Measurement of Liquid Slip on Nanoturf Surfaces

While many new studies have confirmed the existence of liquid slip over certain solid surfaces [1], there has not been a deliberate effort to design and fabricate a surface that will maximize the slip effect and reduce drag in liquid flow in practical conditions. Hydrophobic rough surfaces have been studied experimentally to reduce the friction in liquid flow [2, 3]. The fine grooves [2], trapping air in them, were speculated to decrease the liquid-solid contact area and contribute to the drag reduction. However, the grooves constitute only a fraction of the entire surface and the rest of the surface is also rough, making it difficult to isolate and attribute the effect of the air layer. Although the post structures with large pitches (i.e., over several microns) [3] may be convenient to fabricate and more convincing to demonstrate the drag reduction, they function only under small liquid pressure (e.g., < 5000 Pa), which does not represent most real flow conditions. Although the nano-patterned surfaces have recently been shown to reduce the friction even under high pressure [4, 5], the amount of slip is not large enough to generate a meaningful drag reduction. Hence, our goal, extended from the previous report for droplet flows [6], is to engineer a surface that maximizes the slip effect for continuous flows in real engineering cases when the liquid is considerably pressurized.