Barrio-Zhang Hernán, Ruiz-Gutiérrez Élfego, Armstrong Steven, McHale Glen, Wells Gary G, Ledesma-Aguilar Rodrigo
Smart Materials and Surfaces Laboratory, Northumbria University, Newcastle upon Tyne NE1 8ST, United Kingdom.
Institute for Multiscale Thermofluids, School of Engineering, University of Edinburgh, The King's Buildings, Mayfield Road, Edinburgh EH9 3FB, United Kingdom.
Langmuir. 2020 Dec 15;36(49):15094-15101. doi: 10.1021/acs.langmuir.0c02668. Epub 2020 Dec 1.
Contact-line pinning and dynamic friction are fundamental forces that oppose the motion of droplets on solid surfaces. Everyday experience suggests that if a solid surface offers low contact-line pinning, it will also impart a relatively low dynamic friction to a moving droplet. Examples of such surfaces are superhydrophobic, slippery porous liquid-infused, and lubricant-impregnated surfaces. Here, however, we show that slippery omniphobic covalently attached liquid-like (SOCAL) surfaces have a remarkable combination of contact-angle hysteresis and contact-line friction properties, which lead to very low droplet pinning but high dynamic friction against the motion of droplets. We present experiments of the response of water droplets to changes in volume at controlled temperature and humidity conditions, which we separately compare to the predictions of a hydrodynamic model and a contact-line model based on molecular kinetic theory. Our results show that SOCAL surfaces offer very low contact-angle hysteresis, between 1 and 3°, but an unexpectedly high dynamic friction controlled by the contact line, where the typical relaxation time scale is on the order of seconds, 4 orders of magnitude larger than the prediction of the classical hydrodynamic model. Our results highlight the remarkable wettability of SOCAL surfaces and their potential application as low-pinning, slow droplet shedding surfaces.
接触线钉扎和动摩擦力是阻碍液滴在固体表面运动的基本力。日常经验表明，如果固体表面的接触线钉扎作用较弱，那么它对运动液滴施加的动摩擦力也会相对较小。这类表面的例子有超疏水表面、充满液体的光滑多孔表面以及浸渍润滑剂的表面。然而，我们在此表明，具有共价连接类液体的光滑全憎性（SOCAL）表面具有接触角滞后和接触线摩擦特性的显著组合，这导致液滴钉扎作用非常小，但对液滴运动具有较大的动摩擦力。我们展示了在可控温度和湿度条件下，水滴对体积变化的响应实验，并分别将其与基于分子动力学理论的流体动力学模型和接触线模型的预测结果进行比较。我们的结果表明，SOCAL表面的接触角滞后非常小，在1°到3°之间，但由接触线控制的动摩擦力却出乎意料地大，其典型的弛豫时间尺度约为秒级，比经典流体动力学模型的预测结果大4个数量级。我们的结果突出了SOCAL表面显著的润湿性及其作为低钉扎、慢液滴脱落表面的潜在应用价值。
