Physics of Surface Tension

Surface Tension Physics

A carefully placed pin floats on water despite of it being denser than water. The force that keeps the pin floated is not buoyancy and is called surface tension. It is the property of a liquid surface to minimize its area. Just like a stretched trampoline has the capacity to hold weight, one could assume that the liquid surface is stretched and can bear weight. When a weight is placed on the surface of a trampoline it stretches downwards, this downward stretch increases its surface area.

In a similar fashion when a weight is placed on a liquid surface, the surface needs to be stretched because of the gravitational force acting on the object. But due to this property called surface tension the surface has a potential to maintain minimum surface area and hence does not stretch. Now a legitimate question would be why does a pin pointed downwards or a pin haphazardly placed does not float on the surface although there is surface tension. Surface tension can be quantified as the amount of force acting on the pin per unit length of it in contact with the surface. A pin pointed downwards (perpendicular to the surface) has a very little area in contact with the liquid surface compared with a pin that is tangentially placed and hence there is less force acting on it which eventually leads to its sinking down. A pin haphazardly placed also may not have maximum length of it in contact with the liquid surface always and hence most of the time it sinks down.

Ever wondered why the shape of soap bubbles is spherical? Yes, you have the answer, surface tension. But how exactly surface tension induces spherical shape to a soap bubble needs to be discussed. Of all the shapes a soap bubble can take, from cubical to a polygon, sphere is the one with minimum surface area. Thus soap bubbles always exist in spherical shape irrespective of the solution or apparatus used. And also the shape of raindrops is spherical for the same reason.

Surface tension is affected by temperature, nature of the solution and the impurities present in the solution. The inter-molecular forces between the molecules present on a liquid surface largely govern the surface tension. Weakening or strengthening of these forces decreases or increases the surface tension. An increase in temperature is associated with a decrease in the inter-molecular forces and hence surface tension is lowered at high temperatures.

Detergents contain molecules that occupy a lot of area on the surface when dissolved in liquids like water. These new detergent molecules breakdown the inter-molecular forces between the already existing water molecules (liquid molecules) and lower the surface tension. A soap bubble has more lifetime than a water bubble and the following reasoning will explain the phenomenon. A soap bubbles surface is always under the pressure to be minimized because of surface tension, and hence can cause the collapse of the bubble. But when the surface tension is low the tendency to minimize surface area is reduced and hence soap bubbles have longer lifetime and also larger size.

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