// undine docs
Colliders
A collider is an object that interacts with the fluid and affects its motion.
In the simulation pipeline, colliders are part of scene configuration and continue to matter throughout boundary interaction during each simulation step.
Colliders in the pipeline
The user defines colliders during scene configuration, and the solver later accounts for that geometry during boundary interaction.
This makes colliders one of the clearest examples of how Blender scene objects and solver behavior meet.
| Step | Control | Purpose |
|---|---|---|
| Tagging | Tag Collider | Marks the selected Blender object as a collision participant. |
| Validation | Tagged in Scene: Colliders | Confirms the expected collider count before solving. |
| Preparation | Bake colliders (animated) | Builds animated collider data for the configured frame range. |
| Simulation | Collider SDF controls | Tunes how the solver predicts, bands, and resolves boundary contact. |
Why colliders matter
Collision handling is one of the core simulation subsystems listed in the project overview. Keeping this chapter separate makes it easier to document scene interaction without mixing it into unrelated controls.
Collider SDF
The Collider (SDF) section controls how collision surfaces are represented and how liquid behaves around them.
SDF settings are especially important for thin contacts, moving objects, and shots where fluid needs to slide, stick, or damp against a boundary.
| Control | Purpose | Practical effect |
|---|---|---|
| Predict Band Cells | Predicts collision bands around collider surfaces. | Higher values can help with fast motion at additional cost. |
| Boundary Band (vox) | Sets the boundary interaction band in voxels. | A wider band can make collision response more forgiving. |
| Contact Radius Scale | Scales the contact search radius. | Use when liquid contact appears too loose or too narrow around colliders. |
| Normal Damping | Damps velocity along the surface normal. | Useful for reducing bounce or jitter against collider surfaces. |
| Tangent Friction | Adds resistance along the collider tangent. | Higher values make liquid slide less freely along surfaces. |
| Tangential Capture Speed | Threshold for tangential capture behavior. | Use with no-slip behavior when liquid should adhere to moving surfaces. |
| Tangential No-Slip Strength | Strength of no-slip tangential behavior. | Higher values make the liquid follow collider motion more closely. |
| Slip Capture | Enables slip capture behavior when available. | Only affects slip; it does not increase contact distance. |
Contact enhancement
Contact Enhancement can add particles near collider contact regions to reduce dry gaps and improve refill behavior.
The panel notes that Contact Band Cells must be greater than zero to enable collider-contact refill.
| Control | Purpose | Notes |
|---|---|---|
| Contact Band Cells | Enables and sizes the contact refill band. | Set above zero to enable contact refill. |
| Contact Oversampling | Controls extra sampling in the contact region. | Higher values can improve coverage but increase particle work. |
| Contact Min PPC | Minimum particles per cell near contacts. | Useful when contact zones look under-filled. |
| Contact Mass Bias | Biases mass behavior for contact refill. | Tune carefully to avoid visible density changes. |
| Contact No Reseed Cells | Protects a band from reseeding near contact. | Can reduce unwanted particle churn around colliders. |
Thin sheets and collider-heavy shots
Thin liquid sheets and complex collider interactions are difficult regimes for any solver.
When collider-heavy shots become unstable, first test with shorter frame ranges, higher substep safety, and conservative collider SDF values before increasing mesh quality.