Gavin HochGraduate Student, M.S. Program
Rose-Hulman Institute of
Technology Research Interest: |
 gavin@radke.cchem.berkeley.edu |
Research Summary:
The goal of this research is to develop a model to predict transport rates of water from the post-lens tear film (POTF) through a soft contact lens due to evaporative loss during blinking. Dehydration due to evaporation from the anterior surface of the lens induces a net flux of water through the lens, depleting the post-lens tear film. In addition to evaporation, other factors such as temperature cycling, osmotic withdrawal, lens swelling, changing ambient conditions, and lens properties influence the rate of dehydration. Our proposed model will eventually quantify each of these effects.
Assuming a binary water/hydrogel system, we write the equations of continuity for water and the polymer matrix. The Stefan-Maxwell equations are written relating the driving forces for diffusion to the diffusional fluxes and gradients in chemical potential. Flory-Huggins theory is then used to determine the gradients in chemical potential in terms of the concentrations of the two species and the elastic properties of the lens. The governing equations are solved to determine the time-periodic water concentration profiles across the thickness of the contact lens. These profiles are then used to calculate the flux of water through the lens and POTF depletion rates for various properties of the lens and the environment humidity.
Based upon the proposed transport model, we find reductions in POTF thickness on the order of 0.05 - 1 micron per minute, depending on the initial water content of the lens, the diffusion coefficient for water in the hydrogel matrix, the initial lens thickness, and the ambient humidity. Assuming a typical PLTT of approximately ten microns, our model predicts that without replenishment the POTF can be reduced to zero thickness within a three-hour time. This can, in turn, lead to dryness syndrome and/or lens adhesion.