My question is fairly straightforward, is a curved interface the only way a surface tension-induced pressure jump can form over that interface, or is it also possible to occur over a flat surface? Obviously, the Young-Laplace equation requires a curved interface to balance a pressure jump, which is given by:
$$\Delta p = \sigma \kappa,$$
where $\sigma$ is surface tension and $\kappa$ is interface curvature. This makes total sense for problems like a balloon where the rubber must curve for the tension to generate a net force opposite to the pressure force. However, I recently confused myself a bit after concluding that the balloon problem is not a great analog to a liquid-gas interface. For a liquid interface, the actual molecules on the surface are the same as in the bulk, they are just missing some neighbors on one side, unlike for a ballon where there are rubber molecules on the "interface" (I recognize the thickness is orders of magnitude greater than that of a liquid-air interface) and gas molecules inside. This means that even for a flat interface, there is a net inward force on a fluid-fluid interface, which should generate a balancing pressure in the bulk --- this is different for a balloon where only a tension force tangent to the interface will exist rather than also having a net inward force. Has this pressure been observed in experiments, and if so, how is it expressed mathematically, as the Young-Laplace equation will clearly not capture it?
And on this note, due we ever observe a "pressure" in a solid object due to the net interface forces into the bulk? It seems like even a solid block should be slightly (VERY slightly) compressed by this effect.
What am I missing here?