Reverse Electrowetting Research Articles

Hybrid energy harvesting by reverse di-electric on a piezo-electric generator with thermo-couple and monitoring in WSN

Smart renewable energy harvesting has been implemented from hybrid sources such as solar and wind. The wireless sensor node is created for monitoring surface water. In the intelligent building, electrical energy is harvested from the hybrid source of solar and wind energy. The source energy was selected for the harvesting process by using a fuzzy controller. In this proposed method, piezo-electric reverse electro-wetting on di-electric energy harvesting is proposed where constant DC voltage is generated by a rectifier. A DC-DC converter is designed to power up the remote read-out sensor. The produced charge is transformed by a charge amplifier with the proportion of output voltage that is delivered to the wireless receiver. The harvested DC voltage varies with the temperature and external environmental effect. In our work, we obtained 6 × 10−3 W/m2 of voltage and this harvested energy is monitored using the Internet of Things (IoT) by the proposed EHOR (Energy Harvested Optimized Routing) algorithm.

Mechanical energy harvesting using combined reverse electrowetting and electromagnetic method

Mechanical energy harvesting using combined reverse electrowetting and electromagnetic method

Effect of frequency on droplet actuation in reverse electrowetting

In this work, a theoretical formula based on equivalent circuit model is proposed to describe the influence of frequency on the droplet actuation behavior in reverse electrowetting. To verify the theory, changes of droplet profile are monitored by charge coupled device (CCD) camera and simulated by COMSOL. It shows that the threshold voltage required for the droplet to start contracting inward increases firstly and then decreases with the increasing frequency. When the frequency is 1 kHz, the threshold voltage reaches the maximum value. Once the droplet begins to move inward, its contact angle increases with the working voltage. As for 4 kHz, the contact angle increases linearly from 60° to 154° when voltage ranges from 16 V to 50 V, while the droplet remained stationary when < 16 V. The relative experimental results well accord with the theoretical prediction.

A hybrid piezoelectric and electrostatic energy harvester for scavenging arterial pulsations

Implantable and wearable biomedical devices suffer from a limited lifespan of on-board batteries which require change causing physical discomfort. In order to overcome this, various energy harvesters have been developed as the human body possesses several types of energy available for scavenging through appropriately designed energy harvesting devices, while the cardiovascular system in particular represents a constant reliable source of mechanical energy from vibration. Most conventional energy harvesters exploit only a single phenomenon, such piezo- or triboelectricity, thus producing reduced power density. As an improvement, hybridisation of energy harvesters intends to negate this drawback by simultaneously scavenging energy by multiple harvesters. In the present work, the reverse electrowetting on dielectric (REWOD) phenomenon is combined with the piezoelectric effect in a proof-of-concept hybrid harvester for scavenging biomechanical energy from arterial or other type pulsations. A mathematical model of the harvester was developed; and, an investigation using computational fluid dynamics simulations was carried out using the COMSOL Multiphysics software. The effect of the materials of piezoelectric film and geometrical features of the harvester on parameters such as the displacement, the frequency of pulsations and the energy produced were studied. An experimental setup that could model the time-varying pressures and displacements caused from arterial pulsations was designed and the characteristics of the produced piezoelectrical energy were analysed. A comparison between experimental and computational data was carried out demonstrating a good agreement. The dependencies between geometrical parameters and electrical output were determined and recommendations on piezoelectric materials and design solutions were provided.

REWoD-based vibrational energy harvesting exploiting saline-solutions loaded PAAm hydrogels on micro-structured aluminium oxides electrodes

Nowadays, sensors are among the most exploited systems in everyday life, with widespread applications stimulating increasing research. Usually, they require external power, thus adding issues such as periodic maintenance and size constraints. Energy harvesting (EH) from mechanical vibrations (VEH) can overcome such limitations: in particular, Reverse Electrowetting on Dielectric (REWoD) can provide a high-power density of ∼µW/cm2 by exploiting the mechanical modulation of the capacitance at the liquid/dielectric interface without additional external bias. Compared with other vibrational EH (VEH) techniques, REWoD harvests energy efficiently even from very low-frequency vibrations (<10 Hz,the range of human motion), and it is one of the most promising technologies for miniaturisation. Here we present a feasibility study and proof of concept for a portable VEH device, exploiting low-cost materials such as highly hydrophobic micro-structured Al oxide electrodes combined with off-the-shelf polyacrylamide (PAAm) hydrogels loaded with saline solutions. The PAAm hydrogels, thanks to heat treatment in LiCl solution, exhibit a negligible degradation compared to the typical hydrogel drying time. A laboratory prototype using 3 hydrogel hemispheres simultaneously generated an average power of ∼1.55 μW at 7 Hz with a power density of 9.1 nW/μl.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion.

