If you have made a few slushies you have probably noticed that two drinks poured into the same cup can come out completely different. One turns into smooth slush in forty seconds and the other sits there watery. Here is what is actually going on.
Sugar is doing most of the work
Pure water freezes at a single temperature and turns into one solid block. Add sugar and that changes. The sugar gets in the way of water molecules locking together, so instead of one block you get a slow build up of tiny separate ice crystals with liquid still moving between them. That mixture of small crystals and liquid is what a slushy is.
This is why a regular soda gives a better texture than a diet one, and why unsweetened iced tea comes out icier than a sweetened one. It is not about flavour. It is about what the sugar does to the freezing process.
Why you squeeze instead of waiting
Left alone, the drink closest to the cold wall would freeze into a shell and the middle would stay liquid. Squeezing pushes the warmer liquid from the middle out against the wall, over and over. Every pass forms more small crystals instead of letting a few big ones grow. That is the whole trick, and it is why thirty to sixty seconds of squeezing beats five minutes of waiting.
Why temperature going in matters so much
The cup contains a fixed amount of cold. Every degree the drink has to come down uses some of it. Pour in something at room temperature and most of that budget goes into simply cooling the liquid, with little left for freezing. Pour in something already refrigerator cold and nearly all of it goes into making ice. This is why the same drink can work brilliantly one day and poorly the next.
What to expect
A well chilled sugary drink: smooth slush in thirty to sixty seconds. An unsweetened cold drink: coarser and icier, still drinkable. A room temperature drink: cold, but not slush. None of these mean the cup is broken. They are the same physics behaving differently.
