{"title":"Probing the physical origins of droplet friction using a critically damped cantilever†","authors":"Sankara Arunachalam, Marcus Lin and Dan Daniel","doi":"10.1039/D4SM00601A","DOIUrl":null,"url":null,"abstract":"<p >Previously, we and others have used cantilever-based techniques to measure droplet friction on various surfaces, but typically at low speeds <em>U</em> < 1 mm s<small><sup>−1</sup></small>; 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 <em>U</em> = 10<small><sup>−5</sup></small>–10<small><sup>−1</sup></small> m s<small><sup>−1</sup></small> and identified two regimes corresponding to two different physical origins of droplet friction. At low speeds <em>U</em> < 1 cm s<small><sup>−1</sup></small>, the droplet is in contact with the top-most solid (Cassie–Baxter), and friction is dominated by contact-line pinning with <em>F</em><small><sub>fric</sub></small> force that is independent of <em>U</em>. In contrast, at high speeds <em>U</em> > 1 cm s<small><sup>−1</sup></small>, the droplet lifts off the surface, and friction is dominated by viscous dissipation in the air layer with <em>F</em><small><sub>fric</sub></small> ∝ <em>U</em><small><sup>2/3</sup></small> consistent with Landau–Levich–Derjaguin predictions. The same scaling applies for superhydrophobic and underwater superoleophobic surfaces despite their very different surface topographies and chemistries, <em>i.e.</em>, the friction scaling law derived here is universal.</p>","PeriodicalId":103,"journal":{"name":"Soft Matter","volume":null,"pages":null},"PeriodicalIF":2.9000,"publicationDate":"2024-09-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://pubs.rsc.org/en/content/articlepdf/2024/sm/d4sm00601a?page=search","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Soft Matter","FirstCategoryId":"92","ListUrlMain":"https://pubs.rsc.org/en/content/articlelanding/2024/sm/d4sm00601a","RegionNum":3,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"CHEMISTRY, PHYSICAL","Score":null,"Total":0}
引用次数: 0
Abstract
Previously, we and others have used cantilever-based techniques to measure droplet friction on various surfaces, but typically at low speeds U < 1 mm s−1; 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 U = 10−5–10−1 m s−1 and identified two regimes corresponding to two different physical origins of droplet friction. At low speeds U < 1 cm s−1, the droplet is in contact with the top-most solid (Cassie–Baxter), and friction is dominated by contact-line pinning with Ffric force that is independent of U. In contrast, at high speeds U > 1 cm s−1, the droplet lifts off the surface, and friction is dominated by viscous dissipation in the air layer with Ffric ∝ U2/3 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.e., the friction scaling law derived here is universal.
以前,我们和其他人曾使用基于悬臂的技术来测量液滴在各种表面上的摩擦力,但通常是在低速 U < 1 mm s-1 的情况下;在高速情况下,摩擦力测量会因振铃伪影而变得不准确。在这里,我们使用临界阻尼悬臂消除了振铃噪声。我们在 U = 10-5-10-1 m s-1 的较宽速度范围内测量了超疏水表面上的液滴摩擦力,并确定了与液滴摩擦力的两种不同物理来源相对应的两种状态。在低速 U < 1 cm s-1 时,液滴与最顶端的固体接触(Cassie-Baxter),摩擦力主要由接触线引力支配,Ffric 与 U 无关;相反,在高速 U > 1 cm s-1 时,液滴脱离表面,摩擦力主要由空气层中的粘性耗散支配,Ffric ∝ U2/3 与 Landau-Levich-Derjaguin 预测一致。尽管超疏水性表面和水下超疏水性表面的表面形貌和化学性质截然不同,但同样的缩放规律也适用于它们,也就是说,这里得出的摩擦缩放规律是通用的。