Department of Aeronautics and Astronautics, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
J Phys Chem Lett. 2023 Apr 13;14(14):3561-3566. doi: 10.1021/acs.jpclett.3c00428. Epub 2023 Apr 5.
The pinning effect plays an important role in many fluidic systems but remains poorly understood, especially at the nanoscale. In this study, we measured the contact angles of glycerol nanodroplets on three different substrates using atomic force microscopy. By comparison of the shapes of the three-dimensional images of droplets, we found that a possible origin of the long-discussed deviation of the contact angles of nanodroplets from the macroscopic value is the pinning force induced by ångström-scale surface heterogeneity. It was also revealed that the pinning forces acting on glycerol nanodroplets on a silicon dioxide surface are up to twice as large as those acting on macroscale droplets. On a substrate where the effect of pinning was strong, an unexpected irreversible change from an irregularly shaped droplet to an atomically flat liquid film occurred. This was explained by the transition of the dominant force from liquid/gas interfacial tension to an adsorption force.
钉扎效应在许多流体系统中起着重要作用，但仍未得到很好的理解，尤其是在纳米尺度上。在这项研究中，我们使用原子力显微镜测量了甘油纳米液滴在三种不同基底上的接触角。通过比较液滴的三维图像的形状，我们发现长期以来讨论的纳米液滴接触角偏离宏观值的一个可能原因是由埃尺度表面不均匀性引起的钉扎力。还揭示了作用在二氧化硅表面上的甘油纳米液滴上的钉扎力高达作用在宏观液滴上的钉扎力的两倍。在钉扎效应较强的基底上，一个意想不到的从不规则形状的液滴到原子级平坦的液膜的不可逆转变发生了。这可以通过从液体/气体界面张力到吸附力的主导力的转变来解释。
