How does stirring sugar sideways in coffee sweeten the top and bottom? The fluid is not pushed up and down.?

Answer 1

It's *dissolving* the sugar that takes the most time, not mixing once dissolved. The fluid flow dissolves the sugar faster by dispersing it, thereby exposing more surface area to unsaturated coffee than a lump on the bottom has. Once dissolved and, therefore, fluid, mixing occurs very rapidly with little agitation. The slightest up-down motion of the spoon will suffice.

Answer 2

the shape of the spoon would ensure that your horizontal force is translated into different directions as spoons are usually curved.
Dynamics in liquids arent as rigid as when dealing with solids, and we all know, that eventually the sugar sinks to the bottom anyway so we need a repeat stir about halfway through the coffee :)

Answer 3

Once the sugar dissolves in the coffee, the density gradients tend to equalise because of the higher diffusion of the molecules in liquids and the convection currents.