Slow cross-stream mixing in micro-fluidic devices poses a significant challenge in realising efficient lab on -a-chip technologies. Due to the small dimension and flow velocity, the flow is in the laminar regime, and this results in slow molecular cross-stream diffusion (in contrast to the fast turbulent mixing by cross-stream eddies in industrial applications). Here, we demonstrate a simple and powerful strategy for ultra-fast mixing in a microchannel with one soft wall with height as low as 35 Lim at a Reynolds number as low as 226. There is a spontaneous transition from a laminar flow to a turbulent flow state when the flow rate increases beyond a threshold value, resulting in complete cross-stream mixing. After transition, the mixing time across a channel of width 0.5 mm is smaller, by a factor of 105, than that for a laminar flow, and complete mixing is achieved within a channel length of 2 cm. The increased mixing rate comes at very little energy cost, because the pressure drop is comparable to that required in current microfluidic devices, and it increases continuously and modestly at transition. This is because the channel length required to achieve complete mixing, 2 cm, is much smaller than that used in microfluidic devices that employ diffusive mixing; in addition, the deformation of the soft wall decreases the resistance to flow. (C) 2016 Elsevier Ltd. All rights reserved.