Dry eye syndrome (DES), one of the most common ophthalmological diseases, is typically caused by excessive evaporation of tear fluid from the ocular surface. Excessive evaporation is linked to impaired function of the tear film lipid layer (TFLL) that covers the aqueous tear film. The principles of the evaporation resistance of the TFLL have remained unknown, however. We combined atomistic simulations with Brewster angle microscopy and surface potential experiments to explore the organization and evaporation resistance of films composed of wax esters, one of the main components of the TFLL. The results provide evidence that the evaporation resistance of the TFLL is based on crystalline-state layers of wax esters and that the evaporation rate is determined by defects in the TFLL and its coverage on the ocular surface. On the basis of the results, uncovering the nonequilibrium spreading and crystallization of TFLL films has potential to reveal new means of treating DES.

Computational Physics Laboratory Tampere University FI 33100 Tampere Finland

Department of Physics University of Helsinki FI 00014 Helsinki Finland

Helsinki Eye Lab Department of Ophthalmology University of Helsinki and Helsinki University Hospital FI 00014 Helsinki Finland

Institute of Organic Chemistry and Biochemistry Czech Academy of Sciences CZ 16610 Prague 6 Czech Republic

J Heyrovský Institute of Physical Chemistry Czech Academy of Sciences CZ 18223 Prague 8 Czech Republic

MEMPHYS Centre for Biomembrane Physics

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