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This is to add up a little more detail to the discussion. Water as it exists in the form of $H_3O^+$ and $OH^-$ ions are bound together by "Van der waals forces" which is "the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds, or the electrostatic interaction of ions with one another, with neutral molecules, ...

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A Community Wiki answer to make some other people's comments permanent and tie some loose ends up. To add to Mark Mitchison's Answer, the reason that the prevailing shape is the one that minimises surface energy as he states is that, in the case of water, the liquid's total energy is an (almost) constant offset (the potential and kinetic energy of the ...

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Another way to look at it is the following. The main force on the molecules will come from other water molecules and be due to cohesion. The system will try to minimize it's energy and bond the molecules together as much as possible. This means minimizing the surface which results in a sphere.

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The droplet wants to minimise its surface energy. This energy is proportional to its surface area. So the equilibrium shape is that which minimises the surface area for fixed volume (the bulk density is fixed by the temperature and pressure).

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Let's look at what is going on when there is a liquid-gas interface. We know that in the liquid, there will be some high concentration of liquid particles, and in the gas there will be a very small concetration of particles. This mean as you move from the liquid to the gas you will see a continuous density gradient between the liquid and gas. (In practice ...

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Yes, the direction matters. Take a look at this electronic band structure diagram for gold. Atomic and electronic structure of gold clusters: understanding flakes, cages and superatoms from simple concepts If there was only one spatial dimension (like a wire), the diagram would just have energy vs momentum. In higher dimensions, instead the momentum ...

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As @JánLalinský nicely explains, surface tension is measured between two fluids, while viscosity is measured within one. Say that you have a droplet of some liquid this means that if you change the surrounding medium the liquid-surrounding surface tension changes, while the viscosity of the droplet will not. That said, if you keep the surrounding medium ...

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Both viscosity and surface tension are connected theoretically to inter-molecular forces, but they are still very different concepts. Viscosity force is a force that acts only when the fluid is moving and acts to decrease the gradient of velocity in it. Viscosity is a characterization of the fluid itself. Roughly speaking, it says how fast momentum of ...

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