Laminar Drag Reduction in Microchannels With Slippery Polymer Brush Surfaces

Abstract

Abstract Flow drag reduction plays a significant role in numerous engineering applications. Microfluidics has been studied for various important applications, such as energy conversion, microfluidic sensors, biosensing, and biomedical devices. In laminar flows in microchannels, drag reduction is capable of reducing the required pumping power, enhancing microfluidic cooling, augmenting electrokinetic energy harvesting, enabling better flow control in chemical and biological devices and etc. To reduce fluid drag, researchers have investigated different techniques which include textured slippery surfaces filled with fluid lubricant (i.e., air or oil) in their textures like superhydrophobic or slippery liquid-infused surfaces. For traditional textured slippery surfaces, to retain fluid lubricant, typically additional specific surface textures are required. However, there are two significant challenges associated with textured slippery surfaces: 1) the surface coverage of the fluid lubricant layer is often limited, restricting overall drag reduction; and 2) fluid lubricant suffers from poor durability as the lubricant layer could be depleted by the overlaying shear flow, after which the surfaces will lose their slipping property and even increases the wall friction. In recent studies, a novel type of non-textured slippery liquid-like polymer brush surface has been developed. The slippery liquid-like polymer brush surface has been demonstrated to exhibit macroscope liquid-like properties with an apparent viscosity that can be potentially smaller than that of water and can work as a fluid lubricant layer. Thus, this novel non-textured slippery liquid-like polymer brush surface represents a promising candidate for drag reduction. To investigate the drag reduction induced by the slippery liquid-like polymer brush surfaces, in this work, we fabricated microchannels with slippery polymer brush-grafted surfaces and microchannels made of silicon surfaces and measured the pressure drops along the microchannels under various flow rates and Reynolds numbers. The drag reduction coefficient and corresponding fluid slip length are calculated. The drag reduction coefficient is between 4.53% to 18.77% for Reynolds number between 5.5 to 12 and fluid slip length in micron scale is achieved.