After a morning of grading I needed to get away from the screen for a few minutes. So I went to McDonalds where I could get a coffee, soft drink or tea for a dollar. Being close to lunch I needed more volume and wanted more sweetness than a coffee so I got a tea. They have, as you can see in the picture, both sweetened and unsweetened tea.
I have purchased sweetened tea in the past, but it is SO sweet that I can’t drink it straight. So I usually get a “half sweetened; half unsweetened” tea. It must happen a lot because when I go to the drive-through the cashier often has a special term for it, calling it something like: “One large ice tea; cut.” In any case, this time I was making the tea myself.
Now the problem with this tea is that it does not naturally mix. Recalling some demonstrations as a science teacher, the sweetened tea should be significantly denser than the unsweetened tea. The question arises then; should I put in sugared tea, then unsugared? or vice versa? Or should I add a little of one; a little of the other; a little of the first, and so on until my cup is full? Which sequence is likely to cause the best “mix.”
If I put in the sugared (denser) tea first, it will sit on the bottom while the unsweetened (less dense) tea sits on top. The result will be that the tea will be introduced to the cup in layers that will not spontaneously mix. I will have to mechanically mix them to produce a homogeneous solution. That is, I will have to stir the tea with a spoon or shake the cup. This is not a significant effort, but I hope to find a way to do it without mechanical agitation.
If I add the less dense fluid then it will be on the bottom when the denser solution is added to the cup. The result, I think, is that the denser tea will “fall” through a less dense tea, even as the less dense tea, which is more buoyant, rises. This will produce a modest convection that will mix the tea to some degree.
As an anecdotal conclusion, based on my perception rather than objective, measurable data, I can tell you this method is satisfactory: add the low density tea (unsweetened) then the high density tea (sweetened.)
I also make sure the tea squirts into the cup along its circular, inside surface -as I add both teas – so that a modest vortex does, in full disclosure, provide some degree of mechanical mixing, making the tea as heterogeneous as possible.