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I've read in places that fluids cannot sustain tangential force acting on them. However when reading about surface tension I read the following paragraph on wikipedia

There are two primary mechanisms in play. One is an inward force on the surface molecules causing the liquid to contract.[1][2] Second is a tangential force parallel to the surface of the liquid.[2] This tangential force (per unit length) is generally referred to as the surface tension.

This paragraph clearly mentioned that surface tension is acting tangential force to the surface of the liquid, but it contradicts with the theory above. How is that possible?

Also if it is the result of cohesive and adhesive forces why does it act tangentially to the surface of liquid?

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2 Answers 2

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Surface tension is ignored in the theory you mention in your first paragraph. Surface tension indeed does always act along the surface of the liquid. Also, surface tension has little co-relation with adhesive forces. Its value would be the same if a capillary tube is made of glass or metal, rather it is a property of the liquid, and hence dependent upon cohesive forces alone. So, I believe you are confusing between contact angle and surface tension in your last question.

The surface of a liquid is always at a higher potential compared to the bulk, hence the surface of a liquid is similar to a stretched membrane. This is what causes the force of surface tension to be seen.

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I've read in places that fluids cannot sustain tangential force acting on them.

More precisely, fluids can't sustain a state of shear stress that would result from an unbalanced tangential force. But surface tension must product a tangential force in condensed matter because increasing the surface area increases the system energy (as more bonds are now relatively unsatisfied), which is exactly how we define restoring forces.

Thus, surface tension tends to pull unconstrained condensed matter, most notably liquids, into a sphere, in which configuration the surface energy is minimized and all tangential forces are balanced. Alternatively, the liquid might form a meniscus in contact with a container, for example, precisely because detaching a liquid from a receptive surface also increases energy, providing an opposing force that counteracts the surface-tension-related force to provide a state of equilibrium.

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