Optically clear, allowing an unobstructed view. Easy removal, or no removal required after use The most relevant physical properties for use in ophthalmic surgery are viscoelasticity, viscosity, pseudoplasticity, and surface tension. These physical properties of an OVD are consequences of molecular chain length, and molecular interactions between chains. The rheologic properties of an OVD directly affect its clinical characteristics. An OVD can be chosen that best matches the requirements for a specific procedure, or part of a procedure, and combinations may be useful. Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain when stretched and immediately return to their original state once the stress is removed. This property allows the OVD to retain its shape under low shear stress, and to spring back into shape after a low deforming stress is removed, but also allow instruments to be moved relatively freely when the critical shear stress is exceeded. Viscosity is a measure of its the resistance to shear deformation in a fluid at a given rate, or its resistance to flow. It quantifies the internal frictional force between adjacent layers of fluid that are in relative motion. The viscosity of a Newtonian fluid does not vary significantly with the rate of deformation. Pseudoplasticity is the characteristic of a material to rapidly transform from a gel-like consistency to a liquid and back as the shear stress is varied. A high molecular weight, high viscosity OVD at rest retains its shape well. When under sufficient shear stress it will flow, and alignment of the molecules reduces viscosity to allow rapid flow. Under low shear stress the OVD will quickly revert to an elastic gel, which is a good shock absorber. The highest shear rates occur when the OVD is passed through a cannula, in which state viscosity becomes nearly independent of molecular weight. When the molecules are aligned in the direction of flow, viscosity is determined almost entirely by the concentration. Surface tension: The coating ability of an OVD is determined partly by the intermolecular cohesive forces within the OVD, and partly adhesive forces between the OVD and the contacted tissue, instrument or IOL. The contact angle between a drop of the OVD and the other material on a flat surface is an indicator of