# How does liquid stay in a Pasteur pipette/eye dropper instead of dripping out?

From what I understand about how pipettes work (correct me if I am wrong), when you squeeze the bulb of a pipette, you are removing the air from it and when you dip the end in liquid and release the bulb, the area increases so the pressure in the bulb drops. So due to the atmospheric pressure pushing down on the liquid surface outside the pipette and not having an equal pressure pushing down inside the pipette, the liquid rises into the pipette until the area in the bulb gives an equal pressure to atmospheric pressure.

However, I don't understand how the liquid stays in the pipette once you raise it out of the liquid and doesn't fall out until you squeeze the bulb again. It seems like the liquid should be heavy enough to fall out? Is it due to intermolecular forces holding the liquid there?

There are two factors at play: the first is surface tension, which tends to generalize the action of forces across an area, instead of those forces localizing at one infinitismal point. Second, atmospheric pressure is still in effect. As gravity pulls on the liquid in the pipette, pressure in the bulb diminishes, so external atmospheric pressure pushes back on the liquid. It DOES move down the pipette, but only until those pressures equalize.

If the pipette is long and thin, then the force of the liquid exerted by gravity is not enough to overcome the force exerted by the air as the liquid tries to fight its way out. Because of the short radius of the pipette, the fluid would have to travel very far to get out of the tube. If the pipette were short and fat, then the liquid has a much shorter "Travel" distance and thus it wouldn't take much to get it out. Another reason is that the effect of movement (bumping, shaking, etc.) on a short and fat tube would more easily cause the liquid to clear the opening, allowing air to escape.

If atmospheric pressure is the fact then I can say that even the water of a glass of water reversed downward would not fall due to atm pressure. Moreover, when the pipette is short the pressure of fluid ( $h\rho g$ ) would be smaller and it would stay at the pipette more effectively due to atm pressure.