Micron Pigma Color 005 fine point felt tip pigment ink pen on hot pressed smooth drawing paper (acid free). Roughly 3″ × 3″.
One of a pair of little drawings I did years ago for a friend and colleague who had trained as a Chemical Engineer. he has one of the drawings, this is the other. All those pipes, dimensionless numbers, and pressure drops – clearly managed by a talking to tiny nerdly fairies.