Hands-on Activity Scrub the Scum, Save the Shine:
Engineering Motor-Driven Brushes

Learning Objectives

After this activity, students should be able to:

• Describe how the properties of different surfaces determine the frictional wear between the surfaces.
• Explain how surface tension is related to the cohesion and adhesion between liquid and solid surfaces.
• Evaluate engineering designs based on identified criteria and constraints to identify an optimal solution using the engineering design process.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - English

State Standards

Materials List

Each student needs:

• 1 “What Causes Tooth Damage” Pre-Reading
• 1 Pre/Post Assessment,
• 1 Individual Prototype Testing Worksheet
• 1 6 mm vibrating motor 
• 1 LR44 button cell battery 
• 4 chocolate coins (i.e., foil-covered chocolate candy)
• electrical tape
• strong scissors
• 1 timer/watch
• piece of cardboard
• double-sided tape
• (optional) modeling clay to mount coin while testing

Each group needs:

• Group Prototype Testing Worksheet
• 1 6 mm vibrating motor
• 1 LR44 button cell battery
• 8 chocolate coins (i.e., foil-covered chocolate candy)
• electrical tape
• strong scissors
• 1 timer/watch
• 1 caliper or ruler
• piece of cardboard
• double-sided tape
• (optional) modeling clay to mount coin while testing

For the entire class to share:

• variety of “bristle” materials (e.g., toothbrushes, hairbrushes, sponges, felt, yarn, pipe cleaners, sandpaper)
• dry erase markers
• double-sided foam tape at least 1 cm wide
• cardboard/variety of paper
• pencils/markers
• wire strippers
• scissors
• glue and/or hot glue   
• magic markers
• (optional) craft eyes, decorations, small masses

Worksheets and Attachments

Pre-Req Knowledge

Students should have a basic understanding of properties of matter (i.e., texture, volume, mass, density) and the idea that unbalanced forces (i.e., friction) cause change. This lesson builds on the common experience of tooth brushing and requires students to be aware that liquids (e.g., water) and solids (e.g., teeth) interact in specific ways. Students should have been introduced to the scientific term “surface tension” and be able to identify examples of surface tension in everyday life.

Suggested activities for building background knowledge: How Many Drops? and  Penny Perfect Properties          

Introduction/Motivation

How many times each day do you brush your teeth? Brushing is good for you—right? Discuss with your group your toothbrushing habits: How many times do you brush? What type of toothpaste do you use? What is your favorite toothbrush like? (Allow time for student discussion. All responses are appropriate at this time, as students’ dental habits may vary. This section is just an icebreaker question to get the discussion going.)

Dentists tell us to brush our teeth to prevent damage and keep our teeth strong for our whole lives, but what really causes tooth damage? Today we’re going to learn a little about tooth damage and then learn how we can use our skills as scientists and engineers to create a better toothbrush. (Distribute “What Causes Tooth Damage” Pre-Reading and have students read independently or in groups. After reading, have students complete the initial section of the Pre-Assessment to draw or describe their current understanding of the key vocabulary and begin brainstorming ideas about motor toothbrush design.)

What surprised you in the reading? (Possible answers: teeth are alive, toothbrushing can cause damage.) Where might you see examples of cohesion, adhesion, or mechanical wear in your day-to-day life? (cohesion – water droplets, toothpaste; adhesion – tape, glue; mechanical wear – treads on shoes wearing down) (This would be a good place to insert, build, or reinforce background knowledge with activities such as How Many Drops? and Penny Perfect Properties if needed.)

Properties of cohesion, adhesion, and the effects of mechanical wear play a large role in our lives, and engineers must figure out how to design products to maximize or minimize certain effects. We call these engineering needs criteria, or things we want a product to do, and constraints, or what the limits on our product design are. What might be some criteria and constraints for the design of a toothbrush? (Possible answers: criteria – needs to fit in the mouth, clean teeth well; constraints – not cause damage to teeth, not be too big.) How do engineers know if their product design meets the identified criteria and constraints? (Answer: They perform testing and measurement.)

