You can only learn by trying. There are no tutorials because obviously most people just don't understand how this stuff works and while I understand that stuff, personally I don't care for doing tutorials anymore. There are some secrets I keep to myself...
Try a few tests. Create some gradients in a precomp and use that precomp as your flow map and watch how the particles behave.
Keep the flow map smooth. Sharp edges are bad. Nice, smooth, blurry flow maps are good.
To use flow maps, you can use the Ramp or Gradiant Ramp effect on another layer. (The Ramp and Gradiant Ramp effects are the same, the name was changed as part of a recent update)
To use that effect, or any effect that generates a look, properly for a flow map, you will need to create the solid or apply whatever generating effect you want, then pre-compose the layer with all attributes moved into the new composition. Flow mpas look at the base layer, ignoring any effects applied to that layer. Pre-composing will remove that issue.
The Flow maps will treat the gradient as a gravity map. The white areas are the high points, with the black areas being low. The bubbles will roll down to the low areas.
Not sure if that will be what you need for your effect.
Flow Maps are grayscale images that can be generated in After Effects or Photoshop or any other bitmap editing software. In general, particles generated by the foam affect will avoid blurred boundaries between black and white areas of the Flow Map.
Its the blurred edge which is important.
You'll note a setting under Flow Map called steepness. This refers to how the shift from black to white via the blurred edge in interpreted. You could imagine the blur as a hill between the black and white areas of the map. Bubbles avoid going up or down hills and will tend to remain the black or the white area of the Flow Map, depending on where the Producer Point is.
If you find bubbles escaping the Flow Map, try increasing the steepness and or altering the simulation quality to Intense.
Brian Maffitt created a fabulous tutorial for Foam when he invented the Evolution plugins. That video may be available in the vault or dusti attic section of Total Training or you might call them and ask if ye olde VHS copies can still be purchased.
He did a fabulous job of explaining all the nuances of Foam. Hope you can find it.
The most critical thing about flow maps is, as the others have pointed out, they cannot have hard edges. You don't need to move from whte to black in your gradients or blurs. The effect is quite sensitive to differences in grays. But all of the phyics settings are interactive so a minor adjustment on one can have dramatic effects on others.
Just start playing around. Keep your maps simple and the foam settings low until you get a feel for what you can and cannot do. KNow in advance that Foam, as cool as it is, is not an intelligent flocking system.
Thanks for the responses, but . . . . I'm still confused. First, is there a better way in AE to make bubbles (blood cells) flow through tubes, like in a "U" shape, or a snake shape? Using a mask allows bubbles to escape the "U" shape. Adobe's Foam instructions says to use an "Alpha" path, how would I do that in AE? And then how would I blur the edges of a "U" shape (or a snake shape) Alpha pathto create grey scales along both sides (of an Alpha path)?
I know about precomposing, and ramps, but I can't find anything about Alpha paths or creating grey scales on all sides of a tube. That was my main question. How would I create an Alpha path in AE with grey scale boarders? thanks again, n
PS or is there a better AE tool to create this effect?
Not sure where you are seeing the term "Alpha path". Alpha refers to transparency.
Adobe's help on Foam says the following:
Flow Map controls
The Flow Map controls specify the map that the flow of the foam follows:
Specifies the layer used to control the direction and speed of the bubbles. Use a still image layer; if you select a movie as the flow map layer, only the first frame is used. A flow map is a height map based on luminance: White is high, and black is low. White is not infinitely high; if a bubble travels fast enough, it can travel past a white obstacle. Make sure that the map is a little blurry; sharp edges can create unpredictable results. For example, to make bubbles flow through a canyon, create a flow map with a white canyon rim, a black canyon, and blurry gray walls. Use wind to blow the bubbles in the direction you want them to flow, and the walls of the canyon contain them. You can also use a gentle gradient on the floor of the canyon to control the flow direction.
>>>> Not sure where you are seeing the term "Alpha path". Alpha refers to transparency.<<<<
Could be my fault. The OP may have found it in a search from a post I made way back in 2010. I had just been up to my ears using several of Brian's Evolution effects including Card Dance. The effect's many parameters can be driven with grayscale images or movies that are hidden in RGB and Alpha layers.