Probing the physical origins of droplet friction using a critically damped cantilever.
basic_science · Level V
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- Record sourced from PubMed, PMID 39248408.
- Also identified by DOI 10.1039/d4sm00601a.
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Abstract
Previously, we and others have used cantilever-based techniques to measure droplet friction on various surfaces, but typically at low speeds <i>U</i> < 1 mm s<sup>-1</sup>; at higher speeds, friction measurements become inaccurate because of ringing artefacts. Here, we are able to eliminate the ringing noise using a critically damped cantilever. We measured droplet friction on a superhydrophobic surface over a wide range of speeds <i>U</i> = 10<sup>-5</sup>-10<sup>-1</sup> m s<sup>-1</sup> and identified two regimes corresponding to two different physical origins of droplet friction. At low speeds <i>U</i> < 1 cm s<sup>-1</sup>, the droplet is in contact with the top-most solid (Cassie-Baxter), and friction is dominated by contact-line pinning with <i>F</i><sub>fric</sub> force that is independent of <i>U</i>. In contrast, at high speeds <i>U</i> > 1 cm s<sup>-1</sup>, the droplet lifts off the surface, and friction is dominated by viscous dissipation in the air layer with <i>F</i><sub>fric</sub> ∝ <i>U</i><sup>2/3</sup> consistent with Landau-Levich-Derjaguin predictions. The same scaling applies for superhydrophobic and underwater superoleophobic surfaces despite their very different surface topographies and chemistries, <i>i.e.</i>, the friction scaling law derived here is universal.