Surface Tension

According to Archimedes principle, the buoyant force impacts the object’s apparent weight. The apparent weight loss one experiences in water is equal to the weight of fluid displaced. This is why a person feels like they weigh less in water than they do in air.

SURFACE TENSION

Surface tension is related to cohesive forces, which are the attractive forces between molecules of the same type. Liquid can be held within open containers because of the cohesive forces between the molecules. Adhesive forces are the forces that hold molecules of different types together. Liquid water adheres to panes of glass because of adhesive forces. Both of these are considered attractive forces.

It is the cohesive force between molecules that causes the surface of a liquid to contract to the smallest possible surface area. This is called the liquid’s surface tension. A needle can rest on the surface of water—not because it is less dense and floats—but because it is supported by the surface tension of the water. If the needle were placed on its end, it would sink because the force of its weight would be spread over a smaller surface area of the water. The restoring force is related to the surface tension of the water and results in a net upward restoring force that exceeds the weight of the object. In fact, the restoring force is equal to the surface tension, noted by the Greek letter, gamma.

The greater the cohesive force of the liquid, the greater the surface tension. The surface tension is the force per unit length of a stretched liquid membrane. The SI units for surface tension is units per meter. Soapy water can form bubbles because it has a lesser surface tension than plain water. Water forms droplets that are roughly spherical because of their inward surface tension.

The pressure inside a bubble is four times the surface tension divided by the radius of the bubble. Bigger bubbles have smaller interior pressures. This is why a soap bubble ultimately explodes. The pressure becomes less inside the bubble as it gets bigger. If a hole is placed in the bubble, the bubble would decrease in radius and the pressure will increase within the bubble.

Inside the alveoli of the lungs, the air is naturally forced out because of the shrinkage of the diameter of the alveoli in the exhalation process. Surfactant is a substance in the

alveoli that allows them to expand and prevents total collapse of the alveoli during exhalation. This makes surfactant in the lungs different from that of detergent, which simply lowers the surface tension of a bubble but will not prevent its total collapse.

Adhesive forces are between two molecules of different substances, such as between water and wax, which have little adhesive forces between them. The contact angle is the angle between the tangent to the liquid surface and another surface. The greater the contact angle, the less is the adhesive force between a droplet of water and the surface it’s resting on. Figure 63 shows the contact angle of a droplet with a great adhesive force and a droplet with a lesser adhesive force:

Figure 63.

This brings up the idea of capillary action. This is the tendency of a fluid to be raised inside a narrow tube, called a capillary tube. The actual effect depends on the relative strength of adhesive and cohesive forces. If the contact angle is greater than 90 percent, the liquid will be depressed; if the contact angle is less than 90 percent, the liquid will be