Blender Gas vs Liquid Simulation Differences

Blender utilizes a unified physics framework called Mantaflow to handle fluid simulations, dividing them into two distinct simulation types: Gas and Liquid. While both fall under fluid dynamics mathematically, they represent fundamentally different physical states of matter within 3D space. Understanding the differences in physical behavior, solver mechanics, mesh generation, and rendering requirements is essential for choosing the correct setup for your visual effects pipeline.

Physical Behavior and Properties

The primary difference lies in density, compressibility, and boundary interaction:

  • Gas Simulations: Gases (such as smoke, fire, and steam) have low density and are compressible. They expand to fill their container, disperse into the surrounding air, and are heavily influenced by temperature, buoyancy, and turbulence. Gas physics focus on how particles diffuse and how heat creates upward velocity.
  • Liquid Simulations: Liquids (such as water, oil, and honey) are largely incompressible fluids with a defined volume. They maintain a distinct surface boundary, pool at the bottom of obstacles, splash upon impact, and form droplets due to surface tension and gravity.

Solver Mechanics: Grid vs. Particle Systems

The underlying computation differs significantly based on whether a surface boundary is required:

  • Gas (Voxel Grid): Gas simulations operate on a three-dimensional grid of voxels. The solver tracks attributes such as density, heat, flame, and velocity across this grid over time. It does not calculate discrete particles or create geometry; instead, it updates density values within the volumetric domain.
  • Liquid (FLIP and APIC Particles): Liquid simulations rely on particle solvers, primarily FLIP (Fluid-Implicit-Particle) and APIC (Affine Particle-in-Cell). Blender calculates the positions and velocities of millions of individual particles and then generates a polygonal mesh over those particles to represent the fluid's surface. Liquids can also generate secondary particle systems for foam, spray, and bubbles.

Surface Generation and Geometry

  • Gas: Produces no geometric mesh. The result is purely volumetric data stored within the domain box.
  • Liquid: Requires an explicit surface mesh baking step. The domain generates an animated polygon mesh with customizable smoothing and resolution scale, allowing it to interact geometrically with scene lighting and collisions.

Shading and Rendering

Rendering pipelines reflect the structural differences of the simulation data:

  • Gas Shading: Rendered using volumetric shaders, primarily the Principled Volume node. Shading relies on simulation attributes like density (for smoke opacity) and temperature or flame (fed into emission attributes for fire).
  • Liquid Shading: Rendered using surface shaders, primarily the Principled BSDF or Glass BSDF. Shading relies on surface properties such as transmission, roughness, and the Index of Refraction (IOR) to simulate transparency and reflections.

Performance and Cache Management

  • Gas: Typically lighter on disk space and baking time for simple effects, though high-resolution noise calculations and open VDB export can increase file sizes.
  • Liquid: Computationally heavier due to particle calculations, secondary particle simulations, and surface mesh generation. Liquid caches often require significantly more disk space and processing time.