Why Nanorobots Can't Swim Like a Scallop (2D)

A nanorobot moving through fluid lives in a world with essentially zero inertia — the Reynolds number is around 10⁻⁵, so drag dominates completely and nothing ever coasts. This 2D side-view simulator builds the simplest possible swimmer that actually works in that regime: a rigid body with two hinged paddles, based on Purcell's classic three-link swimmer. Drag the phase-offset slider to 0° and watch the paddles move in lock-step — a perfectly reciprocal stroke that goes nowhere, exactly as Purcell's scallop theorem predicts. Move it toward 90° and the two hinges trace a loop in "shape space" instead of a line, and the robot starts drifting steadily forward. Live readouts track net displacement, drift per stroke cycle, and the Reynolds number; a strip chart plots displacement over time, a mini shape-space plot shows the θ₁–θ₂ loop whose enclosed area is what actually produces thrust, and you can drag the diagram to pan the camera around the swimming nanorobot.