Читать книгу Sticking Together - Steven Abbott - Страница 31

A downside of “thin” and a painful example of pure surface energy adhesion between nicely smooth surfaces is known to those who wear a pair of contact lenses rather longer than they should.

Under normal use, the lenses are held in place by surface tension forces from the liquid on the surface of the eye. You can readily get an idea of the scale of surface tension forces with a well-known hoop-like breakfast cereal. Float one hoop on the surface of your milk and with a pair of tweezers gently pull it upwards. You see the milk clinging to the hoop and you find it pulling a significant weight of liquid up from the surface till gravity exceeds surface tension and the hoop is freed. Those surface tension forces are not large, but for contact lenses they are more than enough to hold them in place. However, such forces alone can easily be overcome with a sudden shock, like blinking, so it requires an extra effect to keep lenses from popping out.

Remember Stefan's squeeze law? If you have a thin layer of liquid, it gets increasingly hard to squeeze it out as it gets thinner. Stefan's law works in reverse, too: if, instead of pushing, you try to pull two surfaces apart, it is hard for the liquid to flow inwards, at least for a while; you have to provide a significant force to allow that flow to happen at a significant speed. Fast, pull forces to the lenses are resisted by the difficulty of liquid flowing through a narrow gap. Together, this means that a normal moist eye has a contact lens floating on a thin and stable cushion of tears. Removing them is all about letting the tears flow easily under the lens as it is pulled away. Squeezing off a soft lens is easy. Blinking off a hard lens probably requires the blink to push liquid under the lens, to fight against the Stefan's law reluctance to flow.

The painful part is when the eye dries out after a long day of wearing lenses. Now you have surface energy contact with the eye. As we know from the gecko, these forces strongly resist a vertical pull and give a modest resistance to shear. If you could peel the lens then it would come off easily, but that isn't viable for most contact lens users: it is exceedingly tricky to catch the edge of a lens. So removing them is a difficult process of applying as much peel and shear (including the trick of holding the lens in place with a finger and rolling the eye away underneath it) and as little butt force as possible, all the while trying to induce tears to re-float the lens.