in impulse ink jet printing , the method of , and apparatus for , controlling the projection of ink fluid droplets towards a printing surface by controlling the viscosity of the printing fluid at an orifice . an entire array of orifices can thereby be driven by a single pump mechanism . in one embodiment , the printing fluid used may have a liquid crystal polymer in suspension . electrical fields can be selectively induced , in one instance , to so orient the crystals as to allow droplets to be projected through the orifice and , in another instance , to so orient the crystals as to prevent droplets from being projected . in another embodiment , heaters are provided at the orifice to heat the fluid sufficiently to allow droplets to be projected and a heat sink adequate to cool the fluid when the heater is turned off to prevent the projection of droplets .