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Fabrication and characterization of a flexible nanogenerator using reverse electrowetting concept

Many researchers are interested in powering sensors and electrical circuits in wireless networks through energy harvesting from environmental waste energies. In this study, a flexible nanogenerator is designed and fabricated based on the reverse electrowetting concept. The performance of the nanogenerator has been investigated in different conditions including various bias voltage, different excitation frequency, and several external loads. The nanogenerator comprises of water droplets, as a strong dipole fluid, and two dielectric layers; polymethylsiloxane polymer. The latter has good hydrophobicity and flexibility. These two dielectric layers are formed on the surface of copper electrodes by using a spin coater. It is shown that increasing the excitation frequency augments the generated power to some extent that the capacitor is not fully discharged. The nanogenerator power output increases with the external load up to equality between the external load and the nanogenerator's internal resistance. The results show that the fabricated nanogenerator can generate a power density equal to 1.08 W/m2 using 1 ml water droplet, 7V bias voltage, and an excitation frequency of 1 Hz.

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

Increasing demand for self-powered wearable sensors has spurred an urgent need to develop energy harvesting systems that can reliably and sufficiently power these devices. Within the last decade, reverse electrowetting-on-dielectric (REWOD)-based mechanical motion energy harvesting has been developed, where an electrolyte is modulated (repeatedly squeezed) between two dissimilar electrodes under an externally applied mechanical force to generate an AC current. In this work, we explored various combinations of electrolyte concentrations, dielectrics, and dielectric thicknesses to generate maximum output power employing REWOD energy harvester. With the objective of implementing a fully self-powered wearable sensor, a “zero applied-bias-voltage” approach was adopted. Three different concentrations of sodium chloride aqueous solutions (NaCl-0.1 M, NaCl-0.5 M, and NaCl-1.0 M) were used as electrolytes. Likewise, electrodes were fabricated with three different dielectric thicknesses (100 nm, 150 nm, and 200 nm) of Al2O3 and SiO2 with an additional layer of CYTOP for surface hydrophobicity. The REWOD energy harvester and its electrode–electrolyte layers were modeled using lumped components that include a resistor, a capacitor, and a current source representing the harvester. Without using any external bias voltage, AC current generation with a power density of 53.3 nW/cm2 was demonstrated at an external excitation frequency of 3 Hz with an optimal external load. The experimental results were analytically verified using the derived theoretical model. Superior performance of the harvester in terms of the figure-of-merit comparing previously reported works is demonstrated. The novelty of this work lies in the combination of an analytical modeling method and experimental validation that together can be used to increase the REWOD harvested power extensively without requiring any external bias voltage.

Charge Trapping-Based Electricity Generator (CTEG): An Ultrarobust and High Efficiency Nanogenerator for Energy Harvesting from Water Droplets.

Strategies toward harvesting energy from water movements are proposed in recent years. Reverse electrowetting allows high efficiency energy generation, but requires external electric field. Triboelectric nanogenerators, as passive energy harvesting devices, are limited by the unstable and low density of tribo-charges. Here, a charge trapping-based electricity generator (CTEG) is proposed for passive energy harvesting from water droplets with high efficiency. The hydrophobic fluoropolymer films utilized in CTEG are pre-charged by a homogeneous electrowetting-assisted charge injection (h-EWCI) method, allowing an ultrahigh negative charge density of 1.8 mC m-2 . By utilizing a dedicated designed circuit to connect the bottom electrode and top electrode of a Pt wire, instantaneous currents beyond 2mA, power density above 160 W m-2 , and energy harvesting efficiency over 11% are achieved from continuously falling water droplets. CTEG devices show excellent robustness for energy harvesting from water drops, without appreciable degradation for intermittent testing during 100 days. These results exceed previously reported values by far. The approach is not only applicable for energy harvesting from water droplets or wave-like oscillatory fluid motion, but also opens up avenues toward other applications requiring passive electric responses, such as diverse sensors and wearable devices.

