When creating a lava stream, we need to consider what type of lava we want to use for reference. A variety of lava types exist depending on terrain, speed, hotness, and material composition. For the purpose of this article, we will create a "pahoehoe"  (pronounced 'paw-hoey-hoey") type of lava (see the reference images above), which creates folds as it moves. However, if you intend to simulate molten lava, you can check the "Solidifying of Molten Lava" tutorial on the official documentation site.

The simulation setup consists of the following:  a highly viscous liquid is emitted from Plane001, flowing down the slope of the Ground_Plane (with a Shelled Modifier applied). The flow then collides with several rocks, creating patterns in its TexUVW. Based on the animated TexUVW that follows the flow, we can then apply diffuse maps, bump maps and displacement to it.

The liquid source is quite simply a standard plane. We mask the source with Stucco Texture, so the emission is not too uniform. A Noise modifier is added to the plane and set to animated. This way the liquid flows in different directions and varies in speed, making the lava collide with itself, generating interesting patterns in its UVs.

Since a plane with no thickness might not voxelize well when running the simulation, a shell modifier is added to Plane001. Then, we select one side of the plane and set the face IDs to ID 2. That's why we set the Polygon ID to 2 in the Liquid source. 

Another useful option of the Liquid Source is the "Inherit TexUVW From Geom". It sets the UVW Grid Channel value for each cell where fluid is emitted to the UV value of the emission geometry in that cell. When combined with the Variation option, it allows you to create an animated UVW pattern. If you are unsure of what it actually does, you can find an example here. 

The first thing to look at here is the "Texture UVW". Be sure to enable the "Texture UVW" channel data in the Output rollout. Otherwise, the liquid won't contain any TexUVW information after simulation. 

The second thing you'll notice is the extremely low value of the Time Scale. This is because we want a slow moving lava flow. We also give some value to the Default Viscosity and Surface Tension options, as these two physical properties simulate thick and viscous liquids like lava.

The third important parameter is the Texture UVW Interpolation. In this example, we give it a value of 0.001. This parameter controls the frequency of Phoenix snapshotting the liquid mesh for UVW. When the Interpolation is set to zero, the TexUVW is determined in the first frame and continues to stretch over time, never to snapshot again. When the Interpolation is more than zero (0.1 for example), Phoenix snapshots the liquid mesh over a period of time and updates its TexUVW. The following table shows the Pros and Cons of these two extreme conditions. We choose in-between values (0.001) in this article for balanced results.

The main topic of this article is TexUVW and we don't want to make it overly complicated. The whole idea of the shading network is simple: we have hot lava material and cold lava material, the two are blended with a VRayDistance Texture map as a mask (as shown in the image above).The hot lava is made of VRayLightMtl and the cold lava is made of standard VRayMaterial. So when the lava touches the rocks in the scene, it appears red hot, while other parts of the lava remain as cold and black. You can also add the ground plane in the VRayDistanceTex Objects list if you like to. The complete shading network can be found in the downloadable scene here.

To keep the scene simple, we don't use a Particle Turner, which allows you to manipulate liquid viscosity when particles age. However, when Particle Turner is used with PhoenixFDGrid Tex, you can get even more realistic molten lava, both in behavior and shading. That is worth another post or tutorial.