Gravity


What Gravity Does:
On a hot summer day, you may notice that if you look just above a large dark object (like a hot road for example), you may see wavy lines rising up off the surface. What is happening is that the air in contact with the hot surface is being heated. And since hot fluid rises, (yes, air is a fluid!) this hot air goes up--just like a hot-air balloon, except there is no balloon to hold the hot air!
In a mold, the same kind of thing happens. There are some regions in the liquid metal that are hotter than others, and these areas try to float to the top. This hot fluid will rise until it gets to the surface, where it will cool a little. But while this hot fluid was rising, it was pushing the cooler fluid out of the way! This makes for a very mixed up liquid after a while. It is this mixing that makes things very hard for scientists to undertand what is going on.
Now, when you get rid of gravity, the hot fluid doesn't want to rise anymore. This is because when there is no gravity, differences in weight don't matter. Everything will just sort of "hang out" without moving unless it is made to move (like by stirring for example).