Iasi Plasma Advanced Research Center (IPARC), Faculty of Physics, "Alexandru Ioan Cuza", University of Iasi , Iasi-700506, Romania.
Langmuir. 2017 May 30;33(21):5188-5196. doi: 10.1021/acs.langmuir.7b01156. Epub 2017 May 15.
Partial wetting on the nanoscale may result in the formation of sessile liquid nanodroplets on flat substrates. In this case, the molecular forces generate a strong interaction between nanodroplet interfaces. This interaction is expressed in the mean-field approximation by the disjoining pressure and determines an important deviation from the spherical cap shape of the nanodroplets. This deviation is observed on the atomic force microscopy images of sessile nanodroplets of oleic acid on glass. The disjoining pressure was manipulated by hydroxylation of the glass surface. This surface modification generated a strong negative disjoining pressure due to structural forces arising from the orientation of oleic acid molecules with their polar heads toward the substrate. As a result, the shape of oleic acid nanodroplets showed large deviations from the spherical cap shape, with the liquid-vapor interface tilting angle with respect to the plane substrate having a maximum (herein considered to be the contact angle) a certain distance from the substrate, followed by its decrease to zero at the droplet edge. The integration of the augmented Young-Laplace equation, where the dependence of the negative structural disjoining pressure on the interface separation distance was assumed to be an exponential decay, yielded height profiles of droplets in good agreement with the experiment.
在纳米尺度上的部分润湿可能导致在平坦基底上形成不流动的液体纳米液滴。在这种情况下，分子力在纳米液滴界面之间产生强烈的相互作用。这种相互作用在平均场近似中由推斥压力表示，并决定了纳米液滴从球形帽形状的重要偏差。这种偏差在油酸在玻璃上的不流动纳米液滴的原子力显微镜图像中观察到。通过玻璃表面的羟化作用来操纵推斥压力。这种表面修饰由于油酸分子的极性头朝向基底的取向而产生结构力，从而产生强烈的负推斥压力。结果，油酸纳米液滴的形状表现出从球形帽形状的大偏差，液体-蒸汽界面相对于平面基底的倾斜角度具有最大值（此处被认为是接触角），距离基底一定距离，然后在液滴边缘减小到零。增广的 Young-Laplace 方程的积分，其中假设负结构推斥压力与界面分离距离的关系是指数衰减，得到了与实验很好吻合的液滴高度分布。
