Kinetics of capillary tear flow in the Schirmer strip.

TLDR: Capillary flow of fluids in filter paper strips in vitro systems for the cases of unlimited supply and limited supply at constant rates, and in vivo using human subjects, who kinetically corresponded to limited supply having variable rates are analyzed.

Abstract: The popular Schirmer test of tearing ability heretofore has never been analyzed kinetically and the interpretation of the results is usually based on implicit, often erroneous assumptions. We have analyzed the capillary flow of fluids in filter paper strips in vitro systems for the cases of unlimited supply and limited supply at constant rates, and in vivo using human subjects, who kinetically corresponded to limited supply having variable rates. The effects of evaporation, surface tension, viscosity, paper pore-size, and the wettability of the cellulose fibers in the paper were also studied. The results show that in the case of unlimited supply, the fluid absorption is kinetically identical to fluid uptake by a horizontal capillary. This implies that the paper strip can absorb fluids only at a certain maximum rate which decreases with time. At lower secretion rates, the rate of wetting length increase is linearly proportional to the secretion rate provided that evaporation is prevented. Evaporation incre...