On tear film breakup (TBU): dynamics and imaging

Abstract

We report the results of some recent experiments to visualize tear film dynamics. We then study a mathematical model for tear film thinning and tear film breakup (TBU), a term from the ocular surface literature. The thinning is driven by an imposed tear film thinning rate which is input from in vivo measurements. Solutes representing osmolarity and fluorescein are included in the model. Osmolarity causes osmosis from the model ocular surface, and the fluorescein is used to compute the intensity corresponding closely to in vivo observations. The imposed thinning can be either one-dimensional or axisymmetric, leading to streaks or spots of TBU, respectively. For a spatially-uniform (flat) film, osmosis would cease thinning and balance mass lost due to evaporation; for these space-dependent evaporation profiles TBU does occur because osmolarity diffuses out of the TBU into the surrounding tear film, in agreement with previous results. The intensity pattern predicted based on the fluorescein concentration is compared with the computed thickness profiles; this comparison is important for interpreting in vivo observations. The non-dimensionalization introduced leads to insight about the relative importance of the competing processes; it leads to a classification of large vs small TBU regions in which different physical effects are dominant. Many regions of TBU may be considered small, revealing that the flow inside the film has an appreciable influence on fluorescence imaging of the tear film.

Dynamics of Fluorescent Imaging for Rapid Tear Thinning

“…Thus both c and f are elevated but f increases more in the thinner tear film because it diffuses slower than c . This is consistent with previous results (Peng et al, 2014; Braun et al, 2017).…”

“…Thus, using FL imaging at initial f′ above the fcr was seen as better for quantifying evaporative tear thinning (Nichols et al, 2012; Braun et al, 2014). Braun et al (2017) extended the model by including solute transport by advection and diffusion and with a spatially-varying evaporation distribution. Initial FL concentration in the self-quenching regime was best able to match computed tear film thicknesses.…”

“…As in our previous glob-driven model (Zhong et al, 2018), we consider four different evaporation distributions based on different hypotheses on the composition of the glob. However, we want to compare our results to results from an evaporation-driven spot model with elevated water loss in a central region (Braun et al, 2017). Thus, we choose a similar evaporation flux where the evaporation rate is high over the aqueous/glob interface and low over the aqueous/air interface, namely J=(1−B)vmax+Bvmin. Here vmax=10μm/min and vmin=1μm/min.…”

A model of tear-film breakup with continuous mucin concentration and viscosity profiles – CORRIGENDUM

“…To rectify the errors, we first reinstate a factor of 6π in the Hamaker constant omitted by mistake; the correct value is A = 6π × 3.5 × 10 −19 J m (Braun et al 2018). Besides, we revise the tear-film thickness to H = 0.5 µm.…”

“…This is based on recent findings that the tear film undergoes a rapid initial thinning owing to factors such as evaporation (Braun et al 2018), lipid clustering on the interface (Zhong et al 2019) and lid-associated thinning, including drainage (King-Smith, Begley & Braun 2018). Thus, within several seconds, the tear film thins from a thickness of about 3.5 µm to 0.5 µm (Braun et al 2018). It is only after this initial phase that the van der Waals force becomes the dominant driving force for tear-film rupture.…”