The tools on Viscous Fluids shelf set up FLIP simulations, which react to temperature changes. The viscous fluid tools set up a temperature attribute that drives the viscosity. For example, as the temperature of lava cools, so does the viscosity.

The Melt Object and Lava From Object tools are very similar, as they both turn geometry objects into a particle fluids with a temperature attribute that affects viscosity. The main difference is that the Melt Object tool doesn’t apply any shader. The Lava From Object assigns a shader to get a basic glowing lava look.

You would typically want to use Melt Object whenever you have an object that will change state over time. For example, a warm chocolate bunny that would slump over and then cool off and become solid. Whereas the Lava From Object is typically used for simulating lava, since it has the lava shader applied.

Both of these tools are affected by the Temperature parameter on the Physical tab of the FLIP Object. This parameter sets the initial temperature of the object. The default value is 0.5, which will cause it to melt immediately since the object starts off quite warm. A higher value will give you very hot, free-flowing liquid which cools over time and a lower value would give you cool, highly viscous fluid that heats up and melts.

The Heat Within Object and Cool Within Object tools are very similar. The main difference is that the heating tool has a high Increase Rate meaning that melting objects will heat up, and the cooling tool has a high Decrease Rate meaning that objects will cool off quickly.

The Increase/Decrease Rate parameters can be found on the POP Attribute from Volume node, and work similar to the way the Temperature Diffusion parameters on the Gas Temperature Update node work. They control how quickly the temperature from your heated or cooled object will affect your viscous fluid.

Use the tools on the Viscous Fluid shelf tab to turn objects into fluids that react to temperature.