Research Progress on Surface Adhesion Regulation under Dry Environment

With the rapid development of micro/nano technology, human beings have stepped into a research field in which scale effect and surface effect play a leading role. In this field, the micro adhesive forces such as van der Waals force, electrostatic force and capillary force become significant compared with the macro forces such as gravity and inertial force. In dry and uncharged environment, the intermolecular van der Waals force is the main source of adhesion. On the one hand, adhesion is one of the main factors affecting the stability and surface damage of micro/nano-mechanical products and needs to be weakened and overcome. On the other hand, in order to develop multifunctional switchable adhesives and micro-manipulation/micro-assembly technologies, the enhancement and utilization of adhesion have attracted the interests of researchers. Under this context, the realization of self-regulation on surface adhesion according to performance requirements has become a research hotspot in recent years. This review attempted to present the research progress of adhesion regulation methods from the aspects of surface morphology modification, material modification and field control. Firstly, the role of surface morphology modification through texturing in reducing adhesion in the magnetic storage devices and microelectromechanical systems was introduced. It was found that the adhesion-reduction mechanisms of textures were the decrease of contact area and the increase of interface interval. The key influencing factors included the shape, area density and orientation of textures. The characteristics of different bionic textures and main parameters affecting the adhesion strength were described. It was demonstrated that the adhesion-enhancement mechanisms of textures were the decrease of effective modulus and the increase of contact area. The adhesion switching effect depended on the structure, alignment state and area density of textures. Secondly, the adhesion regulation performances of material modification through surface coating/film were discussed. It was discovered that hard coatings were often used to reduce adhesion. The adhesion-reduction mechanisms relied on the decreases of surface energy and contact area, and the material type of the coating was the main factor influencing the adhesive response. What’s more, the surface organic film and surface initiated polymerized molecular layer were found to have the effect of regulating adhesion. The switching mechanism could be attributed to the change in surface energy, and the key influencing factor was the polarity of surface functional groups after chemical modification. Furthermore, it was summarized that the adhesion switching mechanisms of material modification through composites were the variation of contact area caused by the change in surface morphology or Young's modulus, the variation of contact interface stress distribution, or the appearance of defects. The constituent, proportion and internal structure size of the composite could significantly influence the adhesion tuning effect. Thirdly, the switching mechanisms between strong and weak adhesion and the key influencing factors under the triggers of velocity, temperature, external force and magnetic field were introduced. It was found that the effect of velocity on adhesion was induced by the energy dissipation of viscoelastic materials, and the regulation performance was controlled by the magnitude of velocity and mechanical properties of viscoelastic materials. Temperature had been utilized to trigger the interfacial thermal mismatch, the change in characteristics of viscoelastic materials or the configuration of the shape memory polymer, thereby influencing adhesion. The magnitude of temperature and the laser parameters could be used to control the adhesion regulation effect. The use of external force could tune adhesion through influencing the contact area, the shape of the microstructured surface or the interfacial cracks, or the formation of negative pressure. The tuning performance relied on the magnitude of force and its applying way. Magnetic field could effectively stimulate the surface deformation or the mechanical properties of the magneto-response materials and thus affect the surface adhesion. The magneto-response characteristics of the material, the surface structure, or the intensity, orientation and acting mode of the magnetic field could significantly influence the adhesion switching performance. In addition, the methods combining multiple adhesion regulation means and the related outstanding achievements were introduced from the perspective of regulation characteristics and the switching effects. Finally, the advantages and application fields of various adhesion-regulation methods were analyzed and summarized, and the existing problems and the future development directions in adhesion regulation were discussed.