Quantitative Characterization And Smart Regulation Of Friction At Liquid-solid Interface

Wetting phenomenon is ubiquitous in nature.For example,water drops on lotus leaves can roll freely,and sharks can swim quickly in the sea,which have attracted great attention of researchers.The friction of liquid-solid interface can significantly affect the self-cleaning,adhesion,anti-icing,drag reduction and other functions of the material surface.The bionic drag reduction surface preparation by studying the liquidsolid interface friction can be widely used in transportation,agriculture and medical fields.Although the current research on liquid-solid interface friction has made considerable progress,the quantitative characterization and intelligent regulation of liquid-solid interface friction still need to be further in-depth.This paper starts from the quantitative characterization of liquid-solid interface friction.First,a simple technique of friction measurement is established to measure the friction force of droplets on high adhesion surfaces,and a kind of self-lubricating surface is prepared by imitating the seaweed in the ocean,which can intelligently control friction.Finally,the surface of the fish-scale structure with excellent drag reduction performance is prepared through additive manufacturing technology,which is expected to play an important role in marine defense and marine economic construction in the future.The main research contents of this paper are as follows:(1)Traditional methods of contact angle and sliding angle measurement for highadhesion surfaces have great limitations.In order to quantitatively characterize the friction of the liquid-solid interface and reveal the detailed characteristics of the movement of the droplets on the solid surface,a simple technique of friction measurement(MPCP)is established.The technology is to transport the droplet to the sample surface through the capillary tube,the capillary tube is kept in the center of the droplet,and the sample is pulled horizontally by the electric displacement platform.During the movement of the sample,the droplet moves relative to the sample under the capillary restraint.By monitoring the real-time offset of the capillary relative to the initial position,the friction force and detailed movement characteristics of the droplet moving on the sample surface can be obtained.The anisotropic stripe structure samples with different periods are prepared on polydimethylsiloxane(PDMS)by photolithography and soft printing technology.The traditional methods of sliding angle cannot distinguish the wettability of samples with different periods along the vertical stripe direction,but MPCP technology can accurately measure the frictional force of droplets moving in various directions on samples with different periods,and can also reveal detailed characteristics of the droplet movement process(such as static friction,dynamic friction,and droplet viscous behavior affected by structure).This technology makes up for the shortcomings that the sliding angle cannot characterize surfaces with high adhesion or can only characterize the resistance of solids to water droplets at the moment the droplets roll off.It is suitable for quantitative measurement of various smooth,rough and structured surfaces,and can be used in many fields such as surface engineering,biomedicine and microfluidics.(2)The kelp in nature can flow with the waves in the ocean without being damaged.Inspired by the undulating structure and self-lubricating characteristics of the kelp,an undulating self-lubricating surface(USS)is prepared by stretching and releasing.The surface has stable self-lubricating properties,and can spontaneously secrete after the surface lubricating layer is damaged under external stimuli.In addition,mechanical stress can be used to control the surface to achieve reversible conversion between the isotropic structure and anisotropic structure.Because USS has good self-lubricating performance and controllable liquid-solid interface friction,the surface can realize the control of droplet movement and the function of anti-icing and drag reduction.(3)Inspired by the structure of fish scales of different fishes in seawater and freshwater,samples with continuous/discontinuous groove structure were prepared through additive manufacturing technology.Adjusting the size,shape and arrangement of the fish-like structure can influence the swimming speed and resistance of different fish scale structures.The results show that the prepared continuous groove structure can increase the maximum speed by about 29% compared with the planar structure.