A model for the contact area and static friction of nominally flat rough surfaces and rough spherical surfaces is presented. The model extends previously published models, which are limited to plasticity index values below 8, to higher plasticity index values by accounting for fully plastically deformed asperities based on finite element results by Jackson and Green [2005, “A Finite Element Study of Elasto-Plastic Hemispherical Contact Against a Rigid Flat,” Trans. ASME, J. Tribol., 127, pp. 343–354]. The present model also corrects some deficiencies of the earlier models at very small plasticity index values below 0.5.

The effect of material properties and surface roughness on the contribution of asperities and sphere bulk displacements to the total displacement of a rough spherical contact is investigated. A dimensionless transition load, above which the contribution of the bulk displacement exceeds the contribution of the asperities displacement, is found as a function of the plasticity index and dimensionless critical interference of the sphere bulk. A criterion is proposed for evaluating the importance of surface roughness in calculating the displacement of a rough spherical contact. Some experimental results with a spherical micro-contact are presented to verify the model.

Microscale intestinal perforation can cause considerable mortality and is very difficult to treat using conventional methods owing to the numerous challenges associated with microscale operations, which require the development of new body-friendly and effective treatment methods. Swarming micro- and nanomotors have shown great potential in biomedical applications in complex and hard-to-reach environments. Herein, we present a wheel-like magnetic-driven microswarm (WLM) with a band-aid imitation to patch microscale intestinal perforations by pasting on the perforation point in mucus-filled environments. A method called “packing under rolling” was applied to make the formed microswarms denser and rounder. Microswarms with variable aspect ratios can be fabricated by tuning the frequency and strength of the external magnetic field. Actuation and navigation in a confined complex environment, locomotion on three-dimensional surfaces, and multiple switchable motion modes have been realized by combining AC and DC magnetic fields. Moreover, we demonstrated WLM controllable navigation, movement, and microscale perforation patching in the chicken intestines ex vivo. The proposed strategy will contribute to the treatment of microscale intestinal perforation and may be applicable to novel, precise topical medication and microsurgery.