  Purpose: To determine whether evaporation, tangential flow, or a combination
of the two cause tear film breakup in a variety of instances; to estimate
related breakup parameters that cannot be measured in breakup during subject
trials; and to validate our procedure against previous work. Methods: Five
ordinary differential equation models for tear film thinning were designed that
model evaporation, osmosis, and various types of flow. Eight tear film breakup
instances of five healthy subjects that were identified in fluorescence images
in previous work were fit with these five models. The fitting procedure used a
nonlinear least squares optimization that minimized the difference of the
computed theoretical fluorescent intensity from the models and the experimental
fluorescent intensity from the images. The optimization was conducted over the
evaporation rate and up to three flow rate parameters. The smallest norm of the
difference was determined to correspond to the model that best explained the
tear film dynamics. Results: All of the breakup instances were best fit by
models with time-dependent flow. Our optimal parameter values and thinning rate
and fluid flow profiles compare well with previous partial differential
equation model results in most instances. Conclusion: Our fitting procedure
suggests that the combination of the Marangoni effect and evaporation cause
most of the breakup instances. Comparison with results from previous work
suggests that the simplified models can capture the essential tear film
dynamics in most cases, thereby validating this procedure as one that could be
used on many other instances.
