Albert Einstein was the first to solve the mystery around why do tea leaves migrate to the center of the cup when stirring it with a spoon. Why don’ t they migrate to the edges? The answer is lied in the centripetal force of any liquid.
Einstein explained the so called ‘tea paradox’ by saying that stirring the liquid makes it spin around the cup, so in order to maintain this curved path, a centripetal force in towards the center is needed.
Near the bottom and outer edges the liquid is slowed by the friction against the cup and the centripetal force is weaker and cannot overcome the pressure gradient.