Liquid marble computer

Liquid marbles are millimetre-scale droplets coated with hydrophobic powder that makes them roll freely without wetting surfaces. Computation is collision-based: two marbles directed at an intersection merge if their relative speed exceeds ~0.29 m/s (AND = 1, carry output) and rebound below that threshold (AND = 0, separate outputs). The three output trajectories encode AND and XOR simultaneously, forming a half-adder in a single interaction gate. By controlling routing channels and gate geometry, all classical gates (AND, OR, NOT, NAND, NOR, XOR) and the reversible Toffoli and Fredkin gates can be constructed. The Fredkin gate conserves marble count — no information is destroyed — making this a physical substrate for reversible and potentially thermodynamically efficient computing. Speed: ~0.1–1 s per gate (marble travel time at cm/s speeds). Capacity: gate-level; multi-cycle datapath demonstrated in simulation.