An apparatus and process for making a foam having a controlled size distribution of gas bubbles in a liquid matrix. The invention utilizes a porous material having a controlled pore size and pore distance to produce a substantially uniform size distribution of gas bubbles； a gas pumping device for directing a flow of gas to and through the porous material to form the gas bubbles； a fluid pumping device for directing a flow of liquid matrix past the porous material and a rotating element moving in the vicinity of the membrane surface causing an additional flow to detach, collect accumulate and entrain the gas bubbles in the liquid matrix to form a foam having gas bubbles of generally uniform size and a substantially uniform gas bubble size distribution. Advantageously, the pore size and pore distance of the porous material, the gas flow from the gas pumping device, the flow field generated by the rotating element and the liquid flow from the fluid pumping device cooperate to provide gas bubbles having a mean diameter X50,0 that is less than 2-2.5 times, preferably less than 1.25-1.5 times the mean pore diameter of the membrane and to provide the foam with a gas bubble diameter distribution ratio X90,0/X10,0 that is less than 5, preferably less than 3.
