A large vertical sheet of acrylic plastic works well. So does styrofoam plastic. Don't use wood for the supports, it's too conductive. Fasten the collectors and inducers to the plastic sheet with screws or silicone caulk, or make holes in the sheet and tie them to the sheet with string or wire. Some people have used plastic rods or plastic strips to support things. Other people use plastic water pipes.
The plastic must be clean and dry. The inducers and collector cans must be spaced away from each other by several inches horizontally and vertically.
The lower collectors must be kept away from the table surface. Bare wires are used to cross-connect the four cans. These two diagonal wires must be far from any other conductive object, and the wires must not touch together.
Use bare wire, this will let you create sparks between the wires, or to later flash a NE-2 neon bulb. Connect the ends of the diagonal wires directly to the metal of the cans. If you use plastic-covered wire, strip off the plastic coating from an inch of each end of the wire. You can use tape to hold the wire against the metal, as long as the wire touches the bare metal directly not, for example against the painted part of a coffee can.
Alligator clipleads bought from Radio Shack stores work well for this. Or poke a hole in the metal near the edge of the can, stick the wire through the hole, and bend it and tape it so it doesn't fall out. The upper rings or cans must be positioned near the place where the water droplets break from the rest of the water.
If the droplets break away right at the tip of the nozzle, then put the nozzles within the upper cans. If a solid stream of water comes from the nozzle and breaks up further down, then move the nozzels up, so that the water-break spot is inside the upper inducer cans.
For the Drippers, you can use glass or plastic "pipettes". You want to have a very small hole, so that the dripper makes lots of tiny droplets. If you cannot get a pipette, try poking holes in a plastic container. See below. Adjust the water flow so it drips VERY fast. The faster, the better. Even better, use lots of drippers instead of just one. After the device runs for a minute, touch one of the coffee cans gently with a finger and listen for the tiny snap of a "static" spark.
If you don't hear a spark, the machine is running weakly or not al all. See the " debugging "" section near the end of this article. If you can't hear any spark, you can try detecting sparks electronically with an AM radio. Place a radio a foot away from your generator, tune it between AM stations or tune it to a very weak station , and turn the volume up. Run your Kelvin machine. Touch one of the cans with your finger and listen to the radio.
You should hear a "snap" noise as your finger touches the metal. Touch one upper can, then the other, then the first one again. You should hear a "snap" each time. Even when sparks are too small to hear or see, a radio will sometimes still detect them. Normal flashlight bulbs won't work, you need a small neon "pilot light" bulb instead. Obtain an "NE-2" or other similar neon light, the kind which looks like a short glass tube with two parallel wires inside and two bare wires sticking out of the glass.
Hold the bulb by one wire, look at the tube, then use the other wire to touch one of the cans of your Kelvin device. You should see a dim orange flash inside the bulb. It might help to turn off the lights and work in a dimly lit room. Hold one bulb wire, then use the other wire to touch the positive can, then the negative can, then the positive, and you should see a tiny orange flash each time. It will short out the generator and prevent high voltage buildup. You can make the generator automatically flash the neon bulb by making a "spark gap".
First twist one wire from the neon bulb around one of the generator's diagonal wires, then bend the bulb wires so other short wire from the neon bulb is very close to the other generator wire but not touching. Small sparks will occasionally jump across the small gap and flash the neon. The smaller the gap, the faster and dimmer the flashing.
If it works, increase the distance to get slow, bright flashes. Also, eliminate the sharp wire tip of the bulb by bending tip over to form a little ring. The sparks then jump to the edge of the little ring. This sometimes lets the voltage rise higher before a spark jumps, which eliminates any air-leakage and lets the bulb flash more brightly.
Put tape only at the top of each strip of tissue so the strip hangs down against the side of the can. When the can charges up, the strips should lift slightly outwards.
The higher the voltage, the farther the strips move. When a spark jumps, the strips jerk because the repulsion force suddenly becomes less. As the stored energy grows, the water has to do more and more work every time it adds a bit more imbalanced charge to the cans. The electrified droplets feel a repulsion force as they fall towards the alike-charged lower cans.
As the voltage increases, the droplets will fall more and more slowly. The sound of the splashing water will change. The droplets may even start bending their paths, even occasionally falling upwards!
How to get the water out of the cans without discharging them? Here's my addition to the classic Kelvin Waterdropper: use the "faraday ice pail" effect, where a conductive hollow object always has no charge-imbalance on its inside. If water falls in a solid stream, the cans will discharge and the generator will stop working.
So, drill lots of tiny drip-holes in a flat plate? Or even better, make your generator look like a VandeGraaff Machine. Make a metal sphere using two 14" Ikea Stainless Steel salad bowls. Carve holes in the top and bottom to pass a "shower" coming down from the inducer-rings. Suspend a plastic bucket inside, using heavy nylon fishline. The center of the poly bucket is perf by many tiny 60 drill holes.
