Fig. 7. Model of claw tip interaction with surface irregularities. (A) Geometry of the contact between the claw tip and a particle of the sandpaper. (B) Dependence of the force ratio F/W on the contact angle () at different friction coefficients between the claw and particles of the sandpaper. (C) Dependence of h/R on the contact angle () at different values of the relationship R_PD=R/r (particle radius/claw tip radius). When the diameter of a particle is comparable to the claw tip diameter (R_PD=1), both structures cannot interlock even at the friction coefficient f=0.5. When R_PD is greater than 4, the structures may interlock even at f=0.2. If the relationship between the immersion depth and particle radius (h/R) is >0.5, the relative maximum force F/W is not larger than 5. The model predicts the relative maximum force depending on the friction coefficient at contact, the diameter of particles and the immersion depth. Broken lines divide ranges of at which interlocking (self-locking) takes place (left side) at a particular friction coefficient; , contact angle; F, leg force; fN, line running perpendicular to the normal line N running through both centers of the particle and the claw tip; h, immersion depth of the hemispherical particle; r, claw radius; PT, contact point; R, particle radius; W, weight acting on the claw.