# Fluid simulation fluid either pass through effector or it floating above the effector

I used a plane as an effector, if I do not check the "is_planer" the liquid will pass through the plane, if I check that option, the fluid just floating above the plane, I can not manage to understand the internals why the fluid will pass through the plane, can someone explain to me and how to fix it.

No, it's not newbie-friendly ;) I was about to write an elaborate answer, but most of the things are quite easy to find out by testing some settings. If you enter a Surface Thickness value (greater than the domain's resolution), you won't need to check Is Planar to make the plane "detectable". If you enable Is Planar it even works with a Surface Thickness of 0.

However, the distance between the fluid and the plane is dependent on both, the domain's resolution and the Surface Thickness (if > 0). Which means the fluid floats above the surface no matter if the thickness is equal or greater than 0. Thickness will not be added on either top or bottom but around the plane. If you rotate it by 90° so it stands vertical, fluid will keep a distance on both sides (and even on the top edge). By the way: using Is Planar and Surface Thickness will result in even larger gaps than using only one of these options.

Since a plane has a thickness of 0 but all settings to make it affect the fluid will result in strange looking invisible borders, it is best to really use a cuboid with a real thickness instead of a plane. And this thickness should at least be slightly larger than the domain's resolution to work properly.

Here's just a few comparisons between using Is Planar or Surface Thickness and the influence of two domain resolutions, 32 and 64 - which are both quite low for a good simulation. On the left side the plane is horizontal, on the right it's vertical (gosh, now it's an elaborate answer after all):

• Thanks a lot, I tried and the rules proved to be correct, which is the effector will be valid only if the resolution( the small cube in the corner) is smaller than the thickness of the plane, but do you know why this happened, I mean why the effector will be invalid if the thickness is smaller than the cube, Commented May 18, 2021 at 9:45
• the cube with a thickness greater than the resolution can be used as effector, do not why the thickness effect the detection of the droplet Commented May 18, 2021 at 9:48
• I'm no developer and don't know what's behind all these simulations, but the most simple reason I can imagine is that with a low resolution you have a wider "grid" of checkpoints so that a smaller thickness falls inbetween and is ignored. Commented May 18, 2021 at 9:49
• Imagine it like that: in a 5 x 5 x 5 m domain, a resolution of 32 means it is subdivided into smaller cubes of 0.16 x 0.16 x 0.16 m. Now if a plane or cube has a thickness of 0.10 then a voxel (these small resolution cubes) can be over and beneath the plane at the same time. And when it's beneath, it will fall to the ground. Commented May 18, 2021 at 9:56
• @Chris It's all try & error :) Commented May 18, 2021 at 10:25

The plane itself is no "real" object, because it has no thickness. So it is treated from Blender in the fluid simulation as it isn't there at all. Either you can give it thickness by applying a solidify modifier or extrude the vertices on z-axis. Then the fluid simulation will recognize it.

• Should the thickness of the "plane" be greater than the "Resolution Division" to make the droplets detectable? Commented May 18, 2021 at 4:29
• Honestly - I don’t know. I just start with a big number to check wether it works at all - then I try out and iterate to lower the number/thickness as less as it can be. It’s trial and error for me - like in most cases in Blender - for me. Commented May 18, 2021 at 5:00
• This flowsim is not newbie friendly Commented May 18, 2021 at 5:03