Guo Zhenjiang, Liu Yawei, Xiao Qianxiang, Schönherr Holger, Zhang Xianren
State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology , Beijing 100029, China.
Physical Chemistry I, Department of Chemistry and Biology & Research Center of Micro and Nanochemistry and Engineering (Cμ), University of Siegen , Adolf-Reichwein-Str. 2, 57076 Siegen, Germany.
Langmuir. 2016 Jan 26;32(3):751-8. doi: 10.1021/acs.langmuir.5b04162. Epub 2016 Jan 11.
Although the morphology of surface nanobubbles has been studied widely with different AFM modes, AFM images may not reflect the real shapes of the nanobubbles due to AFM tip-nanobubble interactions. In addition, the interplay between surface nanobubble deformation and induced capillary force has not been well understood in this context. In our work we used constraint lattice density functional theory to investigate the interaction between AFM tips and pinned surface nanobubbles systematically, especially concentrating on the effects of tip hydrophilicity and shape. For a hydrophilic tip contacting a nanobubble, its hydrophilic nature facilitates its departure from the bubble surface, displaying a weak and intermediate-range attraction. However, when the tip squeezes the nanobubble during the approach process, the nanobubble shows an elastic effect that prevents the tip from penetrating the bubble, leading to a strong nanobubble deformation and repulsive interactions. On the contrary, a hydrophobic tip can easily pierce the vapor-liquid interface of the nanobubble during the approach process, leading to the disappearance of the repulsive force. In the retraction process, however, the adhesion between the tip and the nanobubble leads to a much stronger lengthening effect on nanobubble deformation and a strong long-range attractive force. The trends of force evolution from our simulations agree qualitatively well with recent experimental AFM observations. This favorable agreement demonstrates that our model catches the main intergradient of tip-nanobubble interactions for pinned surface nanobubbles and may therefore provide important insight into how to design minimally invasive AFM experiments.
尽管已经使用不同的原子力显微镜（AFM）模式对表面纳米气泡的形态进行了广泛研究，但由于AFM针尖与纳米气泡之间的相互作用，AFM图像可能无法反映纳米气泡的真实形状。此外，在这种情况下，表面纳米气泡变形与诱导毛细力之间的相互作用尚未得到很好的理解。在我们的工作中，我们使用约束晶格密度泛函理论系统地研究了AFM针尖与固定表面纳米气泡之间的相互作用，特别关注针尖亲水性和形状的影响。对于与纳米气泡接触的亲水针尖，其亲水性有助于它从气泡表面脱离，表现出弱的和中等范围的吸引力。然而，当针尖在接近过程中挤压纳米气泡时，纳米气泡会表现出一种弹性效应，阻止针尖穿透气泡，导致纳米气泡发生强烈变形并产生排斥相互作用。相反，疏水针尖在接近过程中很容易刺穿纳米气泡的气液界面，导致排斥力消失。然而，在回缩过程中，针尖与纳米气泡之间的粘附力会对纳米气泡变形产生更强的拉长效应，并产生强大的长程吸引力。我们模拟得到的力演化趋势与最近的AFM实验观察结果在定性上非常吻合。这种良好的一致性表明，我们的模型捕捉到了固定表面纳米气泡针尖-纳米气泡相互作用的主要梯度，因此可能为如何设计微创AFM实验提供重要的见解。
