Phase Separation in Wetting Ridges of Sliding Drops on Soft and Swollen Surfaces

Open Access

Phase Separation in Wetting Ridges of Sliding Drops on Soft and Swollen Surfaces

Lukas Hauer, Zhuoyun Cai, Artem Skabeev, Doris Vollmer, and Jonathan T. Pham

Phys. Rev. Lett. 130, 058205 – Published 3 February 2023

Abstract

Drops in contact with swollen, elastomeric substrates can induce a capillary mediated phase separation in wetting ridges. Using confocal microscopy, we visualize phase separation of oligomeric silicone oil from a cross-linked silicone network during steady-state sliding of water drops. We find an inverse relationship between the oil tip height and the drop sliding speed, which is rationalized by competing transport timescales of the oil molecules: separation rate versus drop-advection speed. Separation rates in highly swollen networks are as fast as diffusion in pure melts.

Received 12 August 2022
Accepted 11 January 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.058205

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Elastic deformation Phase separation Wetting

Polymer gels

Confocal imaging Imaging & optical processing Scaling methods Sol-gel process

Polymers & Soft Matter

Authors & Affiliations

Lukas Hauer ^1,2,*, Zhuoyun Cai ^2,*, Artem Skabeev ³, Doris Vollmer ^1,†, and Jonathan T. Pham ^2,‡,∥

¹Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
²Department of Chemical and Materials Engineering, University of Kentucky, Lexington, 40506 Kentucky, USA
³Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University Jena, Lessingstrasse 8, 07743 Jena, Germany

^*These authors contributed equally.
^†vollmerd@mpip-mainz.mpg.de
^‡Jonathan.Pham@uc.edu
^∥Present address: Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH, USA.

Article Text

Click to Expand

Supplemental Material

Click to Expand

References

Click to Expand

Issue

Vol. 130, Iss. 5 — 3 February 2023

Reuse & Permissions

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Drop sliding setup and wetting ridge visualization. (a) Macroscopic side view of affixed drop, sliding at $5 μ m / s$ on a swollen PDMS network ( $Q = 14.5$ ). Substrate moves left while drop is stationary. Inset, setup schematic. (b) Temporal averaged ( $n = 158$ ) LSCM image of a phase-separated wetting ridge. PDMS network and silicone oil are dyed separately with fluorescence markers with different emission spectra. Red shows silicone oil and orange shows swollen PDMS network. (c) Extracted interfaces of silicone oil (red points) and PDMS network (yellow points). Inset, blown-up section of the phase-separated zone in the wetting ridge. Standard errors are smaller than the symbol size.
Reuse & Permissions
Figure 2
Dynamic wetting ridge shape for different sliding speeds and swelling ratios (a),(b) $Q = 16$ , (c),(d) $Q = 14.5$ , (e),(f) $Q = 10$ . (a),(c),(e) Dynamic ridge profiles of liquid silicone oil (and network PDMS as insets). (a),(c) For silicone oil, the ridge height gradually decreases for increasing drop speed, $v = 5$ , 10, 50, 100, 300, $800 μ m / s$ . Top row images are representative LSCM images for $v = 5$ and $v = 800 μ m / s$ (scale bar $20 μ m$ ). (b),(d),(f) Maximum height of dynamic ridges of silicone oil (red) and network PDMS (yellow). Data shows average of min. $n = 3$ repetitions together with standard deviations. Dashed lines mark dynamic elastocapillary height $λ \sim γ \sin θ_{adv} / E$ , which are (b) $21 μ m$ ( $E \approx 3 kPa$ ) (d) $16 μ m$ ( $E \approx 3.9 kPa$ ), (f) $13 μ m$ ( $E \approx 4.8 kPa$ ).
Reuse & Permissions
Figure 3
Separation of liquid, silicone oil. (a) Separation height $h_{sep}$ versus $v$ , for $Q = 16$ (cherry red), $Q = 14.5$ (bordeaux), $Q = 12.7$ (dark blue), $Q = 10$ (blue), and $Q = 7.5$ (turquoise). Dashed lines show exponential relaxation per Eq. (6). Inset, measured $h_{sep, 0}$ at zero speed for different $Q$ . The dashed line shows $h_{sep, 0}$ , when the balance Eq. (4) is met. (b) Competing transport mechanisms of phase separating, silicone oil molecules induced by $\nabla μ$ and sliding speed $v$ . The boundary layer where $\nabla μ \neq 0$ penetrates $\approx h_{sep} / 2$ into the network bulk.
Reuse & Permissions
Figure 4
Separation dynamics and mobilities. (a) Normalized tip height $h_{sep} / h_{sep, 0}$ against normalized sliding speed $v / v^{*}$ (colored circles). Dashed line: balance model; and solid line: exponential relaxation. (b) Measured mobility $m^{*}$ against $Q$ . Horizontal line is the mobility of self-diffusion [63]. Inset shows extracted correlation speed $v^{*}$ . Dashed lines indicate trends and error bars correspond to fitted root-square-mean error.
Reuse & Permissions

Physical Review Letters