As you read, you have been challenged to work as engineers to design a motor-driven toothbrush that is able to clean teeth surfaces—that is your criteria—but not damage the surface—that is your constraint. You will build your first prototype individually. Engineers use a process called iteration when designing. Iteration is when you take one idea, test it, and then improve it. You will iteratively improve your individual prototype by working as a team to combine the best features of each prototype to create the best possible product that you will then pitch to the Colgate Corporation to purchase. Who knows, maybe you’ll be the next toothbrush millionaire!

Procedure

Background

Cohesion and adhesion are properties of materials that affect how different substances interact with each other in the world. Cohesion is the property that makes a material “stick” to other particles of the same material. The cohesive forces of liquid molecules contribute to surface tension. Surface tension is the property that allows the surface of a liquid to resist external forces, such as when water beads on the side of a glass or small insects are able to stand on top of a body of water without falling in. Adhesion is the tendency of one material to stick to particles of another material. Adhesive tape uses the adhesion of a sticky glue to attach objects together.

In the human mouth, cohesion and adhesion of liquid saliva interact with the teeth. Saliva, which is mainly composed of water, helps to lubricate the surfaces of teeth. When you brush your teeth, the bristles of the toothbrush and the toothpaste create friction against the hard outer surface of the teeth called the enamel. The cohesive forces between water molecules in saliva help to reduce this friction, allowing the toothbrush to move smoothly over the teeth without causing excessive wear. At the same time, the enamel, which is hard and smooth, resists adhesion from foreign materials that might cause damage.

Abrasion is the special term for friction between rubbing surfaces. Abrasion between the bristles of a toothbrush and the tooth enamel can result in mechanical wear over time, which causes the surface of the enamel to become less smooth. It is easier for bacteria and acids to adhere to the rough surface of the worn tooth, which can further contribute to tooth damage and decay. Changing the material properties of the toothbrush bristles can alter the effects of brushing on the tooth surface and thus either increase or decrease mechanical wear on the tooth surface.

Before the Activity

• Make copies of the “What Causes Tooth Damage” Pre-Reading, Pre/Post Assessment, and Individual Prototype Testing Worksheet. (1 per student)
• Make copies of the Group Prototype Testing Worksheet. (1 per group)
• Practice connecting the vibrating motors to the batteries.
• (optional) Create a sample mechanical brush robot. See examples on Instructables, or see this build example video on YouTube.
• Prepare testing materials by using double-sided tape to mount coins on a solid, nonporous surface.
• Assemble kits of the basic materials for each student, including one motor, one battery, and a battery-sized piece of double-sided tape.
• Assemble kits of the basic materials for each group, including one motor, one battery, and a battery-sized piece of double-sided tape.
• It is suggested that you strip the wires from the motors and/or use a battery housing to make the construction process easier for students. Students struggle with the on/off issue of batteries’ usage, but the procedure of determining how batteries attach to motors is a helpful learning experience.
• Gather assorted materials that could be used as bristles, including toothbrushes, and cut into small, toothbrush-sized pieces.

With the Students

Day 1: (90 minutes)

1. Introduce the activity (see Introduction/Motivation).
2. Give students five minutes to read the “What Causes Tooth Damage” Pre-Reading.
3. Have students fill out the first column of the Pre/Post Assessment.
4. Give students time to sketch their individual designs in the “design space” section of the Pre/Post Assessment. 
5. Restate the challenge from the “What Causes Tooth Damage” Pre-Reading:

“Colgate Corporation, the largest toothbrush manufacturer in America (annual sales of $46.6 million) is seeking innovative designs for an electric brush that makes teeth shiny clean without causing damage to the enamel surface. You and your team are challenged to design a motor-powered brush that meets these criteria and constraints. In other words, you want a brush that scrubs the scum (bacteria and other particles that adhere to the tooth), but saves the shine (smooth surface where cohesion of the liquids in contact is greater than the adhesion of the liquids to the surface).

