This tool will take an object and turn it into a source of cooling when particles pass through. You would typically use this tool if you want to cool a fluid in your scene that has been set up with another one of the tools on the Viscous Fluid shelf. For example, if you are simulating a river of lava, you can create cool zones causing the fluid to harden when it reaches certain areas to control the flow of the river.
Using Cool Within Object
Select the object to be the cooling source.
Click the Cool Within Object tool on the Viscous Fluids tab.
Select the hot fluid to be cooled, and press Enter.
Heat Within Object vs Cool Within Object
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.