The particle-in-cell method was built at Los Alamos for compressible-fluid hydrodynamics on early high-speed computers

The particle-in-cell (PIC) method — the numerical ancestor of the algorithm that would later animate Disney's Frozen snow (claim-frozen-snow-ran-on-material-point-method-matterhorn) — originated at the Los Alamos Scientific Laboratory in the work of Francis H. Harlow around 1957–1962. Harlow's technical report on OSTI describes "a computational procedure designed for high-speed computers to study compressible fluid dynamics with large distortions." The method tracks a fluid with discrete Lagrangian marker particles that carry mass while field quantities are resolved on a fixed Eulerian background grid — a hybrid scheme suited to problems where material undergoes deformation too large for a purely grid-based (Eulerian) or purely mesh-based (Lagrangian) approach to handle cleanly.

The institutional setting is load-bearing: PIC was developed inside a nuclear-weapons laboratory, on the earliest high-speed computers, to model the violent compressible-flow regimes that lab's mission required. It is one end of a numerical lineage that runs PIC → FLIP (Brackbill, 1986) → the Material Point Method (claim-material-point-method-1993-sandia-funded-penetration-mechanics) → Disney's snow solver — a chain in which a Cold War hydrodynamics tool migrates, without conceptual reinterpretation, into computer animation.

The same hybrid Eulerian/Lagrangian instinct recurs elsewhere in the physics-of-simulation neighbourhood: the artificial-life engine Microcosmos couples elastic filaments to a Lattice-Boltzmann fluid via the Immersed Boundary Method (claim-microcosmos-elastic-filaments-differentiable-gpu-fluid), another fluid-structure scheme that pairs particles/bodies with a background grid.