Critical analysis of adhesion work measurements from AFM-based techniques for soft contact

Hypothesis: The work of adhesion is a thermodynamic quantity that is frequently measured by atomic force microscopy (AFM). Determination of the work of adhesion requires quasi-equilibrium measurements, where we address the question of to what extent atomic force microscopy qualifies for quasiequilibrium measurements.Experiment: To measure the work of adhesion, we combined soft colloidal probe AFM (SCP AFM) with reflection interference contrast microscopy (RICM). This allowed us to extract the work of adhesion either from the pull-off force or from the contact radius. With these methods, we investigated the adhesion behavior of poly(N-isopropylacrylamide) (PNIPAM) polymer brushes in the swollen and solventinduced collapsed state by systematically analyzing contact radii and adhesive forces.Findings: In the swollen state, the adhesion to the PNIPAM brush was fivefold larger and exhibited significant time dependencies when measured with SCP AFM. A strong rate dependence of the pull-off force method was indicative of a non-equilibrium process. In order to reliably determine the equilibrium work of adhesion, the contact radius method was found to be the better because it is not rate dependent. Our work highlights the important benefits of using optical measurements to determine the contact radius when deriving the works of adhesion between colloidal probes and polymer brush surfaces.(c) 2023 Elsevier Inc. All rights reserved.

Automated summary

In this study, the adhesion behavior of PNIPAM polymer brushes in different states was analyzed using SCP AFM combined with RICM. It was found that the adhesion force in the swollen state showed strong time dependence and the pull-off force method exhibited rate dependence, indicating a non-equilibrium process. The contact radius method was found to be more reliable for determining the equilibrium work of adhesion. This work highlights the important benefits of using optical measurements when deriving the works of adhesion between colloidal probes and polymer brush surfaces.