Question

Ball pen functions on the principle of

Answer 1

it functions on the principle of surface tension

Answer 2

the working principle of a ball pen:-

A sticky ink goes down a tube obstructed at the bottom end by a tiny little ball. There is a narrow clearance between the tube and the ball so that when the ball is rolled across a piece of paper it entrains a thin film of sticky ink from the other side and deposits it onto the paper. Because the ink is sticky, it feels like the ink is instantaneously dry.  Another advantage is consistent deposition (function of the clearance between tube and ball) regardless of how much pressure is applied by the writing hand. Under normal circumstances also, it does not leak in your pocket. 

It is a relatively recent invention dating back to the late 19th Century.