Flow behaviour of vitreous humour biofluid during saccadic eye movements

Saccadic movements are rapid movements of the eye, which allow the eye to rapidly refixate from one object to another under voluntary control. These movements are considered the most important in inducing fluid motion in the eye. The fluid that occupies most of the eyeball is called vitreous humour (VH) and is a complex, gel-like viscoelastic fluid. It is known that VH is only produced during the embryonic stage and becomes progressively liquefied with age, and as a consequence its rheological properties change. The opensource software OpenFOAM was used to investigate the dynamic response of VH during saccadic movements. Viscoelastic fluid flow solvers were adapted to work with dynamic meshes. The flow behaviour of the biofluid was studied on a simplified vitreous cavity and on a realistic eye chamber geometry. Saccadic movements with rotations between 10 ̊ and 50 ̊ were studied considering both Newtonian and viscoelastic fluid models, where the parameters for the latter were obtained by fitting the Giesekus model to rheological data measured experimentally.
The results show that different degrees of rotation of saccadic movements produce distinct maximum angular velocities and consequently differences in the velocity profiles within the vitreous cavity. Changes in the viscosity of the fluid also affect significantly the results as consequence of the impact of viscosity on the diffusive time scale of the VH velocity field development. Moreover, the elastic behaviour of the fluid affects the velocity field and the stresses acting on the walls. Finally, the shape assumed for the VH cavity also affects the results, mostly in the anterior part of the cavity due to the indentation of the lens.