We create digital sub-millimeter bubble-jet emission in gas–liquid co-flows at a tapered chip zone for moderate Re. Self-similarity features are revealed, giving birth to a local model. Local self-scaling quantities are introduced to scale energies and progresses at onsets of bubble-jets, which gives a highly universal phase diagram and also a scaling law of jet velocity. The phase diagram draws a critical bubble-jetting line and jet-dropping line, as well as orthogonally overlapping flow patterns. The scaling law clarifies the compound mechanisms for bubble-jet emission by the combined competition of interfacial tension, inertia, viscosity, and the tapered geometries. These universal behaviors confirm reciprocally the similarities of the bubble-jet emission processes on behaviors and flow structures.