Hydrophobic and oleophobic nanopillars reduce viscous drag in slit nanofluidic channels

Nanofluidics exploiting 100 nm nanochannels has developed, and ultrasmall analytical applications in single-molecule level are expected. However, with downscaling the channel size, the required external pressure for driving liquids becomes exceedingly high due to increased fluid resistance and causes a problem to miniaturize a fluidic system. In the present study, we developed a drag reduction method of slit nanochannels with micrometers-width and nanometers-depth for both aqueous and organic reagents by integrating hydrophobic and oleophobic nanostructured surface. A fabrication process based on electron beam lithography and dry etching was established to realize inverted-trapezoidal shaped hydrophobically modified nanopillars with 300 nm width and 210 nm height on a fused-silica substrate. We optimized the geometry of nanopillars, and those with an upper base angle of 78° and an interval of 400 nm induced hydrophobic property for water with a contact angle of 139° and also oleophobic property for hexadecane with that of 108° by the Cassie-Baxter state with the pinning effect. By integrating the nanopillars, drag reduction of slit nanochannels for both water and hexadecane by slipping with the gas-liquid interface at the nanopillar interval was achieved with apparent slip lengths of 269 nm (water) and 108 nm (hexadecane). The drag reduction rate increased with decreasing the channel depth, and for a nanochannel with 208 nm depth, rates of 87% (water) and 80% (hexadecane) were achieved. We demonstrated miniaturization and energy saving of nanofluidic system based on the developed method. This work will greatly contribute to advancement of nanofluidics and engineering for dust proof materials.