Surface Tension - Rings, Bubbles, Drops & Plates

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The definition of Liquid Surface Tension is simple.  It's the force that keeps a liquid from flying off into space. However, the measurement of surface tension can take many forms, which can be confusing. In an attempt to get some clarity, we have articulated two principle measurement concepts in this article: 

Drop-Based Concepts of Surface Tension Measurement  One of the concepts is based on the analysis of a drop. This analysis can be:

  1. A complex optical analysis of the drop shape, 
  2. Related to the pressure needed to make a bubble burst, or, 
  3. Associated with the time it takes a series of drops to traverse a tube of small diameter.

These methods either require advanced mathematical analysis, or are hard to perform.

Force-Based Concepts of Surface Tension Measurement  The other concept is based on the force exerted on a foreign body by the surface of a liquid. The underlying task is the measurement of the this force. 

One of these techniques, known as the DuNouy Ring method, starts with the insertion of a ring below the liquid surface. The ring is moved upward through the interface between the liquid and the air (or between the liquid and a lighter or less dense liquid). The force required to get the ring free of the surface or interface is measured.

The other major force method uses a thin rectangle. The rectangle and the test process are referred to as Wilhelmy Plate. When the plate is lowered to touch the liquid, the surface tension grabs the plate and pulls down. The level of this force is used to calculate the surface tension of the liquid.

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