Mechanical model of hook-loop adhesion

The hook-loop like adhesion is a common phenomenon in nature, widely used in our daily life with robust adhesion ability, i.e. the Velcro on our clothes. In general, the hook-loop adhesion can be divided into two stages, i.e., the loop hooked up and pull-out stages. At present, the intrinsic factors that affect the hook-loop adhesion are still unknown, and quantitative models describing the two stages are still lacked. Herein, these issues are rationalized through combining experiments, theoretical analysis, and numerical simulations. The theoretical models for fibrous loops hooked on single hook and pull-out of one loop on curved hook are developed. Furthermore, the loop hooked up stage is further simulated by introducing a spring-sandwich element to describe the resilience and compaction effects of fibrous loops. The results show good agreements among theoretical predictions, numerical simulations, and experiments. It is shown that the number of hooked loops is mainly determined by the length of hook-arm and hook angle, friction coefficient, and expulsion displacement of fibrous loops. While, the pull-out force of single loop on hook mainly depends on the bending stiffness, hook arc angle and arc length. Combining with hooked loop number and pull-out force, a general algorithm is proposed to optimize the adhesion property of Velcro, by considering the parameters of hook interspace, hook arc angle and arc length. The results presented in this work not only explore the adhesion mechanism of hook-loop adhesion, but also provide quantitative models to predict its adhesion behaviors, which may offer a rational design strategy for high-performance hook-loop adhesive.