There's no automatic way to do this in After effects. You would need advanced physics simulation involving inter-object collisions.
Newton ($250) might get you close with some clever blurring and matte choking but really you a need one of the full packages like Maya, C4D, Houdini, ...ect, with those simulation modules.
Sooo...you have to do it by hand (i.e frame by frame animation) You could animate the path of a mask or shape. Or if you'd rather hand draw each frame you can use any traditional animation method. There's a handy $100 plugin called Paint&Stick that would make it pretty easy to pull of right in AE. But if you want the liquid to behave realistically your going have to dig into an advanced particle simulator via any of the major 3D packages or a dedicated particle simulator like Realflow.
I have maya, but it just seemed to hard to pull off the simulation happen, and it wasn t really the style i need anyway. I have to make like a cartoonish style, mr.mercury type of liquid... to look like a b is drawn in real time on the screen by an invisible hand. Is that impossible too in flash or after effects or any kind of adobe software using non frame-by-frame animation ?
Show us the start position, the end position and a reference for the style of animation you want
Is it impossible? No, it's not impossible. However, hand animating it to make it look like Mr. Mercury would be very time consuming even for an extremely talented artist. Mr Mercury uses complex physics simulations. It would be very difficult to reproduce by hand but not impossible if you'r a talented artist.
Roei is right, more details would help get you a more specific answer.
Like I said animating a path from a "bubble of paint" to the shape you want to fill is pretty straight forward.
But it may be irrelevant as I don't even know if you are using shapes or masks or drawings or what?
Be specific about what you've done...You haven't literally created bubbles full of paint that crack open.
So be extremely literal about what you've done so far.
We need to know your process, not your results.
I w finally done it by searching after what you said on youtube and other forums, and i found out that there is a quick and perfect way to simulate it just by tracing over as a shape and animating that one with keyframes.Thank you very much !