Sometimes we want to make the effect where there are objects appearing from a state of invisibility to visibility. MaxwellRender don’t have the phisical possibility to render invisible objects, but we can play with the Opacity channel in the maxwell materials to achieve the same effect.
It can be done for any object, not just for fluids, but depending on the 3D platform we want to use to render will need to do it in a different way.
Using Multiplier Curve for 3Dsmax.
Using Set Driven Key for Maya.
Thanks to, my colleague, Victor Sanchez for explain me the Maya things.