The Guardian, not a journal:

Not all tears are alike. The human body produces three kinds: basal, which form an oily layer over the eyeball to keep it from drying out; reflex, which appear when an eye is bothered by cutting onions or a speck of dust and needs to flush the irritant away; and psychogenic, which are shed for emotional reasons. Notably, emotional tears have a higher protein level than basal and reflex tears, which makes them thicker and causes them to fall more slowly.

This thickness intrigues me. The longer it takes for these tears to travel down a cheek, the greater the chance that they will be noticed by another person and their message perceived. Tears are a social signal.