The Comparison of AFM-Based Techniques to Access the Adhesion in the Case of Soft Contacts

Adhesion affects numerous processes, both natural and industrial: from insects or lizards crawling on vertical walls, to the healing of wounds and the deposition of water-repellent coatings on fabrics, all these processes are manifestations of adhesion. Thus, the study of adhesion as a phenomenon and the peculiarities of the adhesive process are of great importance for many applications. The work of adhesion, the thermodynamic work required to separate two phases, can provide a quantification of surface interactions. At the microscale, such interactions are crucial for bio-medical applications and tribological studies. To access the work of adhesion at the microscale, colloidal probe atomic force microscopy (AFM) is the most suitable way as it allows bringing two surfaces into contact locally. Using hard non-deformable (silica) particle as a colloidal probe, while is relatively easy to realize experimentally, may not be suitable in the case of study on deformable polymeric substrates, as it pushes the system out of the equilibrium by exerting large pressure on the small area. Here, the use of a soft deformable colloidal probe can be a better option. However, it is known that maximal load, retraction rate, and contact time influence the measured values drastically if the force-distance curves are analyzed due to their influence on the pull-off process per se in the case of the soft colloidal probe. The combination of AFM and reflection interference contrast microscopy (RICM) to measure optically the contact radius was shown previously to be a possible alternative to extract the work of adhesion without creating rapid pull-off events. This contribution shows for the first time the direct comparison of the force-distance curve analysis (pull-off method) and the combination of AFM and RICM (AFM/RICM) when studying the adhesion of polymeric responsive brush surfaces. The comparison reveals the necessity to measure the contact geometry experimentally.
Here, the adhesive properties of poly-N-isopropylacrylamide (PNIPAM) brush in swollen and collapsed states were investigated by the soft colloidal probe (PDMS particle) AFM. To directly compare the pull-off method with AFM/RICM for different surface conditions, we chose PNIPAM as the material’s co-nonsolvency allows the characterization of swollen and collapsed states of the brush on one sample, broadening the comparison. This comparison revealed that AFM/RICM is a more reliable way to access the values close to the thermodynamic equilibrium since the measurement is carried out when the colloidal probe is held on the PNIPAM brush during all the contact time (10-100s). The pull-off method, on the other hand, relied on the very rapid pull-off process that occurs in a time scale significantly lower than a second. Both methods show larger adhesion for the swollen state of the PNIPAM brush than the collapsed state, which contradicts the existing hard colloidal probe literature data. To understand this result, we investigated the adhesive properties of a redox-responsive PNIPAM-based hydrogel. Thus, we could control the degree of swelling of the hydrogel in situ by triggering the collapse of PNIPAM employing the co-nonsolvency effect and by reducing the cross-linking density by redox stimulus. The experimental values of the work of adhesion in this case scaled with the degree of freedom of the PNIPAM (from the most cross-linked in the collapsed state to the least cross-linked in the swollen state). Thus, the degree of freedom of the chains of PNIPAM in the uncompressed state resulted in greater adhesion in the swollen state than in the collapsed state.

References:

(1) Sychev, D.; Schubotz, S.; Besford, Q. A.; Fery, A.; Auernhammer, G. K.. J. Colloid Interface Sci. 2023, 642, 216–226.
(2) Hofmann, D.; Sychev, D.; Zagradska-Paromova, Z.; Bittrich, E.; Auernhammer, G. K.; Gaitzsch, J. Macromol. Rapid Commun. 2024, 45 (14).