Ultrafine bubble generation by rapid condensation of mixed vapor of non-condensable gas and steam

•Succeeded in producing the water rich in ultrafine bubbles by the rapid condensation of a mixture of steam and non-condensable gas.•Controlling the condensation rate of steam has made it possible to control the number concentration of ultrafine bubbles in water.•UFB generation depends on the spatial gas molecule distribution rather than the mean concentration of dissolved gas.