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The panel is subdivided into three sections:

Collision

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Enable Collision

Enable or disable the fluid-object collision on demand, e.g. by animating this option.

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• When an object is in "Shell" mode (see "Volume Mode" below), "Continuous Collision Detection" is active by default. 
• If the object's "Volume Mode" is "Solid Inside" or "Solid Outside you have to deactivate "Auto" and check "Continuous Collision Detection" manually.

Interaction

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Friction

A value of 0.0 creates absolutely no friction and a perfectly even surface. Higher values can even stop particles from moving. This parameter accepts all positive between 0.0 and 1.0.

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With 10, for example, particles within a distance 10 units (depends on the actual scene scale) from the object's surface will be affected by parameters like "Friction", Bounce", etc. The value is calculated automatically based on the "Collision Geometry Detail" level. For manual control uncheck "Auto".

Volume

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Cell Size

RealFlow | Cinema 4D's fluid engines are cell-based and therefore, colliding objects have to be rasterized internally. The smaller the cells, the better and more accurate the collision, but at the cost of longer simulation times. By default, "Cell Size" is calculated automatically based on the "Collision Geometry Detail" level. For manual control uncheck "Auto". With "Auto" turned off,  "Cell size" can be animated. This allows you to refine collision detection over time and speed up the simulation, e.g. from coarse to accurate.

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Here are the three possible modes :– you can read more about this topic under → "Fluid-Object Interaction" and → "Filling Objects".

• "Solid inside" makes the fluid collide with object's outer surface.
• "Solid outside" creates a hollow object and fluid interacts with the inner surface.
• "Shell" is normally used with open objects like glasses, vases, but also planes. When active, an invisible layer is added around the object's surface. The thickness of this layer is Cell size * 3. As a consequence, there will always be a gap between the fluid and the object.

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