Dynamic advancing and receding contact angles of adaptive surfaces

• Applicant: Professor Dr. Hans-Jürgen Butt
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Preparatory and Physical Chemistry of Polymers
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 422795072

One of the big challenges in surface science is to quantitatively understand dynamic contact angles of adaptive surfaces. Here, we are concerned with surfaces, which spontaneously change in the presence of the liquid or its vapor. In our recently developed theory, we related the kinetics of adaptation processes to dynamic contact angles. One result of the theory is that adaptation has two important consequences: (a) it leads to contact angle hysteresis and (b) advancing and receding contact angles become velocity dependent. Therefore, surface adaptation can be established as one cause for contact angle hysteresis (in addition to e.g. roughness and heterogeneity) and for changing dynamic contact angles (in addition to e.g. hydrodynamics). Adaptation may in particular explain the observed changes in contact angle at very low speed of the contact line.The aim of our project is to test the theory. To reach this aim, we plan to measure the kinetics of the adaptation process for thin polymer films. With the kinetics and our adaption theory, we can calculate the dynamic advancing and receding contact angles. Finally, the predicted dynamic contact angles will be compared with the velocity-dependent contact angles of moving drops measured on the same polymer films. To measure advancing and receding contact angle versus speed of the contact line we will use our home built Drop Adhesion Force Instrument (DAFI). In DAFI, sessile drops are moved across surfaces at defined velocity by a wire and the advancing and receding contact angles are measured optically in parallel with the force. Since DAFI is limited to velocities < 5 cm/s we plan to build up a tilted plane setup with cameras to monitor the dynamic contact angles of moving drops at velocities up to 1 m/s. As model systems, we plan to use silicon oxide surfaces coated with different polymer brush films. Surface-initiated atom-transfer radical poly¬meri¬za¬tion will be used to synthesize the polymer films. For polar liquids, we will use poly(N-iso¬propyl¬acryl¬amide (PNIPAM) and poly(2-hydroxyethyl methacrylate) (PHEMA). For non-polar liquids, we plan to synthesize polymethyl-meth¬acrylat (PMMA) and polystyrene (PS) brushes. Binary brush films made from PS and poly(2-vinylpyridine)) (PVP) will be investigated, too, as one example of a more complex adaptation. The swelling kinetics and (re)organizations of molecules upon exposure to solvents will be investigated by X-ray reflectometry, surface plasmon resonance (SPR) and laser scanning confocal microscopy.In case we can validate our theory, the measurement of the velocity dependence of the dynamic contact angles will allow calculating the adaption kinetics of surfaces. If successful, we would have a relatively universal framework for quantitative modeling of dynamic contact angles and contact angle hysteresis caused by adaptation processes.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2171: Dynamic wetting of flexible, adaptive and switchable surfaces
• Co-Investigator: Dr. Rüdiger Berger