Abstract
The understanding of friction in soft materials is of increasing importance due to the demands of industries such as healthcare, biomedical, food and personal care, the incorporation of soft materials into technology, and in the study of interacting biological interfaces. Many of these processes occur at the nanoscale, but even at micrometer length scales there are fundamental aspects of tribology that remain poorly understood. With the advent of Friction Force Microscopy (FFM), there have been many fundamental insights into tribological phenomena at the atomic scale, such as 'stick-slip' and 'super-lubricity'. This review examines the growing field of soft tribology, the experimental aspects of FFM and its underlying theory. Moving to the nanoscale changes the contact mechanics which govern adhesive forces, which in turn play a pivotal role in friction, along with the deformation of the soft interface and dissipative phenomena. We examine recent progress and future prospects in soft nanotribology.
Highlights
Friction is the force resisting relative motion between two sliding bodies, spanning many orders of magnitude in length, time and energy scales
We focus on Friction Force Microscopy (FFM) studies carried out on soft materials (Fig. 5), and we discuss how surface interactions and topography affect friction in soft contact mechanics, before we examine the impact of intrinsic properties of the material on friction and other experimental factors, such as loads, scanning distance and sliding speeds
FFM on soft surfaces presents a new paradigm in nanotribology, demonstrating a growing importance with the incorporation of soft materials into electronics and robotics
Summary
Friction is the force resisting relative motion between two sliding bodies, spanning many orders of magnitude in length, time and energy scales. Professor at the School of Physics and Astronomy, has two decades of experience in atomic force microscopy, and currently manages a cross-faculty AFM facility He studies a wide range of systems, with a focus on the structure, dynamics and nanomechanics of soft matter and biological systems, largely in a fully hydrated state. There are no reviews that discuss nanotribology in deformable i.e. low modulus ‘soft’ surfaces, such as polymers, hydrogels, and soft biological interfaces (oral, ocular, dermal, and respiratory), which is one of the outstanding challenges in modern nanotribology In this Review, we provide a critical analysis of FFM focusing on the booming area of nanotribology in soft surfaces, describing the rapid evolution of FFM, the gradual transition from the use of sharp tips[16] to well-defined colloidal probes,[17] and more recently performing friction measurements incorporating flexibility into the material physics and chemistry of the contact surfaces.[18] We explore the frictional laws applying to soft surfaces at the nanoscale, highlighting the effect of adhesion force on the frictional behaviour of soft surfaces, which is an outstanding challenge in modern tribology. Readers might refer to more extensive reviews on polymer brushes, hydrogels and hydration lubrication elsewhere.[19,20,21] we outline the systems where FFM using soft surfaces can be applied, before looking into the future opportunities, including the fabrication of precisely tailored soft probes for FFM along with the growing need for new mathematical models to overcome the current limitations of FFM-based approaches for soft tribology
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