## 1 Introduction  
The air-aqueous tear (AT) surface of the human eye is stabilized by the tear film lipid layer (TFLL), a ~100 nm thick viscoelastic duplex film [1, 2, 3, 4] build of a monolayer of amphiphilic polar lipids (PL) located at the AT interface and a non-polar lipid (NPL) stratum layered on top and extended towards the air [5, 6, 7]. The TFLL is composed predominantly of the lipids of the meibomian gland secretion (MGS, or meibum), a composite oily mixture consisting of >90% NPL (mostly wax (WE) and cholesteryl esters (CE), and triglycerides) and <10% PL (mainly (O-acyl)-\(\omega\)-hydroxy fatty acids) [3, 4].  
Meibomian gland disease (MGD), manifested in the quantitative or qualitative abnormalities of MGS and TFLL, affects up to 86% of all patients with dry eye syndrome (DES) and is widely considered the most common cause of DES [8, 9]. This fact, together with the extended (on the scale of hours) residence time of drug molecules incorporated in TFLL, i.e., a promise for a more durable pharmaceutical action at the ocular surface [10], defines a pressing need for the development of eyedrop formulations that interact favorably with TFLL. One such novel composition is Rohto Dry Aid (RDA), a formulation that utilizes TEARSHIELD TECHNOLOGYM to mimic real tears and restore moisture. The oil phase of the formulation consists of sesame oil (SO) as a mimic of the meibomian NPL, while the PL are emulated via a mixture of Polyoxyethylene Castor Oil 10 (CO-10), Polyoxyl40 stearate (MYS-40) and Menthol. Sesame oil with its diverse fatty acid composition of 40% monounsaturated (oleic acid), 42% polyunsaturated (linoleic acid) and saturated fats (mainly \(\sim\)8% palmitic acid and 4% stearic acid) was recently demonstrated to adequately complement the non-polar lipid stratum of SO supplemented human MGS films [11].  
Therefore, the interactions between RDA and meibomian lipids at the tear-like air-water interface warrant further investigation as it may yield valuable knowledge for the mechanism of action of RDA at the ocular surface. In order to study these interactions pseudobinary meibum/RDA films with controlled RDA content were formed at the air-water surface of the Langmuir trough. Langmuir surface balance measurements have been established as an efficient methodology for the evaluation of MGS films interfacial properties at physiologically relevant blink-like area changes [2] and were successfully implemented in the design and characterization of ophthalmic formulations [12; 13; 14; 15]. The layers (i) reorganization throughout area cycling and (ii) dilatational elasticity were probed via surface pressure (\(\pi\))-area (A) isocycles and stress relaxations, respectively. Brewster angle microscopy (BAM) was used to visualize film morphology at the different MGS/RDA ratios. Furthermore, the impact of the RDA oil phase on the spreading of meibum was characterized by using talcum powder as a tracer [16]. The impact of MGS supplementation with menthol, a specific ingredient of RDA, was also probed with Langmuir surface balance and BAM studies. The attention to menthol was determined by the lack of published information about the impact of such low molecular weight volatile amphiphilic molecules on MGS, while there are already multiple studies of the effects of plant oils and pharmaceutical emulsifiers on the properties of meibomian layers [11; 12; 13; 14; 15].