(D A thermal ink jet printhead ejects ink droplets on demand by utilizing the conservation of momentum of collapsing bubbles in a layer (30) of liquid ink having a predetermined thickness. The printhead has an ink containing chamber (31) with an array of individually addressable heating elements (24) on one chamber interior surface which are aligned with an elongated opening (19) in a parallel, confronting chamber wall. The spacing between the chamber wall with the elongated opening and the chamber surface with the heating elements provide the desired ink layer thickness. Selectively addressed heating elements (24) momentarily produce vapor bubbles in the ink layer (30). When the bubbles collapse radially inward towards their respective heating elements, an oppositely directed force perpendicular to the heating element is generated which is large enough to overcome the surface tension of the ink meniscus (32) in the elongated opening (19) and propel a droplet (18) of ink therefrom towards a recording medium.