Or, how do mermaids cry and sweat, and what it looks like in the water. This is the second in a set of posts for those speculative fiction authors out there who have already–or are planning to–dive into a stories with humans/half-humans that live and breathe underwater. (See the first, How do mermaids hear? on underwater acoustics).
Right off, I’ll say if you’re a mermaid and someone’s trying to sell you the "never let them see you sweat" line, keep your money.
Let’s start with an experiment. Take a glass of fresh water, a glass of saltwater (mix in a few tablespoons of salt into 4oz/118ml of water), and with a teaspoon, pour the saltwater into the fresh a few drops at a time. What do you see? The mixing of fluids of differing salinity affect the refraction, the way light comes through the fluid. Where the two mix, there’s a blurry swirl in the water.
I’ve tried to capture it here in these images. The one on the left is the glass of freshwater, the right has some saltwater mixing in. This also works in reverse. Pour the freshwater water into the saltwater, and you get the same swirls and blurriness.
Close-ups of this:
What’s happening here? It’s all about salinity, or the measure of total dissolved salts in water. (Salts come in many flavors and compound varieties, but we don’t need to go into that here).
The salinity of human tears, sweat, blood plasma, amniotic fluid are around 9PPT (parts per thousand) and seawater is around 35PPT (These numbers vary, for example seawater sampled in the north Atlantic is less saline than water sampled from the Red Sea).
What it comes down to is that even though we have much the same properties as seawater, we are, well, less salty. When a mermaid cries, her tears take some time to blend into the saltier water around her eyes. She may have trouble seeing through a good fit of sobbing.
The lacrimation system, primarily used for cleaning and lubricating the eyes, includes the gland, reservoir, and canals that manage tear production in most land mammals. Tears are salty, but they don’t sting because our eyes are already accustomed to the salt content in the fluid that protects them. This protective fluid for the eyes is actually a set of three different substances that make up the tear film, each layered on top of the other, the outermost lipid layer, aqueous layer, and a mucous layer. (For the different kinds of tears, basal, reflex, and weeping, see the Wikipedia article on this).
There are around 650 sweat glands in an average square inch of your skin, and although the mineral composition of sweat changes with the individual and the source of sweating, the blurring effect of mixing two fluids of differing salinity still applies. In other words, you would be able to see a mermaid sweat, a thin blurry layer of water over her skin.
All of this assumes that your mermaids, mermen, selkies, nereids, people of the sea, have typical human skin and tear functions.
The concentration of sodium in thermal sweat, M. G. Bulmer and G. D. Forwell