Khan Shah H, Hoffmann Peter M
Department of Physics, University of Peshawar, Peshawar, Pakistan.
Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2015 Oct;92(4):042403. doi: 10.1103/PhysRevE.92.042403. Epub 2015 Oct 14.
In this study, we present a detailed analysis of the squeeze-out dynamics of nanoconfined water confined between two hydrophilic surfaces measured by small-amplitude dynamic atomic force microscopy. Explicitly considering the instantaneous tip-surface separation during squeeze-out, we confirm the existence of an adsorbed molecular water layer on mica and at least two hydration layers. We also confirm the previous observation of a sharp transition in the viscoelastic response of the nanoconfined water as the compression rate is increased beyond a critical value (previously determined to be about 0.8 nm/s). We find that below the critical value, the tip passes smoothly through the molecular layers of the film, while above the critical speed, the tip encounters "pinning" at separations where the film is able to temporarily order. Preordering of the film is accompanied by increased force fluctuations, which lead to increased damping preceding a peak in the film stiffness once ordering is completed. We analyze the data using both Kelvin-Voigt and Maxwell viscoelastic models. This provides a complementary picture of the viscoelastic response of the confined water film.
在本研究中，我们通过小振幅动态原子力显微镜对限制在两个亲水表面之间的纳米受限水的挤出动力学进行了详细分析。明确考虑挤出过程中的瞬时针尖 - 表面间距，我们证实了云母上存在吸附分子水层以及至少两个水化层。我们还证实了之前的观察结果，即当压缩速率超过临界值（先前确定约为0.8 nm/s）时，纳米受限水的粘弹性响应会发生急剧转变。我们发现，在临界值以下，针尖平滑地穿过薄膜的分子层，而在临界速度以上，针尖在薄膜能够暂时有序排列的间距处会遇到“钉扎”现象。薄膜的预有序排列伴随着力波动的增加，一旦排列完成，这会导致薄膜刚度峰值之前的阻尼增加。我们使用开尔文 - 沃伊特和麦克斯韦粘弹性模型对数据进行分析。这提供了受限水膜粘弹性响应的互补图像。
