Shear mixing apparatuses and associated methods are disclosed for producing small gas bubbles of a diameter of less than about 0.1 millimeters in a liquid, whereby mass transfer of the gas into the liquid is improved (as compared to the mass transfer achieved by conventional large bubble generators under the same circumstances) in applications benefiting by such improved mass transfer, involving injecting a gas under pressure via one or more orifices into a liquid that is flowing at a velocity sufficient to cause bubbles formed at the orifice(s) to subdivide to the desired small bubble size.