How do geckoes stick to surfaces?
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How do geckoes stick to surfaces?
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Tokay geckos are remarkable for their ability to cling to vertical and even inverted surfaces using an average of 227 mm2 of highly specialized toepads. The toepads are covered with hydrophobic hairs called setae, with a density of approximately 5,300 setae•mm-2, arranged in plate-like layers known as lamellae. Each setae branches into 100 to 1,000 flat, highly deformable tips called spatulae that allow each setae to make multiple contact points with the surface. Gecko toepads are able to produce adhesive forces, drawing the animal toward a surface, and frictional forces, preventing the animal from sliding along the surface.
It is generally agreed that geckos adhere to both hydrophobic and hydrophilic surfaces primarily via attractive van der Waals forces between setae and a surface. Researchers disagree about whether or not capillary adhesion plays a significant role in gecko toe adhesion. Capillary adhesion depends on the formation of a thin layer of water between the two surfaces (a “capillary bridge”), which attracts two hydrophilic surfaces toward each other. The water is thought to arise from normal environmental humidity. Experimental evidence has shown that it takes approximately 4.2 ms for capillary bridges to form, which must occur for capillary adhesive forces to be generated.
To further explore the interplay between van der Waals forces and capillary adhesion, a group of scientists investigated the forces generated by actual setae at different humidities. Arrays of setae were scraped from gecko toepads and artificially attached to a device capable of recording sensitive measurements of force. The setal arrays were dragged 100 nm across various surfaces at a constant velocity of 5 µm per second. The resulting adhesive and frictional forces were measured, as shown in Figure 1.
Figure 1: The influence of humidity on the adhesion and friction of isolated gecko setal arrays during steady-state drag. Each symbol represents the mean of a group of trials (n=6). Silicon dioxide is a hydrophilic semiconductor. Gallium arsenide is a hydrophobic semiconductor. Experiments were carried out at a constant temperature of 25° C.
Adapted from: Puthoff, Jonathan B., Michael S. Prowse, Matt Wilkinson, and Kellar Autumn. "Changes in materials properties explain the effects of humidity on gecko adhesion." The Journal of experimental biology 213, no. 21 (2010): 3699-3704.
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[question] => When a gecko is clinging motionless to a perfectly vertical surface, which of the following must be true?
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[description] => Reason for the Correct Answer:
If you draw a free-body diagram of the gecko, there will be four forces on the gecko.
The four forces are: (1) the force of gravity (downward), (2) the force of friction (upward), (3) the adhesive force (toward the wall), and (4) the normal force (away from the wall).
You can’t determine the relationship between perpendicular forces based on the information you are given. The force of friction is perpendicular to the adhesive force, so we can’t relate these two forces. You can eliminate “The force of friction on the gecko exceeds the adhesive force,” and “The force of friction on the gecko is the same magnitude as the adhesive force.”
If the normal force were greater than the adhesive force, the gecko would accelerate away from the wall; it would no longer be motionless. You can eliminate “The normal force on the gecko exceeds the adhesive force.”
Since the gecko is motionless, each force is canceled out by an equal and opposite force; the gecko is in equilibrium. Therefore, any forces in opposite directions must have the same magnitude. This means that the normal force must be the same magnitude as the adhesive force.
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[each_answer] => A. The normal force on the gecko exceeds the adhesive force.
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[each_answer] => B. The force of friction on the gecko is the same magnitude as the adhesive force.
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[each_answer] => C. The force of friction on the gecko exceeds the adhesive force.
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[each_answer] => D. The normal force is the same magnitude as the adhesive force.
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[question] => Which of the following statements can be concluded from the experimental results shown in Figure 1?
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[description] => Reason for the Correct Answer:
The frictional forces (solid lines in the figure) are higher on the vertical axis than the adhesive forces (dotted lines in the figure). Therefore, you can eliminate “Adhesive forces are generally stronger than frictional forces in the tested conditions.”
There are stronger adhesive forces with silicon dioxide (squares in the figure) than with gallium arsenide (circles in the figure). Therefore, you can eliminate “When all other factors are equal, gecko toepads generally exhibit more attraction toward gallium arsenide than silicon dioxide.”
“Capillary adhesion between gecko toepads and hydrophilic and hydrophobic surfaces is stronger at higher relative humidities,” is tricky. It incorrectly assumes that the adhesive forces are caused by capillary adhesion, even though “researchers disagree about whether or not capillary adhesion plays a significant role in gecko toe adhesion.”
The graph shows that both friction forces and adhesive forces increase at higher humidities. Therefore the correct answer is “Increasing relative humidity increases the magnitude of both friction forces and adhesive forces.”
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[each_answer] => A. When all other factors are equal, gecko toepads generally exhibit more attraction toward gallium arsenide than silicon dioxide.
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[each_answer] => B. Adhesive forces are generally stronger than frictional forces in the tested conditions.
