  Etiologies of tear breakup include evaporation-driven, divergent flow-driven,
and a combination of these two. A mathematical model incorporating evaporation
and lipid-driven tangential flow is fit to fluorescence imaging data. The
lipid-driven motion is hypothesized to be caused by localized excess lipid, or
"globs." Tear breakup quantities such as evaporation rates and tangential flow
rates cannot currently be directly measured during breakup. We determine such
variables by fitting mathematical models for tear breakup and the computed
fluorescent intensity to experimental intensity data gathered in vivo.
Parameter estimation is conducted via least squares minimization of the
difference between experimental data and computed answers using either the
trust-region-reflective or Levenberg-Marquardt algorithm. Best-fit
determination of tear breakup parameters supports the notion that evaporation
and divergent tangential flow can cooperate to drive breakup. The resulting
tear breakup is typically faster than purely evaporative cases. Many instances
of tear breakup may have similar causes, which suggests that interpretation of
experimental results may benefit from considering multiple mechanisms.
