Fluid Slip and Drag Reduction on Liquid-Infused Surfaces under High Static Pressure

Drop Friction and Failure on Superhydrophobic and Slippery Surfaces

The mobility of drops on a surface influences how much water and energy is required to clean the surface. By controlling drop mobility, it is possible to promote or reduce fogging, icing, and fouling. Superhydrophobic and slippery liquid-infused surfaces both display high drop mobility despite being 'lubricated' by fluids having very different viscosities. Superhydrophobic surfaces rely on micro- and/or nanoscale textures to trap air pockets beneath drops, minimizing solid-liquid contact. In contrast, on liquid-infused surfaces, these solid textures are filled with an immiscible liquid lubricant. Over the past few years, innovations in experimental and computational methods have provided detailed new insights into the static and dynamic wetting properties of drops on these surfaces. In this review, we describe the criteria needed to obtain stable wetting states with low drop friction and high mobility on both surfaces, and discuss the mechanisms that have been proposed to explain the origins of friction on each surface. Drops can collapse from the low-friction Cassie state to the high-friction Wenzel state on both surfaces, but the transition follows different pathways: on liquid-infused surfaces, the wetting ridge near the drop edge plays a central role in triggering collapse, a phenomenon not observed on superhydrophobic surfaces. This review emphasizes that a liquid-infused surface cannot be simply viewed as a superhydrophobic surface with the air pockets replaced by lubricant. The wetting ridge surrounding drops on liquid-infused surfaces significantly alters most of the drop's properties, including macroscopic shape, friction mechanisms, and the mechanism of collapse to a Wenzel state.

Slippery liquid-like surfaces as a promising solution for sustainable drag reduction

Drag reduction is crucial for many industries, ranging from aerospace to microfluidics, to enhance the energy efficiency and reduce costs. This work is the first to study drag reduction enabled by novel slippery liquid-like surfaces fabricated from flexible polymers. We experimentally characterized the drag reduction performance of slippery liquid-like surfaces in the laminar flow regime. Our results indicate that liquid-like surfaces can reduce fluid drag regardless of surface wettability and have achieved nearly 20% drag reduction. Furthermore, the durability tests show that these surfaces can maintain slipperiness over a month when exposed to air or water and the drag reduction capability for at least one week under a fluid flow. These findings highlight the potential of slippery liquid-like surfaces as a promising solution for sustainable drag reduction.

Shear Evolution and Slippage of the Liquid-Liquid Interface over a Liquid-Infused Surface: A Many-Body Dissipative Particle Dynamics Study

The liquid-liquid interface (LLI), which is the key to cause flow slippage and thus promote drag reduction of liquid-infused surfaces (LISs), does suffer from the action of flow shear. In the current study, the transverse many-body dissipative dynamics simulation method is applied to explore the shear evolution of LLI and the corresponding slippage over a periodically grooved LIS. Results show that a relatively small shear rate only induces a slight deformation of LLI and the corresponding effective slippage is dependent on the shear rate. With a further increase of the shear rate, LLI deforms apparently and then the downstream three phase contact line depins to move once the balance between the capillary force and the shear force is broken, which results in an apparent increase of the slippage, specifically for a convex LLI. Compared with a convex LLI or a concave LLI, a flat LLI remains relatively stable under the same shear action, and an increase of the viscosity ratio and a decrease of the LLI fraction can both strengthen the shear resistance of an LLI, while they are less effective to promote the slippage. Consequently, the current results not only indicate that the slippage is related to the interface deflection and the shear rate but also suggest that both the shear resistance and the slippage of LLI should be considered when designing an effective LIS.