Abstract: Fine bubbles or droplets of a first fluid are dispersed in a liquid; a stream of a first fluid (10) is injected under pressure into a body of liquid (16), such stream having a lateral dimension relative to the direction of flow of the stream, which lateral dimension is elongate; a stream of a second fluid (12) is injected, under pressure into the body of liquid, which stream similarly has a lateral dimension relative to its direction of flow, which lateral dimension is elongate; the streams are injected such that a large two-dimensional interfacial contact area is established between the two streams; at least the second fluid is a liquid, the first fluid may be a liquid or a gas; the fine bubbles or droplets are produced with a lower energy requirement than prior art devices.