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Fresh water and contact lenses

  1. Jul 1, 2010 #1

    kjl

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    I kitesurf a lot in both fresh water and salt water. I wear well-fitting contact lenses that do not shift around or come out, even if I rub my eyes.

    In SALT WATER: I can crater from 20 feet in the air with a thunderous crash and get dragged through and tumbled by waves with my eyes open underwater and come up with no problems.

    In FRESH WATER: I will often get a few drops of water in my eyes, kicked up from the board or dripping down from my eyebrows or hair, feel that my contact lens feels "funky", blink my eyes once, and the contact lens will have slid off my pupil, perhaps stuck under my eyelid, sometimes folded in half (in which case the next blink helpfully ejects the lens into the river). Sometimes I can get the lens back in my blinking a lot and waiting for it to reseat itself, though it takes much longer than it would if there was no water in my eye.

    Any ideas on why fresh water would want to pry my contact lens out of my eye? I assume there's some boundary layer thing going on between the fresh water and the salty tear fluid in my eye, but I don't really know anything about it, or why it should matter.

    A quick wikipedia search at http://en.wikipedia.org/wiki/Tears says that the film on eyes are actually 3 layers (from inside to outside: a hydrophilic mucus layer that allows for an even coating of the tear film, an aqueous layer with proteins... and salt, I assume, since it tastes salty, and a hydrophobic lipid layer). I don't know where the contact lens fits into the picture, whether it sits on top of all 3 layers, sandwiched in between two of the layers, or disrupts one or more of the layers.

    Anybody have any idea why salt water and fresh water would behave so differently?


    Curious minds want to know. Perhaps I should spray my eyes with Rain-X first! (Kidding)
     
  2. jcsd
  3. Jul 2, 2010 #2
    As a Long-standing windsurfer... :biggrin:

    I wear contact lenses too and I'm well aware of and very grateful for this effect.

    It's so strong, I've had difficulty removing my lenses at the end of the day.

    I always put it down to the lenses being semi-permeable and shrinking onto the cornea.
    But I realise that's a corny explanation. :rofl:(Exits left laughing helplessly.):rofl:
     
  4. Jul 8, 2010 #3

    kjl

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    Huh, interesting - I had been thinking there was some funky process at work that made fresh water want to peel my contacts off my eyes, but perhaps your way of thinking about it is more correct: that the funky process is actually the one that cements the contacts to my eyes in salt water!

    I wonder if it's as simple as osmosis:

    In salt water, the very salty bay/ocean water sucks the water out of the less salty tear film from underneath my contact lenses through the semi-permeable contact lens, basically vacuum-cementing the contact to my eyes (ha). You are right; sometimes it is like they are superglued on there afterwards :)

    In fresh water, either the water simply sloshes around with the tear film and the contact comes off as you would normally expect any two items to come apart when immersed in water, or the slightly saltier tear film pulls water across the semi-permeable contact lens, pushing the contact lens out further from my eye until it wants to slide off.

    Interesting.

    Maybe before kitesurfing in fresh water I should put in super, super salty water eye drops to cement the lens to my eyes first, and hope that it takes a little longer for the water volume back there to build up as I start getting splashed :)
     
  5. Jul 8, 2010 #4

    Andy Resnick

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    I'm not surprised you experience a difference between salt and fresh water, and I appreciate the thoughtful post.

    You are correct, the tear film is more than just water (or saline). It's measured osmolarity of around 315 mOsm is isotonic to most bodily fluids:

    http://www.iovs.org/cgi/content/full/47/10/4309

    should be compared with sea water (2000 mOsm) and fresh water (20 mOsm). Contact lenses are stored in nearly isotonic solutions, so they are essentially 315 mOsm as well.

    Honestly, I would have expected more problems in sea water than fresh water. I suppose the hypotonic fresh water is more likely to disrupt the tear film (absorption of water) than hypertonic sea water (loss of water), but I can't think of a good reason why opening your eyes in sea water causes no problems.
     
  6. Jul 8, 2010 #5
    I dunno about actually opening your eyes underwater, It's rare to spend more than a few seconds actually below the surface and you tend to have your eyes screwed shut because you just hit the water totally out of control and from a height/ moving fast. It's a reflex to clamp every orifice shut.

    Even trapped under a sail as sometimes happens windsurfing, you are too busy frantically clawing for escape to open your eyes and look.

    It's more a case of the odd little splash in the face catching you by surprise...

    And the effect lasts for an hour or two at least. I've driven home after sailing, more than an hour after packing and stowing gear, and still had difficulty prising the darn things off my eyes.
     
  7. Jul 8, 2010 #6

    kjl

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    I rarely open my eyes underwater, but sometimes I've opened them while clawing for my control bar and seen just a huge mess of bubbles and rushing water. I think when you fall on your windsurfing rig you don't need to do much but surface and get back on; when I fall kitesurfing, I need to make sure my kite doesn't fall into the water or get eaten by a wave, so I still have to be manipulating stuff that may or may not be in my hands at the time :)

    But it turns out, opening your eyes under salt water is just fine with contacts. Weird! But the odd splash in fresh water is enough to cause them to fall out.

    I agree that sometimes that trying to take my contacts out multiple hours after having been in the water, they will still be glued to my eyes!


    Thanks for the numbers, Andy. I had not realized tear film was so much, uh, less salty (hypotonic?) in comparison to seawater.

    Perhaps I will experiment a little (the kind of experiment that reinforces ideas but proves nothing :) ) and, next time my contacts are glued to my eyes, try splashing fresh water in them to see if I can get some pulled underneath the lense, and perhaps before kiting in fresh water, putting in a few super salty drops.
     
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