A Controlled-Area Droplet Evaporimeter Reveals Surface Pressure-Dependent Evaporation Resistance of the Tear Film Lipid Layer

Purpose: The purpose of this study was to quantitatively determine the intrinsic evaporation resistance of the tear film lipid layer (TFLL) and resolve discrepancies between in vivo and in vitro findings. Methods: A controlled-area droplet evaporimeter (CADE) was developed based on constrained drop surfactometry. The system integrates three independent feedback control loops to maintain constant droplet surface area, surface temperature (34°C), and environmental relative humidity (30%-70%). A model TFLL composed of representative polar and nonpolar lipids was studied. Evaporation flux was measured under controlled interfacial conditions at defined surface pressures. Atomic force microscopy (AFM) was used to characterize TFLL lateral structure and topography. Results: CADE enabled direct quantification of TFLL evaporation resistance under physiologically relevant conditions. Evaporation resistance increased significantly with surface pressure, from approximately 11% at 20 mN/m to approximately 37% at 35 mN/m, consistent with values reported in vivo. In contrast to a pure air-water surface exhibiting diffusion-limited evaporation, TFLL-covered surfaces displayed interface-limited behavior, indicating that the lipid layer introduces a dominant transport resistance. AFM revealed a concurrent structural transition from heterogeneous nonpolar lipid lenses to a more uniform and continuous dual-layered film with increasing surface pressure. Conclusions: The TFLL provides a substantial and surface pressure-dependent barrier to evaporation, governed by interfacial organization rather than thickness. By isolating intrinsic TFLL properties, CADE reconciles discrepancies between in vivo and in vitro measurements and establishes a mechanistic framework linking lipid organization to evaporation resistance. This platform offers new opportunities for studying tear film biophysics and for developing lipid-based therapies for evaporative dry eye disease (DED).