Physics Chapter: Surface Tension and Capillarity

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Questions and Answers

What happens to the height of liquid in a tube as the radius of the tube decreases?

The height increases. (correct)

The height decreases slightly.

The height remains the same.

The height becomes negligible.

How does the angle of contact influence capillary action?

A smaller angle indicates stronger cohesive forces.

A larger angle indicates weaker adhesive forces. (correct)

A smaller angle leads to a negative meniscus.

A larger angle results in a stronger adhesive pull.

In the formula for capillary rise, what does the variable $h$ represent?

The density of the liquid.

The contact angle between the liquid and tube.

The height the liquid travels up the tube. (correct)

The radius of the tube.

Which of the following statements about cohesive and adhesive forces is correct?

Weak cohesive forces lead to a small contact angle. Signup and view all the answers

In which medical application is capillary action especially significant?

Skin absorption of topical medications. Signup and view all the answers

What is the primary factor that determines the height to which a liquid will rise in a capillary tube?

The liquid's density and surface tension Signup and view all the answers

Which equation best represents the relationship between surface tension and capillary rise?

$h = \frac{2\gamma cos(\theta)}{\rho g r}$ Signup and view all the answers

In the context of blood in the dialysis machine, what does a contact angle of 0 degrees indicate?

Blood spreads out uniformly along the surface Signup and view all the answers

If the contact angle between unknownium and blood is 78 degrees, how does this affect the height of blood in the tube?

It will raise blood to a lower height than a contact angle of 0 degrees Signup and view all the answers

What would be the simplest change to the tube to ensure blood is drawn up at least 3 cm?

Increase the inner radius of the tube Signup and view all the answers

Which of the following factors does NOT directly affect the height of liquid in a capillary tube?

Length of the tube Signup and view all the answers

In an experimental setup involving unknownium, what property would increase its effectiveness as a dialysis machine component?

Lower contact angle with blood Signup and view all the answers

What effect does increasing the tube's inner radius have on capillary action?

Increases the height of liquid rise Signup and view all the answers

If the surface tension of blood were to be decreased, what would be the expected outcome in the dialysis machine?

Higher capillary rise Signup and view all the answers

What is the maximum height to which blood will be raised in a capillary tube with 100 μm radius, density 1050 kg/m³, and surface tension 0.058 N/m?

±0.113 m Signup and view all the answers

What is the impact of adding a surfactant to a bubble's surface tension?

It reduces the surface tension and decreases the bubble size. Signup and view all the answers

What determines the interfacial tension between immiscible liquids?

The size of both adhesive and cohesive forces. Signup and view all the answers

What does a contact angle greater than 90 degrees typically indicate?

Beading of the droplet. Signup and view all the answers

If water does not wet a glass surface, what can be said about the contact angle?

The contact angle is greater than 90 degrees. Signup and view all the answers

In the case of a droplet on a hydrophilic surface, which statement is true?

Adhesive forces are stronger than cohesive forces. Signup and view all the answers

How does the addition of a surfactant affect a liquid's behavior on a hydrophobic surface?

It makes the liquid spread out more on the surface. Signup and view all the answers

What is the gauge pressure inside a bubble if the radius is determined to be 0.0062 m and surface tension is 0.021 N/m?

13.6 Pa Signup and view all the answers

Study Notes

Surface Tension and Capillarity

Cohesion: The attractive force between similar substances.
Adhesion: The attractive force between dissimilar substances.
Cohesive forces drive surface tension.
Adhesive forces between a liquid and a solid cause capillary action.
Surface tension is a property of liquid surfaces, a result of intermolecular bonding.
It minimizes surface area and resists deformation.
Liquids act like they have a thin, elastic skin.
Surface tension reduces surface area in liquids.
Intermolecular forces on liquid molecules from neighboring liquid molecules are stronger than neighboring gas molecules.
Net force on surface molecules is directed inward.
Stronger for more curved surfaces.
Surface tension can be measured using a wire stretched across a liquid film.
Formula for surface tension (γ): Force (F) / Length (L).
Example: A 3cm wide apparatus with 4.8x10⁻³N force has a surface tension of 0.08 Nm⁻¹.
Surface tension affects bubble pressure.
Higher pressure inside a bubble than outside.
Greater gauge pressure inside a smaller bubble.
Air flows from small to larger bubbles when colliding.
Surfactants: Substances that reduce liquid surface tension when added.
Surfactant molecules concentrate near the surface.
Example: Soap in water.
Detergents and soaps have hydrophilic and hydrophobic ends.
Hydrophobic ends are away from water molecules.
Important for lung function.
Calculating bubble radius from surface tension and pressure.
Example: A bubble of water (0.068 Nm⁻¹) with 13.6Pa internal gauge pressure has a radius of 0.020m.

Capillarity I

Interfacial tension considers adhesive forces between liquid and container walls.
In examining surface tension, adhesive forces near the surface can be ignored.
Adhesive forces between immiscible liquids or solids are not negligible.
Interfacial tension depends on adhesive forces between materials.
Examples: Water "wets" glass, mercury does not "wet" glass.

Capillarity II

Water beading on a waxy/oily surface has large contact angles.
Water wetting a hydrophilic surface (like glass) has a low contact angle.

Capillarity III

Contact angle is a quantitative measure of beating tendency.
Angle between liquid-air surface and liquid-solid surface.
Contact angles greater than 90° indicate beading, <90° indicate wetting.

Capillary Action I

Liquids often rise in thin tubes.
Capillary action increases with thinner tubes.
More contact with the tube surface with smaller radii, which supports more liquid mass.

Capillary Action II

Height of liquid in tube is inversely related to tube radius.
Formula for capillarity height (h): 2ycosθ/ pgr
Where:
- y = surface tension
- θ = contact angle
- p = density of liquid
- g = gravity (acceleration)
- r = radius of tube

Capillary Action III

Upward force due to surface tension (y x L x cos0).
This force pulls liquid up until gravity's downward force equals it.
Downward force (weight) is pVg=p(πr²h)g.
Equation for height: 2ycosθ / ρgr

Example III (Dialysis Machine)

An engineer is designing a dialysis machine with a small tube.
Blood surface tension is 0.058 Nm⁻¹.
Blood density is 1050 kg m⁻³.
Tube radius is 100µm.
Maximum height for blood in the tube in a worst-case scenario.
Height range to raise blood in the tube depending on the contact angle (using cosine values of zero, and of 90 degrees).
To obtain a height of at least 3 cm, reduce the radius of the tube.

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Test your understanding of surface tension and capillarity in this quiz. Explore concepts such as cohesion, adhesion, and the forces that affect liquid surfaces. This quiz will help reinforce key principles that govern the behavior of liquids.