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Bernoulli's Equation
Physics Textbooks Boundless Physics Fluid Dynamics and Its Applications Bernoulli's Equation
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Physics
Concept Version 6
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Surface Tension

The tendency of the surface of a liquid to resist a force and behave like a membrane and is a result of cohesion between liquid molecules.

Learning Objective

  • Summarize the cause for different surface tensions at a liquid's surface


Key Points

    • Surface tension is a result of cohesion between the molecules of the liquid. The molecules at the surface of the liquid feel an attractive force pulling them toward the bulk of the liquid more than the solid or fluid at the interface.
    • When a liquid-solid-gas interface is encountered, the contact angle represents a measure of the relative strength of adhesive and cohesive forces.
    • The contact angle determines the wettability of a surface.

Terms

  • adhesion

    The ability of a substance to stick to an unlike substance.

  • cohesion

    Various intermolecular forces that hold solids and liquids together.

  • wettability

    The ability of a solid surface to reduce the surface tension of a liquid in contact with it such that it spreads over the surface and wets it.


Full Text

Surface tension is the tendency of a liquid surface to resist forces applied to it. This effect is a result of cohesion of the molecules of the liquid causing the surface of the liquid to contract to the smallest area possible. This effect is visible in nature with water strider insects that are able to walk on water. Also, a paper clip or pin can be supported by the surface tension at a water air interface .

Surface Tension FBD

Force diagrams showing the direction of forces for water supporting a water strider (insect) foot and a pin. In both cases, the vertical component of the surface tension is enough to support the weight of the object.

In the bulk of the liquid, the molecules are pulled equally in all directions. The molecules at the surface feel a greater attractive force toward the bulk material than the interface material.

The surface of a liquid is an interface between another fluid, a solid body, or both. Therefore, the surface tension will be a property of the interface rather than simply the liquid. Adhesion describes the attractive force between molecules of different types. The surface of a liquid in a container is an interface between the liquid, the air, and the container. Where the surfaces meet, forces must be in equilibrium. This results in a contact angle at the interface. The contact angle is measured in the liquid and depends on the relative strength of cohesive forces in the liquid and adhesive forces between the liquid and interface materials. If liquid molecules are strongly attracted to the molecules of the solid surface (adhesive forces > cohesive forces), the drop will tend to spread out and the contact angle will be close to zero degrees . If the cohesive forces are greater than the adhesive forces, the resulting contact angles will be large and will form a more circular drop .

Water Droplet on Leaf

When a water droplet forms on a leaf, the cohesive forces between the water molecules are greater than the adhesive forces between the water and leaf surface. The leaf is a hydrophobic surface.

Contact Angle

The contact angle is the angle, measured in the fluid, that results when a liquid-gas interface, meets a solid surface.

When the liquid is water, a surface where the contact angle is small is said to be hydrophilic. Large contact angles are present on hydrophobic surfaces. The contact angle determines the wettability of the surface.

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