## 4 Discussion  
As shown by Brewster Angle Microscopy, here and elsewhere, MGS does not spread to a monolayer even at (very) low \(\pi\) (0.5-10 mN/m) due to the very high hydrophobicity of the meibomian lipids. Instead, rough heterogeneous MGS layers are formed at the air-water interface which turn more uniform with the raise in \(\pi\) and preserve liquid extended-like in-plane rigidity for the entire \(\pi\) range [1,2]. This is similar to the behavior of meibum-like mixtures of triglycerides and cholesterol esters, which also form non-collapsible and reversible multilayers at the air-water interface [26,27]. An inherent property of such hydrophobic lipids rich thick layers is that upon area compression the polar lipid molecules (positioned at the interface with the aqueous subphase), escape compression to condensed and collapsed states, and instead move to the upper NPL stratum of the layer [1,2,26]. During the subsequent area expansion, this upper stratum of the film serves as a reservoir from which polar lipids rapidly return back to the surface, thus rendering the stability of the layers at consecutive blink-like surface compressions and expansions. The so-described duplex film structure, i.e., a polar lipid monolayer with a continuous oily fluid (with some crystallites dispersed within) on top of it [6], ensures the non-collapsibility and the resistance to the loss of material which are characteristic of MGS layers upon continuous and consecutive blinks. These unique features offer plausible insights into how TFLL is able to maintain its composition and structure _in vivo_ at the tear surface for hours, as manifested by the TFLL turnover rate of 0.93 \(\pm\) 0.36%/min (i.e., much slower than the AT turnover of 10.3 \(\pm\) 3.7%/min) thus minimizing the necessity for the meibomian glands to deliver new lipids to the TFLL [10]. RDA films displayed behavior typical for other oil-in-water ophthalmic nanoemulsions with similar compositions [27,28]. The nanoemulsion forms highly reversible noncollapsible surface films. As already shown, SO can supplement the nonpolar lipids of MGS layers and lead to the formation of a more uniform and ordered NPL stratum [11]. It is worth discussing the possible impact of the eyedrop surface-active constituents (CO-10, MYS-40, Poloxamer 407 and menthol) on the interactions of RDA with MGS films and how they facilitate the interplay of the formulation with the meibum layers as compared to pure SO. In a recent study [29] it was found that surfactants with high hydrophilic-lipophilic balance,  
Figure 10: Stress relaxation transients of MGS films supplemented with extra amounts of MNT. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Film Composition** & **E, mN/m** & \(\tau\), s & \(\beta\) & **E\({}_{\text{EQ}}\), mN/m** \\ \hline MGS & 71.705 & 59.86 & 0.486 & 40.03 \\ \hline MGS + MNT (0.344 \(\times\) 10\({}^{16}\)) & 88.57 & 54.42 & 0.63 & 40.03 \\ \hline MGS + MNT (0.862 \(\times\) 10\({}^{16}\)) & 108.30 & 50.02 & 0.698 & 49.70 \\ \hline MGS + MNT (1.723 \(\times\) 10\({}^{16}\)) & 112.96 & 53.84 & 0.712 & 51.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Kohlrausch–Williams–Watts (KWW) equation parameters obtained from the fits of the relaxation transients (Figure 10). For all the fits the adjusted R\({}^{2}\)\(\geq\) 0.98.  
such as MYS-40, incorporate in the TFLL at the aqueous interface, and can compensate for the moderate deficiency of polar lipids. Thus, it enhances the spreading and distribution of the RDA oil phase (and overall of the non-polar lipid stratum of the layers) in the pseudo-binary MGS/RDA films. The combination of high and low HLB surfactants (CO-10 and MYS-40, respectively) ensures optimal miscibility of the formulation with TFLL [13, 29].