We borrowed from Fluid–Solid Interaction mechanics to build a formal model of human decision-making. An individual is a solid body with defined structure. A decision is a fluid — it flows, exerts pressure, and reshapes what it contacts. The boundary between them determines everything.
In classical Fluid–Solid Interaction, a rigid body is immersed in a flowing medium. The fluid exerts force on the solid; the solid deforms the fluid's path. Neither remains unchanged by contact.
We apply this directly to behaviour. The individual is the solid — a structured entity with measurable density, rigidity, and surface properties. The decision is the fluid — dynamic, pressurised, seeking the path of least resistance through the boundary layer.
The boundary condition governs permeability: what can pass through, what is blocked, and at what threshold. The system is the containing medium — the organisation, the team, the environment — that must absorb and equalise whatever pressure the interaction generates.
Marcus is a senior security analyst with privileged access. A financial stressor has elevated his risk surface. A decision approaches: exfiltrate proprietary client data to a competitor. Step through each phase of the FSI model.
The individual model is never static. Like a material under repeated stress, properties shift: agency hardens or softens, risk tolerance expands under financial pressure, loyalty weakens under prolonged threat of displacement.
Each pending decision is scored on viscosity, urgency, and emotional temperature. High-temperature, low-viscosity decisions move fast and penetrate weak boundaries easily. We compute the pressure differential at each boundary layer.
Resolution generates a pressure wave. We model its propagation through the containing system — who absorbs it, who amplifies it, and where fracture points exist. Second and third-order effects are the hardest to detect and the most consequential.