Microfluidic devices capable of efficiently mixing two or more fluid are provided. Two or more microfluidic inlet channels defined in different sheets of material meet at an overlap region in fluid communication with an outlet channel. The channels are defined through the entire thickness of stencil sheets. The overlap region may include an aperture-defining spacer layer, and/or an impedance element, such as a porous membrane, adapted to distribute at least one fluid across the entire width of the outlet channel to promote reliable fluid mixing.