Molecular adsorption to the microfluidic device surfaces can be passively and actively mitigated by mixing certain hydrophilic polymers (organic polymers with repeating hydrophilic groups—the preferred polymers being amphipathic surfactants—with the sample liquid during or prior to relevant microfluidic operations. Nonionic surfactants such as polyoxyethylene sorbitan monooleate and polyoxyethylene octyl phenyl ether are especially effective. High molecular weight polyethylene polymers are also effective. The hydrophilic polymers appear to prevent binding of the fouling molecules to the microfluidic by occupying the surface sites in place of the fouling molecules or by interacting with the fouling molecules to prevent binding of the fouling molecules the surface. When surface adsorption is thus mitigated, microfluidic devices can readily handle samples containing biomolecules to enable active sample concentration, filtering, washing, transport, mixing and other sample handling operations.