Dynamics of an Artificial Tear Film on Contact Lenses in Response to a Moving Force Mimicking Fingertip Application.

Globally, it is estimated that more than 140 million people wear contact lenses. As a result, users demonstrate varying levels of dexterity and consistency when inserting lenses, often requiring fine adjustments. Moreover, the diverse structure and quality of individuals' precorneal tear films lead to different tear film responses under various insertion conditions. In this work, we mimic such tear film dynamics using a force probe coupled with reflection interference contrast microscopy. We use an in-house prepared artificial tear film solution that mimics human tear film composition deposited on commercially available soft contact lenses. The probe is used to replicate contact forces that resemble the pressure and motion of a human fingertip during lens insertion. Our findings reveal that contact-induced deformations and resulting film morphologies are sensitive to the volume, i.e., thickness of the tear film: from localized wetting ridges with memory effects for thicker films to traveling deformations with permanent wrinkling for thinner films. In extremely thin films, we observe that film evaporation dominates over contact-driven dynamics, although mobile contact forces can locally reverse rupture spots in the tear film.