Run a wire between the inside of the metal sphere and the water inside the bucket. When the charged droplets from a "shower head" pour through this metal sphere, they're intercepted by the plastic bucket and wadded plastic screening to prevent splashing, and all their electric charge is grabbed by the metal sphere. The bucket then streams the water out in a narrow shower of small uncharged droplets, which pass through the hole in the bottom.
Water never touches the metal sphere. Problem: if the sphere gets charged up to huge voltage, the top stream of droplets will repel and fly outwards. The upper hole may need to be larger than the lower.
Cover the holes' sharp edges with slitted hose filled with RTV silicone. Or, even simpler, install a cone-shaped piece of metal window screen inside a bottomless can, so the water droplets touch the screen and continue through.
Make sure the screen is centered vertically within the can, so that the point of the cone doesn't extend past the lower lip of the can. Don't let the water drip from the edge of the can, otherwise it will carry charge away with each drop. Or, you can stack all four parts of one Kelvin device in a single row, for an in-line waterdropper generator. Note that the Inline version is more tricky to make work.
Build the above device first before attempting the one below. The water supply need not be a "dripper", it can be a high velocity spray, as long as the water jet divides into droplets, not a contiguous stream. And multiple jets can be used, sort of like a shower head. The faster the flow and the larger the number of separate streams, the higher the total output current. Higher current gives faster recharge rate after a spark, and it lets the generator self-start more reliably when humidity is high.
Use halves of VandeGraff spheres, the halves with the holes. Or maybe use metal gal drums. But those drums may have sharp edges, and we can't attain millions of volts if the edges aren't bulbous. A foil-covered truck innertube should support about a million volts before air-corona leakage stops the voltage from rising any higher.
Or do as Tesla-coilers do, and make a skeletal torus from bent, coiled pipe. I formed cone-shape screens, then punched in successive circles to form a concentric "ripple" pattern in the screen. This stops water from running off the cone-tip as a long stream which would discharge the whole thing. Also keeps the sharp cone tip hidden within the metal cleft: electrically shielded.
DON'T let water run along the torus and drip from the bottom. NEWS: I suspended a bundt-pan by fishlines, then sprayed water through the center, so that the water did not touch the metal.
I used a garden hose with a "water breaker" a sort of shower head attachment. I charged up the bundt pan with a 10KV power supply, and then measured the electric current between a collector pot and ground. It was 2. This doesn't sound like much, but it's at lease as much as some VandeGraaff generators can create.
I found that if I disconnected the power supply from the bundt pan, the current did not vanish. The charge on the bundt pan stayed the same as I watched for about 30sec. Invented in the s, it is an ingenious sort of electrostatic generator. In this device, water from a single source is directed into two separate Metal Buckets via tubing ending in small nozzles.
The flow of water is adjusted precisely so that the water rapidly falls in droplets, rather than in a continuous stream, through Metal Rings located above each bucket. The rings are electrically attached to the bucket on the opposing side; there is no contact between the wires.
Each bucket also is connected to a ball-tipped Discharge Rod positioned so that it is only a short distance away from the rod on the other bucket.
Periodically a spark will jump across the gap in the conductors when the potential difference between the two buckets can no longer be maintained. This gap can be adjusted using the Discharge Rod Separation slider; the closer the rods, the more quickly the build-up of charge will be discharged. The difference in potential that develops is related to the ions charged particles present in water. Water does not exhibit an overall charge under normal circumstances, but contains many ions from salts dissolved in the liquid and the dissociation of water itself.
Some of the ions are positively charged cations and others are negatively charged anions , so that they typically balance each other out. Figure 3 shows the outputs of the MKWDs with two different microchannel diameters using various flow rates. The duration of each experiment depends on the volumetric capacity of the syringe pump and the flow rate. Figure 3 only shows the data collected from the negative electrodes of MKWDs. Two phenomena were observed from the experiments.
First, for both MKWDs the output voltages increased as the flow rate increased. The charge collectors in the MKWD can be considered as a capacitor.
Therefore, in the experiments, higher flow rate led to a larger charging current and faster charging rate. As a result, a higher voltage gain and shorter charging time were achieved. Second, the output voltages plateaued at certain voltage levels. As the voltage increased, the charge leakage became more and more significant. The output voltage can only reach a point where the leaking current counters the charging current.
This means that the net gain of charges vanishes and therefore the output voltage remains constant. The highest output voltages from both MKWDs were obtained with the highest flow rate at It was an unexpected and interesting phenomenon that the microchannel dimension was found to have a huge impact on voltage output. A possible explanation for this is that, for electrostatic sprays, a larger flow rate results in a much larger droplet size, and thus may cause a lower amount of charge carriers [ 36 ].
However, the sizing effect on the output of MWKDs requires further investigation. Nevertheless, it was successfully demonstrated that MWKDs can provide ultra-high and stable voltage output for electrowetting. Output voltages of the negative electrodes of two MKWDs over a period up to s. The calculated average flow speeds were 2. It was found that the charge leakage becomes a dominant factor that limits the ultimate output voltage of the MKWD.
