Abstract: Apparatus and methods for the controlled introduction of submicron-sized bubbles into a fluid stream. The gas-micronizer apparatus of the present invention operates in a manner that maximizes the turbulence of the liquid flow stream at the interface of the porous surface while flowing essentially parallel to the porous surface through which gas enters to form bubbles, thereby ensuring that the gas streams through the pores are dispersed quickly into the bulk liquid flow and/or are of sufficiently small size as to behave as rigid spheres that do not easily coalesce. In a preferred embodiment, the membrane is of generally tubular shape with a nominal surface porosity of 25-30%, and an average pore size below 1 micron (and ideally about 0.2 micron). Fluid is provided to the inlet of the membrane element (by means of a pump) through an inwardly tapered approach conduit. An outwardly tapered exit conduit extends from the outlet end of the element, thereby forming a venturi arrangement.