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[each_answer] => C. Capillary adhesion between gecko toepads and hydrophilic and hydrophobic surfaces is stronger at higher relative humidities.
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[each_answer] => D. Increasing relative humidity increases the magnitude of both friction forces and adhesive forces.
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[question] => A previous hypothesis attempted to explain gecko toe adhesion by postulating that gecko toes created an area of low pressure between the toes and the substrate surface, similar to a suction cup. Which of the following experimental findings would most challenge this hypothesis?
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[description] => Reason for the Correct Answer:
Suction cups work because of a difference in pressure. The normal, external pressure is much higher than the low-pressure area created inside the suction cup. The normal, external pressure pushes the suction cup against the surface much more than the low-pressure area pushes back out–so the suction cup is forced against the surface.
The suction-cup mechanism does NOT have any obvious connection to humidity based on the information provided in the passage. Eliminate both choices that mention high-humidity.
The suction-cup mechanism would no longer function without external pressure. In other words, they would no longer function in a vacuum. If the suction-cup mechanism was really how gecko toes adhered to a surface, we would expect them to lose their adhesion abilities in a vacuum–so this confirms the hypothesis rather than disproving it.
If the gecko toes were able to adhere in a vacuum, without any external pressure forcing them against the surface, then something else besides the suction-cup mechanism must explain how geckos stick to surfaces. This would disprove the suction-cup mechanism.
Therefore, the correct answer is, “Gecko toes are able to adhere in a vacuum.”
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[each_answer] => A. Gecko toes are able to adhere in a high-humidity environment.
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[each_answer] => B. Gecko toes are able to adhere in a vacuum.
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[each_answer] => C. Gecko toes lose their ability to adhere in a vacuum.
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[each_answer] => D. Gecko toes lose their ability to adhere in a high-humidity environment.
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[question] => What can be concluded about the possible role of capillary adhesion and capillary bridges in the experiment shown in Figure 1?
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[description] => Reason for the Correct Answer:
The answer choices all mention capillary bridges. In order to determine if they might have formed in the experiment, you need to know how long the setal arrays were in contact with the surfaces, because the passage says “experimental evidence has shown that it takes approximately 4.2 ms for capillary bridges to form.”
The average velocity equation (average velocity = distance / time) can be used to determine how how long the setal arrays were in contact with the surfaces. Be sure to rearrange the equation to solve for time: time = distance / average velocity.
Be careful about unit conversions! The distance each array was dragged is 100 nm, or 100×10⁻⁹, while the average velocity is 5 µm•s⁻¹, or 5×10⁻⁶m•s⁻¹
Plug into your re-arranged average velocity equation:
Solving the equation gives you 20×10-3 s, or 20 ms. This is more than enough time for capillary bridges to form (they only need 4.2 ms to form). Therefore, capillary adhesion could have occurred in the experiment, because capillary bridges were able to form.
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[each_answer] => A. Capillary adhesion did not occur in the experiment, because capillary bridges were unable to form.
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[each_answer] => B. Capillary adhesion could have occurred in the experiment, because capillary bridges were able to form.
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[each_answer] => C. Capillary adhesion did not occur in the experiment, because capillary bridges were able to form.
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[each_answer] => D. Capillary adhesion could have occurred in the experiment, because capillary bridges were unable to form.
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[question] => Based on the results in Figure 1, what is the most plausible explanation for the changes in adhesion and friction forces that occur as a result of changes in relative humidity?
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[description] => Reason for the Correct Answer:
If the coefficient of friction increased, we would only see increases in friction forces. However, Figure 1 shows that both friction forces and adhesive forces increase.
If capillary bridges were more likely to form, we would see a difference in behavior on hydrophilic and hydrophobic surfaces. However, Figure 1 shows a similar increase in forces on both hydrophilic and hydrophobic surfaces.
If only hydrophilic surfaces developed greater ionic character, we would see a difference in behavior on hydrophilic and hydrophobic surfaces. However, Figure 1 shows a similar increase in forces on both hydrophilic and hydrophobic surfaces.
If setal arrays have more contact with surfaces at higher relative humidities, we would expect stronger adhesive forces on all surfaces at higher relative humidities. This is exactly what Figure 1 shows, making this the most plausible explanation.
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[each_answer] => A. Capillary bridges are more likely to form at higher relative humidities, creating stronger capillary adhesion forces.
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[each_answer] => B. Setal arrays are more deformable at higher relative humidities, creating more contact between setae and surfaces.
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[each_answer] => C. The coefficient of friction increases at higher relative humidities, creating stronger friction forces.
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[each_answer] => D. Hydrophilic surfaces develop greater ionic character at higher relative humidities, creating stronger adhesion forces.
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Adapted from: Puthoff, Jonathan B., Michael S. Prowse, Matt Wilkinson, and Kellar Autumn. "Changes in materials properties explain the effects of humidity on gecko adhesion." The Journal of experimental biology 213, no. 21 (2010): 3699-3704.
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