Mathematical Medicine and Biology Advance Access originally published online on July 14, 2008
Mathematical Medicine and Biology 2008 25(3):187-214; doi:10.1093/imammb/dqn013
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An overset grid method for the study of reflex tearing

Department of Mathematical Sciences, University of Delaware, Newark, DE 19711, USA
College of Optometry, The Ohio State University, Columbus, OH 43218, USA
Email: braun{at}math.udel.edu
Received on February 19, 2008. Revised on May 29, 2008. Accepted on May 30, 2008.
We present an overset grid method to simulate the evolution of human tear film thickness subject to reflex tearing. The free-surface evolution is governed by a single fourth-order non-linear equation derived from lubrication theory with specified film thickness and volume flux at each end. The model arises from considering the limiting case where the surfactant is strongly affecting the surface tension. In numerical simulations, the overset grid is composed of fine boundary grids near the upper and lower eyelids to capture localized capillary thinning referred to as black lines and a Cartesian grid covers the remaining domain. Numerical studies are performed on a non-linear test problem to confirm the accuracy and convergence of the scheme. The computations on the tear film model show qualitative agreement with in vivo tear film thickness measurements. Furthermore, the role of the black lines in the presence of tear supply from the lid margins, reflex tearing, was found to be more subtle than a barrier to tear fluid flow between the anterior of the eye and the meniscus at the lid margin. During reflex tearing, tears may flow through the region normally containing the black line and drift down over the cornea under the influence of gravity.
Keywords: tear film; reflex tearing; black line; overset grid; lubrication theory