The tear film on the human eye is a thin liquid layer — roughly 3 to 5 micrometers thick — that must remain continuous between blinks to protect the cornea and maintain optical clarity. When it ruptures, the eye dries, vision blurs, and discomfort follows. Models of tear film breakup have traditionally treated the corneal surface as smooth, but corneas are not smooth. Their topography — epithelial cell ridges, microvilli, surface irregularities — interacts with the thin film dynamics.

Surface roughness significantly accelerates tear film rupture. The mathematical model incorporates not just the roughness geometry but also slip effects (how the fluid moves at the corneal surface), van der Waals molecular forces (which thin the film from below), lipid layer transport (which stabilizes the film from above), and the mucin layer (which governs wettability). The roughness creates local thin spots where molecular forces can overcome surface tension earlier than they would on a smooth surface. The film breaks at the peaks of the roughness, not in the valleys.

The structural insight is about where vulnerability concentrates. A uniform thin film on a smooth surface breaks uniformly — the whole film thins at the same rate. On a rough surface, breakup localizes. The roughness does not create the instability (that comes from van der Waals forces), but it selects where it initiates. The topography acts as a template for failure, converting a global instability into a local event.

This has clinical implications for contact lens wear. Contact lenses change the effective roughness of the surface. If the lens surface is smoother than the cornea, it might stabilize the film; if rougher, it accelerates breakup. The geometry of the surface, not just its chemistry, determines how long the film survives.

(arXiv:2603.18491)