a drop - on - demand ink jet printing system includes an ink channel having a nozzle orifice through which ink droplets are ejected when ink in the ink channel is subjected to a momentary positive pressure wave . an ink feed passage opens into the ink channel to transport ink into the channel from an ink reservoir . a selectively - actuated valve , associated with the ink feed passage , restricts the flow of ink through the ink feed passage when actuated . the valve is actuated in timed association with the momentary pressure wave , whereby flow of ink past the valve from the ink channel towards the reservoir is inhibited . the ink feed passage may be a microfluidic channel , and the selectively - actuated valve a heater in thermal contact with at least a portion of the associated microfluidic channel , whereby thermally - responsive ink in the ink feed passage can selectively be heated by the heater such that the thermally - responsive ink will be caused to increase in viscosity to thereby restrict backward ink flow through the ink feed passage . the ink may be comprised of a carrier having a tri - block copolymer of poly - poly - poly .