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When you have an object with a small opening, such as a pipette, water droplets form and hang easily. However, when you have a larger opening such as a pipe, the same water droplets do not form. Is this to do with surface tension? If so, what about small openings makes water droplets form?

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It has to do with the balance between surface tension and gravity. If a droplet seperates from the opening, the exposed area of the fluid becomes greater, which increases the energy due to the surface tension. Let's call the difference $\Delta E_s$ On the other hand, seperating the droplet decreases the potential energy due to gravity, call it $\Delta E_g$. The droplet will seperate when $\Delta E_s < \Delta E_g$.

As the opening size increases, the seperation of the droplet gives a larger $\Delta E_s$. However, the droplet that forms is also bigger and heavier, so $\Delta E_g$ also increases. The volume and weight of the droplet happen to increase faster than the area increases. If we keep increasing the opening size, gravity will win sooner or later. This is why fluids are less likely to stick to large openings.

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  • $\begingroup$ Indeed classical nucleation theory does a pretty good job answering such questions (it does so probably too often :D). $\endgroup$ – Phonon Feb 25 '16 at 2:43

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