identified that an exposed ground electrode is not required in electrowetting. Moreover, this research work suggests that for applications where precise control of the contact angle is paramount, a dielectric-coated ground electrode should be used since it prevents the increase in the contact angle when increasing the applied potential from zero to the threshold voltage. This study also identified that contact angle hysteresis is lower with a Cytop-coated ground electrode and DC voltage than with a bare ground electrode using AC or DC voltages.

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.

Ultrastretchable Organogel/Silicone Fiber-Helical Sensors for Self-Powered Implantable Ligament Strain Monitoring

Implantable sensors with the abilities of real-time healthcare monitoring and auxiliary training are important for exercise-induced or disease-induced muscle and ligament injuries. However, some of these implantable sensors have some shortcomings, such as requiring an external power supply or poor flexibility and stability. Herein, an organogel/silicone fiber-helical sensor based on a triboelectric nanogenerator (OFS-TENG) is developed for power-free and sutureable implantation ligament strain monitoring. The OFS-TENG with high stability and ultrastretchability is composed of an organogel fiber and a silicone fiber intertwined with a double helix structure. The organogel fiber possesses the merits of rapid preparation (15 s), good transparency (>95%), high stretchability (600%), and favorable stability (over 6 months). The OFS-TENG is successfully implanted on the patellar ligament of the rabbit knee for the real-time monitoring of knee ligament stretch and muscle stress, which is expected to provide a solution for real-time diagnosis of muscle and ligament injuries. The prepared self-powered OFS-TENG can monitor data on human muscles and ligaments in real-time.

Self-Powered Galvanic Vibration Sensor

The development of the IoT demands small, durable, remote sensing systems that have energy harvesters and storage. Various energy harvesters are developed, including piezoelectric, triboelectric, electromagnetic, and reverse-electrowetting-on-dielectric. However, integrating energy storage and sensing functionality receives little attention. This paper presents an electrochemical vibration sensor with a galvanic cell (Zn-Cu cell) as energy storage and a vibration transducer. The frequency response, scale factor, long-term response, impedance study, and discharge characteristics are given. This study proved the possibility of integrating energy storage and vibration sensing functionality with promising performance. The performance of the sensor halved within 74 min. The longevity of the sensor is short due to the spontaneous reactions and ions drained. The sensitivity can be restored after refilling the electrolyte. The sensor could be rechargeable by changing to a reversible electrochemical system such as a lead–acid cell in the future.

A self-powered wireless motion sensor based on a high-surface area reverse electrowetting-on-dielectric energy harvester

This paper presents a motion-sensing device with the capability of harvesting energy from low-frequency motion activities. Based on the high surface area reverse electrowetting-on-dielectric (REWOD) energy harvesting technique, mechanical modulation of the liquid generates an AC signal, which is modeled analytically and implemented in Matlab and COMSOL. A constant DC voltage is produced by using a rectifier and a DC-DC converter to power up the motion-sensing read-out circuit. A charge amplifier converts the generated charge into a proportional output voltage, which is transmitted wirelessly to a remote receiver. The harvested DC voltage after the rectifier and DC-DC converter is found to be 3.3 V, having a measured power conversion efficiency (PCE) of the rectifier as high as 40.26% at 5 Hz frequency. The energy harvester demonstrates a linear relationship between the frequency of motion and the generated output power, making it highly suitable as a self-powered wearable motion sensor.

Advancing Reverse Electrowetting-on-Dielectric from Planar to Rough Surface Electrodes for High Power Density Energy Harvesting

Reverse electrowetting-on-dielectric (REWOD)-based energy harvesting has been studied over the last decade as a novel technique of harvesting energy by actuating liquid droplet(s) utilizing applied mechanical modulation. Much prior research in REWOD has relied on planar electrodes, which by its geometry possess a limited surface area. In addition, most of the prior REWOD works have applied a high bias voltage to enhance the output power that compromises the concept of self-powering wearable motion sensors in human health monitoring applications. In order to enhance the REWOD power density resulting from an increased electrode-electrolyte interfacial area, high surface area electrodes are required. Herein, electrical and multiphysics-based modeling approaches of REWOD energy harvester using structured rough surface electrodes are presented. By enhancing the overall available surface area, an increase in the overall capacitance is achieved. COMSOL and MATLAB-based models are also developed, and the empirical results are compared with the models to validate the performance. Root mean square (RMS) power density is calculated using the RMS voltage across an optimal load impedance. For the proposed rough electrode REWOD energy harvester, maximum power density of 3.18 μW cm^-2 is achieved at 5 Hz frequency, which is ≈4 times higher than that of the planar electrodes.

UV-Vis Spectrophotometric Analysis of DNA Retrieval for DNA Storage Applications

Informational Deoxyribonucleic Acid (iDNA) has gained the attention of many researchers and pioneer companies for the development of novel storage systems for the long-term and high-density storing of information. This research focuses on the physical storage of iDNA strands to address some of the current challenges by evaluating the accuracy of the process of iDNA retrieval from the surface after the dehydration process. For this aim, a UV-Vis spectrophotometric technique was used to measure the concentration of the DNA samples. Although spectroscopy has been widely employed for the evaluation of DNA concentration and contamination in a solution, it has not been used to investigate dry-state DNA, which is one of the preferred storage formats for the long-term retention of information. These results demonstrate that the UV-Vis spectrophotometric technique can be used to accurately measure dry-state DNA before the retrieval and its residues after the DNA retrieval process. This paper further examines the storage/retrieval process by investigating the relationship between the storage time and the amount of retrieved DNA or the DNA residue left on various surfaces. Based on the experimental results demonstrated and discussed in this paper, UV-Vis spectrophotometry can be used for monitoring dry-state DNA with a high accuracy larger than 98%. Moreover, these results reveal that the hydrophilicity and hydrophobicity of the surface do not significantly affect DNA retrieval over a one-month time period.