A bristle of a liquid-dispensing hairbrush designed to deliver low-viscosity liquids to a user's scalp during hair-brushing only by gravity and not capillary action. The bristle has a tube with an internal channel with a maximum diameter M and a length L. There is a roller-ball nozzle having a ball with diameter B and a ball seat into which the ball movable fits. The ball seat has radially spaced apart inwardly directed protrusion portions with protrusion portion spaces therebetween. The protrusion portions have apexes, wherein a central hole is defined by an imaginary circle which contacts the apexes of the protrusion portions, and the central hole has a diameter, and the ball seat has a minimum cross-sectional area S which is free for liquid flow. The protrusion portion spaces are large enough so that liquid can flow through the protrusion portion spaces and through the central hole only by gravity, and not by capillary action. A ratio of 8 8 is satisfied. Furthermore, the ratio of M/B