You will create and test a prototype, then work with a team to iterate, or change, your design to combine the best features of each design into the ultimate brushing machine. Be sure to gather data to justify your design to the Colgate Corporation and, who knows, you could be the next toothbrush millionaire!”

6. Ask: Guide students through a discussion to “Define the Problem.” (Colgate Corporation needs a new motor-powered brush that cleans teeth but does not damage teeth.
7. Research: Use the Brainstorming and Planning KWL Chart sheet to have students work individually or in small groups to generate ideas about topics they “need to know” to be successful in solving the problem. If time allows, have students to use internet tools or classroom materials to research answers to the “need to know” section.
8. Imagine/Plan: Have students individually brainstorm and sketch an initial design in the “Brainstorming: My Design Sketch” section of their Brainstorming and Planning KWL Chart sheet. 

   

9. Demonstrate the materials available. 

   

10. Create/Build: Allow students to build their own individual prototypes. Walk around the room and be available for students as they build them. 

    

11. Test: Go over the Prototype Testing Procedure Presentation for Test 1.
12. Distribute one Individual Prototype Testing Worksheet for students to record data and analyze results.
13. Have students sketch the prototype they built—not necessarily the one they initially sketched—at the top of their Individual Prototype Testing Worksheet.
14. Give students time to test their individual prototypes.
15. Have students record their data in their Individual Prototype Testing Worksheet.
16. Improve: Encourage students to redesign and complete a new test for the next design if time allows.
17. Reflection: Have students answer the “Post-Testing Reflection” questions in their Individual Prototype Testing Worksheet. (Optional: If there is not enough time, these questions can be assigned as homework.)
18. Store bristle bots for Day 2 testing. Be sure students label their prototype with their names.

Day 2: (90 minutes)

1. Return the individual student prototypes and testing record worksheets from Day 1 to their respective students.
2. Group students into teams of 3 or 4.
3. Have students assign the following group roles (See Slide 11 in Prototype Testing Procedure Presentation): materials manager, data recorder, measurement guru, and electrician.
4. Imagine/Brainstorm: Have each student share their initial design, prototype, and test results with their group.
5. Have students work with their team to identify the strengths of each individual design.
6. Plan: Give students time for each group to sketch a new group design (one per group) that incorporates the best features of each individual design.
7. Have each group sketch their group design in the Group Prototype Testing Worksheet.
8. Create/Build: Give groups time to construct their group prototype. Follow the procedure from Day 1 for monitoring student prototype construction and testing. 

   

9. Test: Have each group do Test 1 in the Prototype Testing Procedure Presentation. Have them record their collected data in their Group Prototype Testing Worksheet.
10. Have each group do Test 2 in the Prototype Testing Procedure Presentation. Have them record their collected data in their Group Prototype Testing Worksheet.
11. Have students answer the reflection questions.

Day 3: (90 minutes)

1. Introduce the requirements for the Product Pitch Assignment and Product Pitch Rubric.
2. Have students work with their teams to create and practice their pitch.
3. Optional: Have student groups present their pitches the same day, as time allows, or give groups additional time for further practice and refinement of their pitches.
4. Optional: Have students record or electronically document their pitches for later evaluation.

Vocabulary/Definitions

abrasion: The wearing, grinding, or rubbing away caused by friction.

adhesion: The tendency of particles of one substance to stick to particles of a different, dissimilar, substance.

cohesion: The tendency of particles of a single substance to stick together.

enamel: The hard outer surface of a tooth; the part of the tooth that is exposed to mechanical wear from brushing or chemical wear from toothpaste.

iteration: The process of continual redesign based on testing and refinement of an original design.

mechanical wear: The damage to a surface caused by interaction with another material, such as by rubbing or other abrasive processes.

surface tension: A property of liquids in which molecules of one substance are more attracted to each other than to molecules of another substance.

