Gas to Liquid Interfaces Many paths used for pressure determination include a passage from a reqion of fluid as gas to one of fluid as liquid. One might logically ask "How does pressure change along a path that passes through an interface from fluid "A" to "B." The quick answer - if the interface is flat, there is no pressure change. However, for spherical droplets in air and for the very small diameter jets of machines that cut with water, the pressure of water inside is substantially greater than atmospheric.

We consider water that issues downward from a faucet. The sketch shows our system, taken as half of a horizontal slice of the falling water. We view the half-slice from above. By the momentum equation for the half-slice of a jet diameter, D, (as sketched) we have:

To interpret our result, realize that the surface tension of water (σ) is very small. Thus, the pressure change across the air to water interface of a jet equals zero except for jets of very small diameter (as with water jet cutting tools). A simplar calculation will show that the pressure of water within small droplets can be considerable.