Configure Liquid Boundary Conditions in Blender

Configuring boundary conditions for liquid simulations in Blender's Mantaflow engine controls how fluid interacts with domain walls and external objects. Properly setting these parameters determines whether fluid escapes the scene, sticks to surfaces, or bounces off obstacles, which is essential for achieving physically accurate and visually convincing fluid behavior.

Configuring Domain Border Collisions

The primary boundary in any simulation is the Domain itself. By default, the domain's outer bounds can either act as solid walls or open space.

  1. Select your Domain object and open the Physics Properties tab.
  2. In the Liquid settings panel, locate the Border Collisions section.
  3. Toggle the checkmarks for the individual axes (-X, +X, -Y, +Y, -Z, +Z):
    • Checked: The domain wall acts as a solid container, reflecting and containing the liquid.
    • Unchecked: The domain wall acts as an open boundary (outflow), allowing any liquid touching the edge to exit the simulation permanently.

Setting Up Effector Objects as Boundaries

To make fluid interact with custom objects inside the domain (such as cups, pipes, or floors), you must define them as Effectors.

  1. Select the obstacle mesh.
  2. In the Physics Properties tab, click Fluid, set the Type to Effector, and ensure the Effector Type is set to Collision.
  3. Configure the boundary behavior settings:
    • Surface Thickness: Adds an invisible collision buffer around the mesh. Increase this value if fluid particles leak through thin geometry.
    • Is Planar: Enable this only if the obstacle is a single, zero-thickness plane. For solid meshes with volume, keep it disabled.
    • Guide Mode / Velocity Factor: Controls how moving obstacles impart velocity to the fluid upon collision.

Adjusting Boundary Slip and Friction

Boundary friction dictates how liquid moves across solid surfaces:

  • Free Slip: Fluid moves across boundaries with zero resistance, ideal for low-viscosity liquids like water on smooth glass.
  • No Slip / Partial Slip: Fluid slows down or clings near the boundary surface, essential for thick liquids like honey, oil, or paint.

To adjust surface interaction:

  1. Select the Domain object.
  2. Under the Liquid settings, enable Viscosity if simulating thick fluids.
  3. In the Effector object settings, adjust the Friction value to determine how much the boundary slows down the adjacent fluid layer.

Preventing Fluid Leaks at Boundaries

If liquid passes through boundaries unexpectedly, apply these adjustments:

  • Apply Transforms: Ensure all objects have their scale applied (Ctrl + A > Apply All Transforms) to prevent coordinate distortion.
  • Recalculate Normals: In Edit Mode, select all faces (A) and press Shift + N to ensure face normals point outward toward the fluid.
  • Increase Resolution Divisions: In the Domain settings, increase the Resolution Divisions to make the collision grid finer.
  • Increase Timesteps: In the Domain > Settings panel, increase Timesteps Maximum and lower the CFL Number to calculate collisions more frequently between animation frames.