Study on the morphology and mechanical properties of solid, liquid and gas nanoscopic soft matter in liquid phase

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.