Hi @carneiroart ,
Sorry, I didn’t see your message.
Actually, there isn’t any sprite sheet at all, juste one texture with transparency. This is how it’s work :
In PopcornFx, there is a type of shader called AlphaRemap. This is basically a alpha erosion shader. There is a link of a PopcornFx tutorial for you to understand better how it’s work in PopcornFx.
But basically, it’s a simple erosion effect, work like a mask, like explained here :
I will show you how I achieve to find the shape I want.
In photoshop, I create a texture with a gradient. The gradient will define wich part of the texture will disappear first.
This is a texture and how it work. To previsualise it, I use the threshold effect in Photoshop, so I can ajust it directly to what I want :
And here, a more compicated gradient to refine the shape as I want :
So you know all of it now 
You can see upward the look of the texture I use on my effect. Feel free to comment if you have any other questions or if you don’t understand something !