Tunable Slippage on Bubble Mattresses
Hydrodynamic slippage is advantageous for drag reduction. Superhydrophobic substrates providing gas-liquid interfaces with shear-free boundary condition provide slippage. Hydrodynamic friction can potentially be tailored by controlling the interface geometry. Therefore, establishing stable and optimal interfaces is crucial but rather challenging. Here, we design and fabricate hydrophobic microfluidic devices, allowing stable and controllable micro-bubbles at the boundary of the micro-channels. We experimentally and numerically examine the geometric effect of the micro-bubbles on the slippage at high resolution. The effective slip length is measured for a wide range of protrusion angles of the micro-bubbles into the flow using micro-particle image velocimetry. Our measurements and simulations reveal a maximum effective slip length for approximately 10 degrees protruding bubbles. In addition, the results show a decrease in slip length with increasing protrusion angles above this protrusion angle. Such microfluidic devices with tunable slippage are essential for the amplified interfacial transport of fluids and particles.