Provided herein is a microfluidic device and related method for controlling flow of different fluid components of a fluid. The microfluidic device comprises an input channel, focusing channel and an assaying channel. The microfluidic device is adapted to separate a fluid into at least two fluid components, and is further adapted to detect a target material comprised within one of the fluid components. The method comprises providing a channel, the channel having a dimension which is a function of a dimension of one of the fluid components and deliver the fluid through the channel at a set flow rate.