High Surface Area Reverse Electroactuation Energy Harvesting

Manipulating a liquid droplet with applied electrical fields has long been studied and is known as electrowetting on dielectric (EWOD). In recent years, a new approach of energy harvesting that is related to EWOD has been demonstrated and it is termed reverse electrowetting energy harvesting or simply reverse electroactuation. Reverse electroactuation is opposite to electrowetting in which energy is generated by converting mechanical energy of the liquid motion into electrical energy. The amount of energy generated using this technique mainly depends on the contact area of the electrode-electrolyte interface. The contact area can be varied by applying a bias voltage to decrease the surface tension of the electrolyte resulting into a surface area increase. As an alternative, the contact area can also be varied by applying pulsating external pressure. In this study, we plan to model and experimentally demonstrate an AC current generation by applying pulsating external pressure to move the electrolytic fluid in and out of porous materials with high surface to volume ratio. The model is based on a simple solution to the Washburn equation which establishes the pressure dependency of fluid height in the pores. Various porous materials with both uniform and non-uniform pores are used in the analysis.

Energy harvesting from aperiodic low-frequency motion using reverse electrowetting.

Mechanical energy harvesting can provide a promising alternative to electrochemical batteries, which are currently widely utilized to power mobile electronics. In this work we present a theoretical analysis of a recently proposed method of mechanical energy harvesting, which combines a reverse electrowetting phenomenon with the fast self-oscillating process of bubble growth and collapse. We investigate the details of the bubble dynamics and analyze the dependence of the energy generation process on the system parameters. The results demonstrate that self-oscillation frequencies of several kHz are possible, which can lead to very high power generation densities in excess of 104 W m-2. The obtained results indicate the possibility of high-power energy harvesting from mechanical energy sources with very low frequencies, well below 1 Hz.

Self‐Powered Multimodal Temperature and Force Sensor Based‐On a Liquid Droplet

AbstractHerein we report a self‐powered multimodal temperature and force sensor based on the reverse electrowetting effect and the thermogalvanic effect in a liquid droplet. The deformation of the droplet and the temperature difference across the droplet can induce an alternating pulse voltage and a direct voltage, respectively, which is easy to separate/analyze and can be utilized to sense the external force and temperature simultaneously. In addition, an integral display system that can derive information from external temperature/force concurrently is constructed. Combined with advantages of excellent sensing properties and a simple structure, the droplet sensor has promising applications in a wide range of intelligent electronics.

Self-Powered Multimodal Temperature and Force Sensor Based-On a Liquid Droplet.

Herein we report a self-powered multimodal temperature and force sensor based on the reverse electrowetting effect and the thermogalvanic effect in a liquid droplet. The deformation of the droplet and the temperature difference across the droplet can induce an alternating pulse voltage and a direct voltage, respectively, which is easy to separate/analyze and can be utilized to sense the external force and temperature simultaneously. In addition, an integral display system that can derive information from external temperature/force concurrently is constructed. Combined with advantages of excellent sensing properties and a simple structure, the droplet sensor has promising applications in a wide range of intelligent electronics.

High-performance reverse electrowetting energy harvesting using atomic-layer-deposited dielectric film

Reverse electrowetting-on-dielectric (REWOD), which is a novel technology for energy harvesting, was demonstrated by depositing Al2O3 via atomic layer deposition (ALD). Specifically, thin layers of Al2O3 about 100nm-thick were successfully formed by ALD to obtain dense films with high capacitances. REWOD with the ALD Al2O3 thin films exhibited high performances under low-voltage and low-excitation-frequency conditions. The maximum power density was 11mW/cm2 with a DC bias of 24V and an external excitation frequency of 2Hz. Moreover, ALD and sputtered thin films were compared by investigating their capacitances and leakage current densities. Due to the differences between the film densities and growth methods, the ALD thin films exhibited higher capacitances and lower leakage current densities. It was also determined that the leakage current affects the REWOD energy harvesting performance. By analyzing these features, it was confirmed that ALD thin films are suitable for REWOD energy harvesting.

