A micromixing apparatus includes a mixing microchannel formed in a top surface of a substrate having channel length and a variable channel width defined by a first sidewall surface and a opposing second sidewall surface. The channel width varies from a minimum channel width h to a maximum channel width H in a ratio of H:h greater than or equal to 1.1:1.0. A first inlet is for injecting a first fluid into the mixing microchannel and a second inlet for injecting a second fluid into the mixing microchannel. The first and second fluid flow in a flow direction in the mixing microchannel along the channel length. The first sidewall surface includes first curved surface portions and the second sidewall surface includes a second curved surface portions. The plurality of first curved surface portions and plurality of second curved surface portions are non-overlapping to provide a variable channel width
