Study on the morphology and mechanical properties of solid, liquid and gas nanoscopic soft matter in liquid phase
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摘要: 为保持稳定，纳米软物质在液相环境中通常呈球冠状，这对液相下不同纳米软物质的形貌表征和辨识具有一定挑战性。本文利用原子力显微镜（AFM）对水下微纳米水泡、聚合物液滴和表面气泡进行高分辨成像，通过改变扫描力来研究分析它们的形貌变化及形态特征；通过纳米压痕测试，分析了探针与固、液、气界面的相互作用，并获得了它们的力学特性。研究结果表明：在0.5 nN扫描力下，水泡、液滴和气泡的形貌均呈球冠状；在大扫描力（5.0 nN）下，水泡形貌不变，液滴体积减小，气泡消失。在3.0 nN载荷的压痕实验中，顶点处的力–距离曲线表明三者均发生了弹性形变。探针脱离液滴需克服较大黏附力，气泡则呈现两段式弹性变形。此外，由于锚定效应的影响，球冠边缘位置抵抗变形的能力更强。聚甲基丙烯酸甲酯（PMMA）纳米膜的模量（约为3.38 GPa）和水泡弹性变形时的载荷无关。小尺寸PDMS液滴在水下的界面张力约为37.3 mN/m，表面气泡的气–液界面张力约为32.5 mN/m。Abstract: In liquid environments, nanoscopic soft materials typically adopt a cap-like shape to maintain their stability. Therefore, their morphology characterization and identification remain challenging in the liquid phase. The present study employs Atomic Force Microscopy (AFM) to achieve high-resolution imaging of subaqueous micro-nano blisters, polymer droplets, and surface bubbles. By analyzing the morphological changes in various scanning forces, the morphological characteristics of these nanoscopic soft materials are investigated. Subsequently, nanoindentation tests are conducted to analyze the interaction between the probe and the solid-liquid-gas interfaces, and their mechanical properties are obtained. The results show that under a scanning force of 0.50 nN, all blisters, droplets, and bubbles exhibited cap-like shapes. Under a higher scanning force (5.0 nN), the blister morphology remained constant, the droplet volume decreased, and the bubble disappeared. Force-distance curves at the vertex under a load of 3.0 nN indicate that all the three experienced elastic deformation. The probe has to overcome greater adhesion force to detach the droplet, while bubbles display a two-stage elastic deformation. Furthermore, due to the influence of anchoring effects, the considered objects exhibit stronger resistance to deformation near the edge of the spherical cap. The modulus of the poly (methyl methacrylate) (PMMA) nanofilm is independent of the applied load during elastic deformation caused by blister and it is estimated to be around 3.38 GPa. The interfacial tension of small-sized PDMS droplets underwater is approximately 37.3 mN/m, while the gas-liquid interfacial tension of surface bubbles is approximately 32.5 mN/m.
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图 1 水下纳米软物质的AFM形貌和力学测量
Figure 1. AFM morphology and mechanical measurements of suba-queous nanoscopic soft materials
图 2 水泡、液滴和表面气泡的接触半径与接触角的关系
Figure 2. Plot of the contact angle as a function of the contact radius of blister, droplet and surface bubble
图 3 水泡、液滴和表面气泡顶点位置的力–距离曲线
Figure 3. Force-distance curves acquired around the apex of blister, droplet and surface bubble
图 4 水泡、液滴和表面气泡的力–距离曲线
Figure 4. Force-distance curves of blister, droplet and surface bubble
图 5 水泡、液滴和表面气泡沿中心轴线处的刚度
Figure 5. The stiffnesses of blister, droplet and surface bubble at the central axis
图 6 水泡刚度与载荷的关系
Figure 6. Plot of the stiffness of the blister as a function of the indenting force
图 7 水泡弹性变形时的载荷和PMMA薄膜杨氏模量的关系
Figure 7. Plot of Young's modulus of PMMA film as a function of the loading during elastic deformation of the blister
图 8 液–液界面张力与曲率半径的关系
Figure 8. The relationship between liquid interface tension and curvature radius
图 9 气–液界面张力与曲率半径的关系
Figure 9. The relationship between liquid interface, gas-liquid interface tension and curvature radius
表 1 0.5 ～ 5.0 nN 扫描力下的水泡形貌特征
Table 1. Morphological characteristics of blister under the scanning force of 0.5 ～ 5.0 nN
扫描力FS /nN 接触半径a/${\text{μ}} $m 高度H/${\text{μ}} $m 曲率半径R/${\text{μ}} $m 接触角θ/(° ) 体积V/${\text{μ}} $m3 0.5 0.69 0.330 0.88 51 0.27 1.0 0.69 0.320 0.90 50 0.26 3.0 0.70 0.330 0.90 51 0.28 5.0 0.69 0.320 0.91 50 0.26 
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表 2 0.5 ～ 5.0 nN 扫描力下的液滴形貌特征
Table 2. Morphological characteristics of droplet under the scanning force of 0.5 ～ 5.0 nN
扫描力FS/nN 接触半径a/${\text{μ}} $m 高度H/${\text{μ}} $m 曲率半径R/${\text{μ}} $m 接触角θ/(° ) 体积V/${\text{μ}} $m3 0.5 0.53 0.120 1.2 26 0.053 1.0 0.51 0.120 1.1 27 0.052 3.0 0.43 0.096 1.0 25 0.029 5.0 0.43 0.096 1.0 25 0.029
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表 3 0.5 ～ 5.0 nN 扫描力下的表面气泡形貌特征
Table 3. Morphological characteristics of surface bubble under the scanning force of 0.5 ～ 5.0 nN
扫描力FS/nN 接触半径a/${\text{μ}} $m 高度H/${\text{μ}} $m 曲率半径R/${\text{μ}} $m 接触角θ/(° ) 体积V/${\text{μ}} $m3 0.5 0.32 0.043 1.23 15 0.0070 1.0 0.32 0.037 1.40 13 0.0060 3.0 0.19 0.016 1.08 10 0.0009 5.0 — — — — —
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