Viscosity is the “thickness” of a fluid. For example, water has low viscosity, honey has higher viscosity, and raw bread dough has very high viscosity.
By default, FLIP fluids have no viscosity. These parameters let you add uniform viscosity or varying viscosity across the fluid.
Tip
The value of the Viscosity parameter depends on the scale of the particles. At the default scale, you will need values around 1000 for a thick fluid, and around 10000 for a doughy fluid.
| To... | Do this |
|---|---|
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Set the uniform viscosity of a FLIP fluid object |
In the or Use the |
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Simulate varying viscosity |
You can use a per-particle viscosity attribute to control a varying viscosity field.
or If you override the Viscosity field through the Volume Velocity input to the FLIP Solver, you can arbitrarily edit the viscosity field. |
FLIP Solver node
Viscosity tips ¶
If Under-Resolved Particles is set to Treat as Ballistic, the FLIP Solver will detect particles that are leaving the surface and are too small to be resolved on the grid and treat them ballistically. This works well for fluid with no viscosity, but for viscous liquids it can lead to particles creeping off the surface even with very high viscosity. Setting the Under-Resolved Particles method to No Detection turns off this behavior and should stop particle creep.
However, if your sim is only slightly viscous, individual flying
particles might freeze in air if they are too small to be resolved
on the grid. Increasing the Particle Radius Scale on the 
FLIP
Object to >= sqrt(3) or decreasing the Grid
Scale to about 1.4 should alleviate this problem.
The FLIP Solver computes the number of substeps to calculate based on velocity. While the viscosity solver should always be stable, it will be more accurate with smaller substeps. A slow moving, highly viscous sim might need more substeps than the 1 that the FLIP Solver will give it based on its velocity. Increase Minimum Substeps until you get sufficient viscosity.
Metaballs might provide better results than Average Position
for surfacing goopy, viscous fluids with the 
Particle Fluid Surface
SOP.
When surfacing very slow moving viscous particles, occasionally particles newly introduced by the reseeding process can be apparent. Turning off Reseeding should alleviate this problem.
The Surface visualization on the FLIP
Object can be a good way to preview viscosity
sims.
Choosing Float 32-bit for the precision on the Viscosity tab will speed up the viscosity solver and is sufficient accuracy for most simulations. If you have a fast GPU, enabling OpenCL acceleration via the Use OpenCL parameter on the Solver tab can further accelerate high resolution simulations of viscous fluids.