subfluid

/sʌbˈfluɪd/
noun
  1. A fluid that is part of or beneath another fluid layer, especially in scientific or technical contexts.
    • The subfluid in the tank was denser and settled below the main liquid.
    • In the experiment, the subfluid layer remained cooler than the surface liquid.
    • Geologists study the subfluid beneath the Earth's crust to understand magma movement.