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# 7.1: Investigating Static Forces in Nature: The Mystery of the Gecko

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Activity: Topography of the Unknown

Slide 2

What are some instruments (probes) you have used in previous science classes?

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Slide 3

Purpose:

Scientists use probes to identify the properties of objects, even if they cannot see the objects! In this activity, you will use a height probe to identify and analyze the height differences in an object that you cannot see.

Materials:

One set of materials per group of 2–3 students:

• Observation box
• Height probe: straightened paperclip at least 6 inches long
• $3$ pieces of scan paper (provided) or graph paper
• Thumbtack
• Ruler
• Pen
• Glue-stick
• Scissors

For the class:

• Glue gun

Procedure:

Measuring With the Height Probe

1. How will you measure the height of the object inside the box using a ruler and height probe with different colored marks?

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Sketch Space

2. Measure the height of the box. Systematically measure the distance from the probe hole to the object for each probe hole. Subtract this distance from the height of the box and record your data on the scan area below or on a larger scan paper. These measurements represent the various heights of the object inside the box.

A B C D E F G H I J K L M N
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2
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5
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7
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9

Slide 4

Making a Three-Dimensional Representation of the Objects in Your Box

1. Open a new spreadsheet in Excel.

2. Select the cells with data. Go to the chart wizard (chart symbol on the toolbar). Choose a surface plot: 2-D or 3-D (try both). Select NEXT at each dialogue box (Don’t worry about labels).

Notes:

• Once you get the chart, you can change the scale of the coloring by double clicking on the legend, going to the scale tab and changing the value for the major unit.
• You can rotate a 3-D chart by clicking on the chart and then dragging the corner dots.

1. Look at the 3-D representation you have created. Describe your object based upon your 3-D representation. Do you see any pattern in the numbers that gives a clue to the structure of the object inside the box? What does it look like?

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2. Optional: Open your box and compare to the graph you just created.

3. Optional: Print and staple your graph to this journal page.

Activity: Modeling an AFM with a Refrigerator Magnet

Procedure:

Use a refrigerator magnet to simulate and AFM probing a surface. The magnetic probe strip represents the scanning probe tip of an AFM. The refrigerator magnet represents a nanoparticle sample.

1. Drag the probe strip across the surface of the magnet, and notice any deflections. You will need to hold the probe strip nearly horizontal to the surface. Probe the magnet on various sides and in various directions.

2. As you know, the north pole of a magnet will attract the south pole of another magnet, and two magnets with either their north poles or their south poles pointed at each other will repel.

3. With this in mind, draw a representation of the magnetic poles in the box below.

• Be sure to include the number of up and down deflections that you observe as you

• What is the distance between the up and down deflections?

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• Are these distances uniform all the way across the surface, and in all directions?

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Top View Looking Down on the Magnet Surface

Slide 6

AFM Box Model
What characteristic is measured by the probe?
How is the characteristic measured by the probe?

Slides 8–11

1. Describe using words how the spatula-shaped tips come into contact with a surface. How does the number of model spatulas on a gecko seta affect the amount of surface contact?

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2. Look back at your previous methods (see Student Journal page 5–4, 5–5) as to how the gecko adheres to a wall. Are there some methods that you could eliminate because of the new information you now have? What evidence do you now have that supports the remaining possible adhesive methods?

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3. What new questions do you have about surface-to-surface interactions?

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Slide 13

Image 7.20a Force graph of one seta on a probe

Image 7.20b Location of one seta on a probe surface

1. What is being shown on the $X-$axis and $Y-$axis?

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2. What units are being used?

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3. What does the oval represent? (It should be noted that adhesion is enhanced during this timeframe on the graph.)

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4. What is the maximum force that is measured for adhesion?

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5. Knowing that there are $1 \;\mathrm{million}$ setae on all four feet, is this enough force to hold up a $2.2 \;\mathrm{Newton}$ gecko?

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What’s Happening to the Seta? (Diagrams below the graph) What’s Happening to the Force of Adhesion? (Slope of graph)

Between Points A and B

Between Points A and B

Between Points B and C

Between Points B and C

Between Points C and D

Between Points C and D

At what point does the seta completely leave the probe surface? Explain your answer in terms of time and amount of force shown on the graph.

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Investigating Static Forces in Nature: The Mystery of the Gecko

Lesson 7: How Do We Measure Forces at the Nanoscale Level?

Student Journal

## Date Created:

Feb 23, 2012

Apr 29, 2014
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