Assessment

Pre-Activity Assessment

Brainstorming: In small groups, have students engage in open discussion about how often they brush their teeth, what kinds of toothpaste they like, and what characteristics make a good toothbrush. After providing discussion time, challenge students to think of possible negative effects of tooth brushing.

Pre-Reading: Distribute the “What Causes Tooth Damage” Pre-Reading and have students read individually or in pairs. Encourage students to identify and discuss key vocabulary terms, as indicated by underlined items in the text.

Vocabulary Pre-Teaching: After pre-reading, distribute the Pre/Post Assessment and have students record their current understanding of the key vocabulary terms, using either words or pictures.

Initial Design Sketches: Review the design challenge (found in the Introduction/Motivation section and in the text on the worksheet) and have students complete initial design sketches for a motor-driven toothbrush in the “design space” section of the worksheet.

Activity Embedded (Formative) Assessment

Prototype Testing Worksheet (Individual): After students design their initial prototypes, distribute the Individual Prototype Testing Worksheet to each student to record testing data. Review testing procedures and clarify units of measurement to use to record. After students have tested and recorded data for their original prototype, ask them to reflect on the strengths and weaknesses of their current design and how to improve it in the future.

Prototype Testing Worksheet (Group): After students have created the second version of their prototype by combining the best ideas from each individual member, they share their responses to the reflection questions on the back of their Group Prototype Testing Worksheet and identify one or two strengths of their design. Students discuss the individual design strengths and how these strengths could be combined into a single prototype that, by combining multiple beneficial design features, will (hopefully) have an overall improvement in performance. After designing and building the new group prototype, students test the new prototype and record testing results. Students reflect as a group about strengths and weaknesses of the new design and how the new design compares to the original designs in terms of performance.  

 Post-Activity (Summative) Assessment

Product Pitch: Students create a presentation, poster, or short video/audio recording that explains the benefits of their prototype to the Colgate Corporation. Presentations should include design details, scientific vocabulary, and reasoning, as well as creative effort. (See Product Pitch Rubric.)

Post-Assessment: Students revisit their Pre-Assessment vocabulary models and initial design sketches. Students draw or describe their refined understanding of the vocabulary terms in the second column. Students draw or describe their final design in the “design space” on the back and justify redesigned components related to improvements over initial design.

Safety Issues

• Use eye protection (goggles or safety glasses) when working with liquids during this activity.
• Use caution when handling LR44 cell batteries; keep batteries away from liquids.
• Dispose of batteries in school-identified battery disposal receptacles; do not put them in the trash.
• Keep batteries away from small children and pets; they are a choking/swallowing hazard.
• Use caution with sharp materials such as scissors and wire bristles.
• If students choose to do online research to gather ideas for designs, encourage digital safety and responsible online use. Resources are available at Common Sense Media.

Troubleshooting Tips

• Make sure the chocolate doesn’t melt!
• If students cannot measure the change in thickness, you can determine the wear by measuring mass before and after.
• Duct tape can also work well to stick batteries and motors to brush heads if double-sided tape is insufficiently adhesive.
• Turning the battery horizontal when attaching it to the base can be helpful for students with limited manual dexterity.

Activity Extensions

• Discuss the properties of different adhesive materials used to hold the designs together (i.e., which type of tape or glue holds best?)
• Use CAD software to design and 3D print a body/chassis for the motor-driven brush.
• Use decision matrix for engineering design-based decision making for prototype improvement.

Activity Scaling

• For ELL or students with special needs, have students keep a vocabulary journal to refer to during the lesson.
• For elementary school students, you could design, create, and test a device as a class.

Additional Multimedia Support

• To go more in-depth in explaining surface wear, use PhET friction simulation.
• For presentations, students may use Google Slides, Canva, or other multimedia technology.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This material is based upon work supported by the National Science Foundation under grant no. ECC-1160494 - a Research Experience for Teachers program titled “Nanomanufacturing Systems for Mobile Computing and Mobile Energy Technologies or NASCENT.” Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.