Lubrication and surface chemical properties of ophthalmic solutions.

Surface chemical and lubrication parameters such as contact angle, surface tension, and coefficient of friction, together with viscosity, were measured for commercial ophthalmic solutions. A sensitive friction-testing instrument was employed to measure the coefficient of friction between low-energy polymeric surfaces, e.g., polymethylmethacrylate (PMMA). The viscosity of ten commercial tear substitutes employed in the present study ranged from 2 to 25 cp. The results showed that there is no correlation between the coefficient of friction and viscosity, surface tension, or contact angle of these tear substitutes. The coefficient of friction of tear substitutes appears to depend upon the structure, conformation, and adsorption characteristics of polymer as well as surface characteristics of sliding surfaces. A systematic study was undertaken to evaluate the effect of speed and load on the coefficient of friction between different sliding surfaces, namely PMMA/nylon and PMMA/PMMA. The analysis of data established that our system was operating in the region of "boundary lubrication." In general, a PMMA/PMMA system exhibited a lower coefficient of friction as compared with the PMMA/nylon system.