Disclosed herein is a device comprising a pair of bellows pumps configured for efficient mixing at a microfluidic scale. By moving a fluid sample and particles in suspension through an aperture between the paired bellows pump mixing chambers, molecular collisions leading to binding between the particles and ligands in the sample are enhanced. Such devices provide an alternative for mixing that does not use a vent and can be used with a variety of particles in suspension such as magnetic beads to capture or purify useful cells and molecules.