Journal
MATHEMATICAL MEDICINE AND BIOLOGY-A JOURNAL OF THE IMA
Volume 32, Issue 2, Pages 209-238Publisher
OXFORD UNIV PRESS
DOI: 10.1093/imammb/dqu001
Keywords
tear film; thin film; porous material; evaporation; partial differential equation; evolution equation; method of lines
Categories
Funding
- U.S. National Science Foundation, Computational Science Training for Undergraduates in the Mathematical Sciences [DMS-0639300, DMS-1107848]
- REU Program [DMS-1062633]
- Department of Defense ASSURE program
- Division Of Mathematical Sciences
- Direct For Mathematical & Physical Scien [1107848] Funding Source: National Science Foundation
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A contact lens (CL) separates the tear film into a pre-lens tear film (PrLTF), the fluid layer between the CL and the outside environment, and a post-lens tear film (PoLTF), the fluid layer between the CL and the cornea. We examine a model for evaporation of a PrLTF on a modern permeable CL allowing fluid transfer between the PrLTF and the PoLTF. Evaporation depletes the PrLTF, and continued evaporation causes depletion of the PoLTF via fluid loss through the CL. Governing equations include Navier-Stokes, heat and Darcy's equations for the fluid flow and heat transfer in the PrLTF and porous layer. The PoLTF is modelled by a fixed pressure condition on the posterior surface of the CL. The original model is simplified using lubrication theory for the PrLTF and CL applied to a sagittal plane through the eye. We obtain a partial differential equation (PDE) for the PrLTF thickness that is first-order in time and fourth-order in space. This model incorporates evaporation, conjoining pressure effects in the PrLTF, capillarity and heat transfer. For a planar film, we find that this PDE can be reduced to an ordinary differential equation (ODE) that can be solved analytically or numerically. This reduced model allows for interpretation of the various system parameters and captures most of the basic physics contained in the model. Comparisons of ODE and PDE models, including estimates for the loss of fluid through the lens due to evaporation, are given.
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