Ballpoint pens are the world’s most-used pens – millions are made and sold each day. They are beautifully simple devices that work by dispensing ink over a metal ball at the pen’s point; i.e. the ballpoint. Nonetheless, they don’t always perform as they should. Sometimes the ink stops coming out, and other times ink builds up unevenly on the metal ball, leading to smudges on the page and on our fingers.
These effects are influenced at least by a couple of factors. These include the precision and of the ball and socket assembly, as well as its tribological properties, and also the rheological and tribological behavior of the ink. For instance, if the ink’s viscosity decreases, either due to shear or temperature, more ink flows through the socket and onto the tip of the ball. Fatigue of the ball-and-socket mechanism can also lead to such increases or decreases in ink flow.