DIY triboelectricity experiments
By Matthew D. Stilwell, Chunhua Yao, Dale Vajko, Kelly Jeffery, Douglas Powell, Xudong Wang, and Anne Lynn Gillian-Daniel
In response to COVID-19, we have found these nanogenerators to be an inexpensive and engaging at-home science activity.
What if “every breath you take, every move you make” () could be harnessed to produce renewable energy? Triboelectric nanogenerators (TENGs) are state-of-the-art devices researchers are studying to do just that—convert kinetic energy into electrical energy at the source (). This type of electrical energy is called triboelectricity, commonly experienced as static electricity, and is produced when two materials, such as a rubber balloon and human hair, come into contact, exchanging and separating charges, producing a voltage (Figure 1).
TENGs create triboelectricity through the use of two materials, one that develops a positive charge and one that develops a negative charge. Triboelectricity is produced when the materials within the TENG are repeatedly brought into contact and separated. Typically, the kinetic energy from movement like walking or dancing is dissipated as heat and sound (), but TENGs convert this kinetic energy into useful electricity that can power small electronics such as cell phones. This article details how students can build a simple, state-of-the-art, renewable energy device; experiment with circuits; and explore how scientists and engineers exploit surface-area-to-volume ratios to achieve desired results.
We begin our discussion with a simple, eye-catching, and familiar demonstration of triboelectricity, i.e., static electricity. Students can rub a balloon with hair or fur and observe how the two objects attract one another. Alternatively, students can run a plastic comb through hair or fur, charging the comb and enabling it to pick up small pieces of plastic or paper in close proximity. Van de Graaff generators produce triboelectricity and allow for exciting demonstrations (please follow all safety considerations with your Van de Graaff generator!). See for additional ideas and an excellent PhET simulation that demonstrates how electrical charges are exchanged when two materials come into contact and the effect the charge exchange has on the material’s behavior.
Two important concepts of triboelectricity that we want to emphasize are that: (1) when two materials with different charge affinities come into contact with one another, they donate or accept electrons, and (2) some materials are more amenable to accepting electrons, while others are more amenable to donating electrons. To drive these concepts home, students recreated a small section of the triboelectric series (), a ranking of materials’ affinity for positive or negative charges.
We gave groups of students six different materials: three rods (wood, glass, PVC) and three cloths (cotton, wool, polyester). Using a DIY electroscope (Figure 2; see for original source and our instructions) that we built prior to the activity and that students can build themselves, students tested every combination of rod and cloth in an attempt to create a subsection of the triboelectric series. Rubbing a rod with a cloth separates charges, and then bringing the rod close to the electroscope coil induces a charge gradient in the electroscope, causing the aluminum leaves to repel one another and enabling the students to measure the angle of deflection, and thus electrification. The larger the angle, the greater the amount of charge that was separated, and the further apart the materials are on the triboelectric series. Students record their results (Figure 3) for each combination of rod and cloth.
After the demonstration of triboelectricity and investigation into the triboelectric series, we activate student’s prior knowledge around energy and charge carriers. We ask students to explain where the energy to separate the aluminum leaves in the electroscope came from (kinetic energy from rubbing causes triboelectrification, which, when the rod is brought close to the coil, causes the leaves to separate). Additionally, we discuss the atomic charge carriers (protons carry positive charges, neutrons carry no charge, electrons carry negative charge), and charge carrier mobility, highlighting that electrons are the most mobile and can be transferred between atoms, while protons and neutrons are bound in the nucleus of an atom (Saurabh Rathore, Swain, and Ghadai 2018). We then confirm that like charges repel and opposite charges attract.
After completing the electroscope activity, we show students how a nanogenerator operates and the steps for constructing one from a plastic egg (see ). When the nanogenerator is shaken, a tape-covered ball inside bounces against the aluminum foil covering the inside of the egg, accepting electrons from the aluminum foil and then transferring them to the other side of the egg (Figure 4). The electrons conduct through the aluminum foil and connected wire on the other side, through the LED (producing a flash of light), the next wire, and finally back to the first piece of foil, rebalancing the charges. The materials and instructions to build a triboelectric nanogenerator are listed below. This method has worked for nearly all of the students (>95%) who followed the instructions; additional tips are included in the “Feedback for successful TENG construction” section.
Several options are available for inquiry-based modifications. As an engineering optimization problem, students can use the triboelectric series to predict which combinations of metal foil, such as copper or nickel, and tape, such as frosted office tape, polytetrafluoroethylene (PTFE) tape, or fluorinated ethylene propylene (FEP) tape, will produce TENGs with the highest voltage. Students can then build the different TENGs and use a voltmeter or oscilloscope to measure the voltage output. See Figure 5 for a table of voltage outputs from different combinations.
In addition, students can alter the size ratio of ball to plastic egg, change the plastic egg to a container of another geometry, or use multiple smaller balls. In addition, TENGs can be created with alternate geometries, such as the ones produced by Zhang et al. (). Several types of TENGs can be seen in Figures 7 and 8. For advanced groups, students can attempt to power a small electronic device such as a wristwatch using their TENGs, similar to the work done by Cheng et al. ().