Ambient humid air and insulating materials are the main causes of charge leakage. Water vapor in air has a tendency to remove charges from objects. Higher humidity means higher water content in air, which results in an increase of its electrical conductivity, making discharging easier [ 37 ].
People usually observe electrostatic sparks or shocks in winter, because the humidity reaches its lowest level in winter, which minimizes discharge. The insulating materials and their thickness also affect the voltage output.
Although the Ohmic resistance of an insulator is high, the leaking current may become significant if the voltage increases to a significant level. In addition, if dielectric breakdown of the insulator occurs, it may cause a catastrophic failure of insulation and thus generate unsteady voltage output.
Therefore, in order to obtain a high voltage and stable output, humidity control and appropriate electrical insulation are critical. Specifically, in our experiments, humidity was maintained using an electrical fan and monitored by a humidity meter. Thick layers of liquid electrical tape were applied to provide good electrical insulation.
Water droplets on a dielectric substrate will deform under an applied electric field. The deformed shape is attributed to several factors, including the strength of the electric field, the thickness of the dielectric substrate, the placement of electrodes, and the ambient environment that may cause discharge.
In the present research, note that the voltage measurements were conducted beneath the counter electrode Ti pellet instead of the working electrode steel needle , where accurately measuring the surface voltage was difficult. Figure 4 shows the voltage data blue line recorded from the counter electrode during a typical electrowetting test. The operating conditions are shown in Experiment B in Table 1.
The dotted line was obtained from the curve fitting of the experimental data to obtain a time-dependent function of voltage for simulation. Eight check points were selected to compare the experimental versus simulation results of the droplet deformation during the electrowetting test.
Voltage measurements solid blue line during electrowetting and a curve dotted black line that fits the experimental data. The left halves of the photos in Figure 5 show the time-lapse images of the electrowetting experiment, corresponding to the eight check points selected.
It was observed that the droplet remained almost unchanged until time passed point b, after which the deformation became more and more obvious. This was the reason why more check points were selected towards the end. Physically, electrowetting begins when the electric repulsion force starts to overcome the initial surface tension between the water and the PDMS substrate.
The MWKD was used to charge the water droplet continuously through the needle, resulting in a hike of electric repulsion force. The net outcome was that the water droplet kept wetting more areas of the PDMS substrate. Another phenomenon observed in most experiments was that the electrowetting process only lasted up to 60 s, after which the droplet was ejected out like a bullet shown in the supplemental video.
This was because the cumulative charge imbalance overcame the overall attraction from the PDMS substrate. Even though the needle was carefully placed into the center of the droplet, ejection could hardly be prevented. In real applications, constraints such as a retention wall can be fabricated around the desired wetting area to retain the liquid. Time-lapse images of the experimental results left halves of the images versus the simulated water droplet deformation right halves of the images during the electrowetting test shown in Figure 4.
The right halves of the photos in Figure 5 show the CFD simulation results of those eight selected timepoints shown in Figure 4. The animation of the whole simulation can be find in Supplemental Video S1. Because the applied voltage for electrowetting was obtained from the MWKD, a time-dependent voltage function was adapted in the simulation, unlike other research that used constant voltage values [ 25 ].
It can be seen that the simulation results closely match the experimental observations. The slight discrepancy between two images at point b was believed to be caused by the initial tension between the needle and water, which was not considered in the model. In general, the present model well predicted the behaviors of the droplet during electrowetting.
Figure 6 shows the change in the contact angle from the experiment and the simulation shown in Figure 4. The errors were calculated as follows:. The errors in the simulation were more likely caused by the discrepancies from the curve fitting of the experimental data points.
Change of the contact angle during electrowetting in the experiment versus the simulation, shown in Figure 4. Microfluidic Kelvin water droppers were built in house to evaluate their feasibility for use in electrowetting. Ultra-high and stable voltage outputs were obtained from the MKWDs. The best performed MKWD was then used to conduct electrowetting tests.
A combined analytical and experimental investigation was performed to characterize the droplet deformation during electrowetting. It was demonstrated that the MWKD is indeed a feasible and low-cost power supply that can be used in electrowetting. Furthermore, the experimental observations were well predicted by the numerical model presented in this paper.
For future applications, electrowetting using MKWDs can be further optimized by environmental control e. The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
National Center for Biotechnology Information , U. Journal List Micromachines Basel v. Micromachines Basel. Published online Feb Find articles by Elias Yazdanshenas. Find articles by Xiaoyu Zhang. Author information Article notes Copyright and License information Disclaimer. Received Dec 16; Accepted Feb Associated Data Supplementary Materials micromachiness Abstract The Kelvin water dropper is an electrostatic generator that can generate high voltage electricity through water dripping.
Introduction The Kelvin water dropper was invented by William Thompson aka. Methods 2. Open in a separate window. Figure 1. Analytical Method In the present work, an analytical model was built to simulate the time-dependent deformation of a water droplet during electrowetting using MKWDs.
Figure 2. Results and Discussion 3. Figure 3.
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