REVERSE ELECTROWETTING

SUBMITTED BY:

TARUN DODA

WHO DISCOVERED IT?

Professor Tom N. Krupenkin:- Tom Krupenkin is Associate Professor, Nanotechnology Cluster and Mechanical Engineering, at University of Wisconsin-Madison.

Dr. J. Ashley Taylor:- J. Ashley Taylor received his BA in mathematics at the University of Texas in 1971 and his Ph.D. at the University of Houston

WHY WAS IT NEEDED?•Over the last decade electrical batteries have emerged as a critical bottleneck for portable electronics development. •High-power mechanical energy harvesting can potentially provide a valuable alternative to the use of batteries.• Existing methods of mechanical-to-electrical energy conversion such as electromagnetic, piezoelectric, or electrostatic are not well suited as their output remains in the microwatt to hundreds of milli watt range. •This new approach of reverse electrowetting has a number of significant advantages, over existing mechanical-energy-harvesting technologies, including very high power densities, up to 103 W m−2.

ELECTROWETTING CONCEPT

The electrowetting effect has been defined as "the change in solid-electrolyte contact angle due to an applied potential difference between the solid and the electrolyte". The fringing field at the corners of the electrolyte droplet tend to pull the droplet down onto the electrode, lowering the macroscopic contact angle and increasing the droplet contact area. In simple words there is electrical-to-mechanical energy conversion.

ELECTROWETTING

VIDEO SHOWING ELECTROWETTING

WHAT IS REVERSE ELECTROWETTING?

•Here we employ a new approach that allows one to run this process in reverse, converting mechanical energy of liquid motion into electrical current.•The droplet and the electrode are connected to the external electrical circuit that provides a constant bias voltage between the droplet and the electrode.• External mechanical actuation is used to move the droplet in such a way as to force a decrease of its overlap with the dielectric-film-coated electrode.• This results in the decrease of the total charge that can be maintained at the droplet liquid–solid interface.• The excessive electrical charge then flows back through the electrical circuit that connects the droplet and the electrode, generating electrical current that can be used to power the external load.

Shows in greater detail schematics of reverse-electrowetting-based energy generation process in a microchannel geometry.

DROPLET ACTUATION METHODS

a) DROPLETS BETWEEN OSCILLATING PLATES.

b) DROPLETS BETWEEN SLIDING PLATES.

c) DROPLETS IN A MICROCHANNEL.

There are three methods of droplets actuation:-

1) DROPLETS BETWEEN OSCILLATING PLATES

2) DROPLETS BETWEEN SLIDING PLATES

3) DROPLETS IN A MICROCHANNEL

CIRCUIT TO IDENTIFY ENERGY GENERATION

The electric circuit used to investigate energy generation is identical for all three set-ups. This circuit includes a source of a constant bias voltage V, a resistive load R and a variable capacitor C, (the REWOD unit, which represents a harvester set-up, that is, a set of droplets in contact with the electrode grid). The voltage drop across the resistive load was captured by the data acquisition board and converted into electrical current allowing direct calculation of the generated power. A battery (with internal resistance of about 1 Ω) was used to provide a bias voltage.

APPLICATIONS OF REVERSE ELECTROWETTING

a) Walk to charge:- The power that can be produced by a footwear-embedded microfluidic harvester using the REWOD process. The average power per foot can exceed 2 W for bias voltages in excess of 35 V and 10 W for bias voltages in excess of 75 V. The bias voltage can be substantially reduced by increasing the capacitance of the dielectric film stack.

There is enough power, according to the researchers, to charge a standard mobile phone or laptop. Getting the energy from the device to the handset presents another challenge. One way is to plug a USB cable into the shoe.The military could put it in boots and cut way back on the number of batteries a typical soldier has to carry. Right now they have to walk around with up to 20 pounds worth to power various electronic devices such as night vision goggles, laptops and GPS units.

b) Harvesting Of Mechanical Vibration Energy:-• Currently, the majority of experimental vibration harvesters have output power in the range from 10−6 to 10−2 W.

•One example of the REWOD-based vibration harvester device consists of an array of conductive droplets squeezed between two dielectric-coated electrodes

•Mechanical vibration of the load device causes periodic change in the solid–liquid contact area and, thus, electrical current generation.

ADVANTAGES

The above examples illustrate new possibilities in portable high-power energy harvesting that can be opened by utilizing the REWOD process, thus its advantages are:

• High-power energy harvesting can potentially provide a valuable alternative to the use of batteries.

• Even though energy harvesting is unlikely to completely replace batteries in the majority of mobile applications, it can have a very important role in reducing cost, pollution, and other problems associated with battery use.

ANY QUERIES…???