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Wetting ridge dynamics during spreading on a soft solid studied by x-ray imaging

S. J. Park1, Joshua B. Bostwick2, and J. H. Je1*
1 X-ray Imaging Center, Department of Materials Science and Engineering, Pohang University of Science and Technology, South Korea.
2 Department of Engineering Science and Applied Mathematics, Northwestern University.

Dynamic wetting behaviors on soft viscoelastic solids are important to interpret complex biological processes resulted from cell-substrate interactions. When a droplet sits on a soft surface, its surface tension deforms the contact line, creating a ‘wetting ridge’, which causes characteristic spreading behaviors. The key to understand the underlying mechanisms is to investigate wetting ridge dynamics during spreading. However, it is challenging to explore wetting ridge dynamics, mostly due to limitations in observation. Here, we directly visualize wetting ridges in real-time during spreading using x-ray microscopy with a high spatio-temporal resolution. We identify two ridge growth stages characterized by early ‘broadening’ and later ‘heightening’ that affect the ridge geometry, which we use to interpret two observed different mechanisms of contact-line pinning/depinning transitions: ‘stick-slipping’ and ‘stick-breaking’. The two growth mechanisms control the ridge-geometry and determine the spreading behaviors. Finally, we clarify two different mechanisms of pinning/depinning transitions: ‘stick-slipping’ and ‘stick-breaking’. The fundamental results reported here can give substantial inspiration to elucidate a number of dynamic wetting behaviors, as well as puzzling biological issues such as cellular differentiation, proliferation, and morphogenesis.

Park Figure

Figure. X-ray images of wetting ridges in three spreading regimes. (A) Continuous spreading. (B) Stick-slip. (C) Stick-breaking. The scale bars, 5 μm.

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