Reducing the friction of liquid flows on solid surfaces has become an important issue with the development of microfluidics systems, and more generally for the manipulation of fluids at small scales. To achieve high slippage of liquids at walls, the use of gas as a lubricant--such as microbubbles trapped in superhydrophobic surfaces--has been suggested. The effect of microbubbles on the effective boundary condition has been investigated in a number of theoretical studies, which basically show that on flat composite interfaces the magnitude of the slippage is proportional to the periodicity of the gaseous patterns. Recent experiments aiming to probe the effective boundary condition on superhydrophobic surfaces with trapped bubbles have indeed shown high slippage in agreement with these theoretical predictions. Here, we report nanorheology measurements of the boundary flow on a surface with calibrated microbubbles. We show that gas trapped at a solid surface can also act as an anti-lubricant and promote high friction. The liquid-gas menisci have a dramatic influence on the boundary condition, and can turn it from slippery to sticky. It is therefore essential to integrate the control of menisci in fluidic microsystems designed to reduce wall friction.

Etertec HQ-6100 dry film photoresist was used in this work to fabricate soft-lithography masters applied to microfluidic applications. We demonstrated that the use of this photoresist was a convenient alternative to conventional microfabrication approaches based on DRIE and liquid photoresists for fast-prototyping of microfluidic structures. Our method was at least two times faster than conventional processes and required limited investment for equipments. Finally, this approach was applied to the design and fabrication of microfluidic networks used for gradient generation in bulk solution.