Partially substrateless microchannels for direct monitoring of interfacial dynamics in hydrophobic surfaces

Superhydrophobic and liquid-infused surfaces are the most prominent techniques to achieve drag reduction in microchannels. However, they have specific drawbacks such as costly fabrication of complex and mechanically sensitive surfaces, surfaces susceptible to lubricant abrasion or involve hazardous chemicals. We present a partially substrateless microchannel whose upper wall features a large no-shear air/water meniscus at atmospheric pressure. On this wall, a self-assembled monolayer of hydrophobic alkyl silane was bonded covalently. Flow experiments reveal a drag reduction of up to 25% although only 4% of the wall fulfils the no-shear condition. These experiments demonstrated long-term stability and self-healing properties. Furthermore, White Light Interferometry (WLI) was used for direct monitoring of interfacial dynamics. By optical investigation of the full meniscus topography the contact-free evaluation of the spatially resolved static pressure distribution was possible. Conducted numerical simulations are in good agreement with the experimental findings and illustrate the drag reduction mechanism.