Flexible dry adhesions

This review outlines the development of classic adhesion theories and identifies two key factors: dimension length and modulus. Current research replicates multi-scale microstructures with soft materials, demonstrating successful adhesion in space capsules through Van der Waals forces at the nanometer scale. Soft materials, like silicon rubbers (~10 MPa elastic modulus) are typically preferred to enlarge contacted areas and adhesions. Principally, all materials could be soft and bendable just as thin enough in a dimension, which practically is the case of gecko setae, where hierarchical compliance and modulus gradients enable a stiff intrinsic material (~2 GPa) to exhibit a low apparent modulus (~100 MPa) at the contact interface. Thus, we define flexible dry adhesion to clarify that materials’ flexibility figure-of-merit (thickness/modulus) governs adhesion strength. Decreasing microstructure dimensions and improving modulus are equally important to enhance adhesion. Adhesion strength σ_ad is quantified by materials flexibility figure-of-merit f_FoM and effective contact radius R^*of microstructures. A σ_ad-R^*-f_FoM adhesion map is developed, encompassing various classic adhesion models, and suggests nonpolymeric materials like metallic alloys for extreme space environments. The current challenges of flexible dry adhesion are also discussed at the end.