I understand that surface tension is caused by an imbalance of forces for molecules on the surface in comparison to those that lie within. As a result, this creates a net inward force that seeks to reduce surface area. However, if this force acts inwards, why does surface tension act tangentially? I've seen explanations that state bonds on the surface become stronger when the surface area is minimised. If this is true then why is this the case? It doesn't seem clear to me why a net downward force causes the surface to act as an elastic membrane.
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1$\begingroup$ I’m voting to close this question because this is not a chemistry question. Please check Physics.SE $\endgroup$– Nilay GhoshCommented Jan 8, 2023 at 2:20
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$\begingroup$ Consider that in the interior, all molecules are surrounded by equal attractive forces, but at an interface, the attractive forces are missing (or greater, in the case of adhesion to another substance). $\endgroup$– DrMoishe PippikCommented Jan 8, 2023 at 2:21
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$\begingroup$ This question is related to soft-matter chemistry, so solid grasp of the phenomenon in physics is indeed relevant, just like how you got struggle enough to understand introduction of quantum mechanics for quantum chemistry. $\endgroup$– làntèrnCommented Jan 8, 2023 at 2:38
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4$\begingroup$ @NilayGhosh Surface tension is an essential concept taught in physical chemistry. $\endgroup$– Buck Thorn ♦Commented Jan 8, 2023 at 8:33
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$\begingroup$ Good point. An imbalance of forces (a net force) can arise where there is uneven curvature or three phases meet. A small volume of water suspended in air meets the requirement until becoming a tidy sphere. $\endgroup$– Buck Thorn ♦Commented Jan 8, 2023 at 8:38
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1 Answer
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Each molecule at the surface of a drop of water is pulled towards the others around it (by cohesion, i.e. in this case hydrogen bonds), those at the surface, and those below. There is nothing above it, so the net force is pointing towards the center of the drop.
In the orthogonal surface area, the forces cancel out, except if the surface was previously disturbed/deformed, i.e. the area increased from the planar/spherical ideal.
When you increase the surface area by deforming the drop, that means you pull out water molecules from the body, against that net downwards force.
Ergo, surface tension and that perpendicular force are essentially the same thing.
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$\begingroup$ It's a pity this post has been closed, because nobody has ever answered the fundamental question : How and why a radial force produces a tangential force ? $\endgroup$– MauriceCommented Jan 10, 2023 at 20:50