A coffee ring forms when a drying droplet pins at its contact line: evaporation is fastest at the edge, outward flow carries suspended particles to the rim, and they deposit in a ring. But this is the room-temperature story. Heat the substrate and the internal flow changes character.
The paper on evaporative cooling and deposition patterns of alumina nanofluid droplets (arXiv: 2603.24367) maps the transition between deposition patterns as substrate temperature increases. At low temperatures, particles form interconnected irregular polygonal networks at the periphery — labyrinthine structures driven by slow, symmetric evaporation. At moderate temperatures, the classic coffee ring appears. At higher temperatures, dual-ring formations emerge with central particle deposits, as thermocapillary Marangoni flow competes with the evaporation-driven capillary flow.
The surface temperature is not uniform across the droplet. It remains higher near the contact line, where the thin film conducts heat from the substrate most efficiently. This temperature gradient drives Marangoni flow inward along the surface, opposing the outward capillary flow that builds the coffee ring. The internal circulation becomes asymmetric and multi-vortex, and the particles follow.
A dimensionless parameter captures the transition: the ratio of thermocapillary to capillary effects. The deposition pattern is a phase diagram indexed by this single number.
The through-claim: the coffee ring is not the default — it's a special case in which one transport mechanism dominates. Add a second mechanism (thermocapillary flow) and the deposition pattern becomes a competition. The pattern you see records which mechanism won, and the transitions between patterns mark the boundaries where neither dominates.
2603.24367. Fluid dynamics / evaporation / nanofluid / coffee ring / Marangoni flow.