Enhancing heat transfer in microchannels usually means adding structure. Fins, grooves, ribs, turbulators — geometric features that disrupt laminar flow, create secondary vortices, and mix the fluid. The mixing brings fresh coolant to the hot wall. The cost is pressure drop. Every geometric perturbation that helps heat transfer also impedes flow, requiring more pumping power to push the same volume of coolant through the channel.
Chej, Carusela, Monastra, Harting, and Malgaretti (arXiv:2603.09607, March 2026) show that the mixing can be achieved without geometry. The channel is straight. The walls are smooth. Nothing protrudes. But the walls are patterned: some regions are slippery (high-slip boundary condition), others are sticky (no-slip). The alternating pattern breaks the symmetry of the velocity profile, generating secondary flows — swirl — in a straight, unmodified channel. The swirl mixes the fluid transversely, bringing cool fluid to the wall and pushing warm fluid to the center. Heat transfer improves.
The result requires no additional pumping power. The pattern changes where the fluid moves fast and slow, but the total volumetric flow rate is unchanged. The swirl is free — not free in the thermodynamic sense (it is driven by pressure), but free in the engineering sense: it costs nothing beyond what the unpatterned channel already requires. The mixing that geometric features buy with pressure drop, the boundary condition pattern provides at no incremental cost.
The mechanism is symmetry-breaking at the boundary, not in the bulk. A uniform channel with uniform walls has a parabolic velocity profile — no transverse component. A channel with alternating slip and no-slip regions has a velocity profile that varies along the wall, generating lateral pressure gradients that drive transverse flow. The swirl is a three-dimensional flow pattern in a two-dimensional geometry, created entirely by the surface condition.
The through-claim: the fin is in the boundary condition, not in the wall. What matters for mixing is the spatial variation of the flow velocity near the surface, and this variation can be achieved by patterning a property — slipperiness — rather than a geometry. The channel has no fins. The channel does not need fins. The boundary condition is the fin.
Chej, Carusela, Monastra, Harting, and Malgaretti, "Swirl flow in microchannels: patterned slip walls enhance heat transport," arXiv:2603.09607 (March 2026).