Spherical grains and sacrificial particles are mixed in a suspension. The sacrificial particles are larger than the spherical grains. The suspension is injected into a channel in a microfluidic chip, and the spherical grains form microporous structures in the channel. The microporous structures are sintered in the channel. A solvent is injected into the channel, and the solvent dissolves the sacrificial particles and forms macropores between at least some of the microporous structures, thereby forming a mixed-porosity microfluidic chip.