This study aims at the mechanism of drag reduction in turbulent boundary layer （TBL） with superhy- drophobic surface. Comparing the time-resolved particle image velocimetry （TRPIV） measurement results with that of hydrophilic surface, the drag reduction rate over a superhydrophobic surface is approximately 10%. To investigate the characteristics of coherent structure in a drag-reduced TBL with superhydropho- bic surface, a modified multi-scale spatial locally-averaged structure function is proposed for detecting coherent structure. The conditional sampling and spatial phase-lock average methods are employed to obtain the topology of physical quantities like the velocity fluctuation, spanwise vorticity, and Reynolds stress during eject and sweep process. The results indicate that the suppression of coherent structure burst in the near-wall region is the key mechanism in reducing the skin friction drag for TBL over super- hydrophobic surface.
Inspired by nepenthes pitcher plants,Lubricant-Impregnated Surfaces(LISs)are surfaces with lubricant infused in the textures which form slippery interfaces.In this paper,we investigated slippery properties and the robustness of LISs with different micro-texture topologies(i.e.,no textures,micro-pillar textures and micro hole textures),including original LISs and LISs rinsed by water.We measured the static contact angle,sliding angle and droplet motions on the LISs,using a contact angle instrument and a Particle Image Velocimetry(P1V)system.Similar contact angles and small sliding angles were observed on all original LISs,which indicated that an oil-layer existed on each LIS's interface.After rinsed by water,the sliding angle increased obviously and the slip velocity decreased,which meant that the LISs slippery properties deteriorated in different degrees.Among all the LISs in our experiments,LIS with micro pillar textures has the best slippery performance and robustness before and after rinsed.We found that the LISs'slippery properties were closely related to the states of the oil-layer on the interface,which were changed after rinsed.For the LIS with micro pillar textures,a thin layer of lubricant can sustain on the interface even after rinse,which made the droplet slide smoothly on the surface,with a lower sliding angle and a larger sliding velocity.This indicates that the proper micro-texture will enhance the slippery property and make it last longer.