Bubbler: A Novel Ultra-High Power Density Energy Harvesting Method Based on Reverse Electrowetting.

We have proposed and successfully demonstrated a novel approach to direct conversion of mechanical energy into electrical energy using microfluidics. The method combines previously demonstrated reverse electrowetting on dielectric (REWOD) phenomenon with the fast self-oscillating process of bubble growth and collapse. Fast bubble dynamics, used in conjunction with REWOD, provides a possibility to increase the generated power density by over an order of magnitude, as compared to the REWOD alone. This energy conversion approach is particularly well suited for energy harvesting applications and can enable effective coupling to a broad array of mechanical systems including such ubiquitous but difficult to utilize low-frequency energy sources as human and machine motion. The method can be scaled from a single micro cell with 10−6 W output to power cell arrays with a total power output in excess of 10 W. This makes the fabrication of small light-weight energy harvesting devices capable of producing a wide range of power outputs feasible.

Charge injection in dielectric films during electrowetting actuation under direct current voltage

Electrowetting-on-dielectric technology suffers from a lack of accuracy and reliability under DC voltage. The root of this phenomenon is related to charge injection within dielectric materials polarized and in contact with electrolytes. Correlation between contact angle measurements and transient current is used to investigate electrowetting stability under DC voltage. In particular, nature of ionic species and polarization effect are scrutinized. Asymmetric mechanisms appear between positive and negative polarizations from both wetting stability and leakage current, attributed to charge injection and allowing electrets-type actuation. A first occurrence of large amplitude reverse electrowetting is also observed.

Visualization study on the behaviors of oscillating liquid bridge between two parallel plates with surface treatments

For more than a decade, energy harvesting devices have been rapidly developed based on the electrical energy conversion techniques from kinetic energy using piezoelectric, electrostatic, and electromagnetic principles. However, the amount of power is insufficient for practical products. Recently, a new concept of energy generation, the reverse electrowetting technology has been invented. This method demonstrates that an oscillating water bridge between two parallel conducting plates can generate an AC electric power. The electrical double-layer capacitors formed on the two interfacial areas of a water bridge and plates are continuously charged and discharged when the shape of the bridge between two plates while the bottom plate is oscillating. For this research, we treat surfaces of each plate hydrophilic and hydrophobic, give variety to vibration frequency, amplitude and droplet size. Though the images of liquid bridge taken by a high-speed camera, the size of contact area is measured and flow field is observed by PIV, then we find a correlation with output voltage signal. This observation could contribute to more accurate prediction of the behaviors of oscillating droplets and development of micro-fluidic power generation devices.

A Parametric Study of Microfluidic Power Generator Based on Reverse Electrowetting in a Microchannel Geometry

The micro power generators that depend on converting the ambient energy into usable electrical energy for portable electronic devices have attracted much attention in recent years. A novel microfluidic power generator which is based on reverse electrowetting in a microchannel geometry was presented by Tom Krupenkin and J. Ashley Taylor. This generator overcomes the defects such as low output and low resource utilization of traditional energy converters. Understanding the parameters influencing output of this generator is a key to enhance the power output of it. This paper discussed the effects of factors on generator output by numerical simulation. The results show that promising performance of REWOD-based power generator can be achieved by choosing a thinner dielectric film with high dielectric constant and high break strength, and increasing the droplet oscillation frequency.

Correction: Corrigendum: Reverse electrowetting as a new approach to high-power energy harvesting

Nature Communications 2, Article number: 448 (2011); Published: 23 August 2011; Updated: 7 February 2012 This article was published without any declaration of competing financial interests. A revised competing financial interests statement is provided below: The authors declare competing financial interests.

Microfluidic circuits harvest mechanical energy

Embedded in a pair of shoes, circuits composed of a train of conductive droplets could generate a few watts of power—enough to charge a cell phone from a casual stroll.