We used formative assessments while designing this activity to gauge what students knew and learned, and what aspects of the activity proved challenging. These assessments led to iterative design improvements of the TENG. We led this activity during 14 field trips to a large research university for over 330 upper-middle school and high school students. Two local high school teachers also led the activity in their classrooms. We found a few common sources of error that prevent the proper functioning of the TENG:
Many options for assessments exist, such as quizzes related to electricity or student reflections on the design and importance of the work. Students can draw diagrams explaining the mechanism of the TENGs and how the current flows through the device. Questions such as “why does there need to be a gap in the foil between the two sides of the TENG?” and “why did we have to take care not to cover the wires with tape?” can be used to assess students’ understanding of circuits, conductivity, and current flow.
Additional questions such as “Could these devices be used to power the school, and how would you accomplish that task?” “What changes would you make to the TENGs to increase their power output?” “Where might this technology be best implemented?” can stimulate student’s creativity and design thinking. If voltmeters or oscilloscopes are available, students can demonstrate proficiency in HS-PS3-3 by refining the device while satisfying a set of constraints such as power output, cost, and size of device.
Students should observe that the brightness of the LED in these TENGs is relatively low (the black straws aid in visualizing the LEDs; green LEDs are easiest to see), which can lead to an interesting discussion on electronics and surface-area-to-volume ratios. The brightness of an LED is, within limits, linearly dependent on the current flowing through it. These TENGs produce a voltage large enough to illuminate the LED, but a low current, on the order of tens of microamps, that causes the LED brightness to be low.
We asked the students how they might design nanogenerators to produce more current and thus brighter lights. For this TENG design, the only part of the ball that contacts the aluminum foil is the tape-covered surface. Therefore, the surface of the ball is responsible for triboelectrification. Since the ball is spherical, we know that the surface area (SA) and volume (V) depend on the radius of the sphere (r):
SA = 4πr2
V = (4πr3)/3
The surface-area-to-volume ratio is then:
SA/V = (4πr2)/((4πr3)/3) = 3/r
Without employing different materials or changing the TENG geometry, altering the surface-area-to-volume ratio is our only option for optimizing charge separation, and thus electricity generation, within the device. With the above equation, we see that as the radius, r, becomes larger, the surface-area-to-volume ratio becomes smaller, and that as r becomes smaller, the surface-area-to-volume ratio becomes larger.
In order to maximize the charge separation with these materials and this geometry, we want to maximize the surface-area-to-volume ratio. Scientists and engineers greatly increase the surface-area-to-volume ratio by working at the nanoscale (on the order of 1–100nm), enabling greater charge separation with the same amount of materials. Since the nanoscale is so much smaller than the TENGs outlined in this activity, scientists and engineers can essentially pack hundreds of thousands of nanogenerators into the same volume, drastically increasing the total charge separation and energy generation. Scientists and engineers are currently designing nanoscale triboelectric nanogenerators to maximize the energy generation for myriad uses, from flooring that harvests the energy of footsteps (Yao et al. 2016) to bandages that electrically stimulate wounds, accelerating healing times (Long et al. 2018).
Triboelectric nanogenerators provide another tool for electricity generation in a world with increasing demands for renewable energy. These cutting-edge devices promise to reduce the need for batteries in small electronics and make a valuable addition to humanity’s portfolio of sustainable, nonpolluting energy generation technologies. Global challenges such as climate change require us to keep moving, and to harness the kinetic energy at the same time!
This research was primarily supported by NSF through the University of Wisconsin Materials Research Science and Engineering Center (DMR-1720415). Any opinions, findings and conclusions, or recommendations expressed in this report are those of the authors and do not necessarily reflect the views of the Foundation.
Original electroscope instructions: https://www.youtube.com/watch?v=ViZNgU-Yt-Y
Written electroscope instructions and materials list:https://education.mrsec.wisc.edu/do-it-yourself-electroscope/
Background on triboelectricity: https://education.mrsec.wisc.edu/triboelectricity/
Written TENG instructions, materials list, and construction guide: https://education.mrsec.wisc.edu/triboelectric-nanogenerator/
Matthew D. Stilwell () is the Associate Director of Education at the University of Wisconsin–Madison (UW) Materials Research Science and Engineering Center (MRSEC) in Madison, Wisconsin, Chunhua Yao is a Research Assistant at the UW in Madison, Wisconsin, Dale Vajko is a Chemistry and Math Teacher at Pembine High School in Pembine, Wisconsin, Kelly Jeffery is a Biology, Chemistry, and Physics Teacher at McFarland High School in McFarland, Wisconsin, Douglas Powell is a Project Assistant at the UW MRSEC in Madison, Wisconsin, Xudong Wang is a Materials Science & Engineering Professor at the UW in Madison, Wisconsin, and Anne Lynn Gillian-Daniel (email@example.com) is the Director of Education at the UW MRSEC in Madison, Wisconsin.
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