A microfluidic device has features that reduce the speed of flow of higher speed diffusing molecules. The features are used to separate various components of a mixture in the microfluidic device. The channel is sized to ensure laminar flow of the mixture to promote only diffusive mixing of the components. Wells extend from the channel. Components having higher diffusing velocities are slowed by the wells as the mixture proceeds through the channel. The result is that slower speed diffusing components exit the channel prior to the lower speed diffusing components. The device is formed by use of photolithographic techniques or other techniques capable of forming such features.