A blocking material is injected into a microfluidic chip that includes microscale-porosity microchannels etched in a substrate, filling at least a portion of the microchannels. Silicon dioxide spheres are injected into the microfluidic chip. The blocking material prevents the silicon dioxide spheres from entering the portion of the microchannels filled with the blocking material. The silicon dioxide spheres form a region of nanoscale porosity in a portion of the microchannels not filled with the blocking material. A solvent is injected into the microfluidic chip, the solvent operable to dissolve the blocking material and thereby providing a region of microscale porosity adjacent to the region of nanoscale porosity.