Compared to the bulk properties of material, our fundamental understanding of the interactions and dynamics at the liquid-air, liquid-solid, and even solid-solid interfaces are still very limited. The lack of progress is partially because the interfacial interactions are extremely sensitive to the nanoscale distance at which the measurements are made, and the interfacial dynamics are easily disturbed by defects and impurities at the interface. Well-controlled experimental systems with tunable interfacial properties together with surface-sensitive experimental tools and analysis methods are needed for the study of the interactions and dynamics at interfaces. Over the past 30 years, atomic force microscopy (AFM) has become one of the most important tools for the study of interface...[ Read more ]

Compared to the bulk properties of material, our fundamental understanding of the interactions and dynamics at the liquid-air, liquid-solid, and even solid-solid interfaces are still very limited. The lack of progress is partially because the interfacial interactions are extremely sensitive to the nanoscale distance at which the measurements are made, and the interfacial dynamics are easily disturbed by defects and impurities at the interface. Well-controlled experimental systems with tunable interfacial properties together with surface-sensitive experimental tools and analysis methods are needed for the study of the interactions and dynamics at interfaces. Over the past 30 years, atomic force microscopy (AFM) has become one of the most important tools for the study of interfaces. In this thesis, I present the development of new AFM probes and techniques and their applications in the investigation of novel interfacial phenomena and understanding of the underlying physics. Three interesting problems are studied at different interfaces.

In the first experiment, we carry out direct AFM measurements of capillary force hysteresis (CFH) and relaxation of a circular moving contact line (MCL) formed on a long micron-sized hydrophobic glass fiber intersecting a liquid-air interface. The measured CFH and contact line (CL) relaxation show an asymmetric speed dependence in the advancing and receding directions. A unified model based on force-assisted barrier crossing is developed to find the effective energy barrier E_b and size λ associated with the defects on the fiber surface. The experiment demonstrates that the pinning (relaxation) and depinning dynamics of the CL can be described by a common microscopic framework, and the advancing and receding CLs are in influenced by two different sets of relatively non-wetting and wetting defects on the fiber surface.

The second experiment studies the enhanced optical force acting on a nano-structured plasmonic resonant cavity using dynamic mode atomic force microscopy (DM-AFM). The plasmonic cavity is made of an upper gold-coated glass sphere and a lower quartz substrate patterned with an array of subwavelength gold disks. In the near-field when the sphere is positioned close to the disk array, plasmonic resonance is excited in the cavity, and the induced force from a 1550 nm infrared laser is found to be increased by an order of magnitude compared with the photon pressure generated by the same laser light. The experiment demonstrates that DM-AFM is a powerful tool for the study of light-induced forces and their enhancement in plasmonic nanostructures.

In the third experiment, we conduct a non-contact measurement of the viscoelastic properties of nanometer-thin films in a liquid medium using frequency modulation atomic force microscopy (FM-AFM) together with a newly developed long-needle probe. The probe contains a long vertical glass fiber with one end adhered onto a cantilever beam and the other end with a sharp tip placed near the liquid-film interface. The nanoscale flow generated by the resonant oscillation of the needle tip provides a precise hydrodynamic force acting on the soft surface of the thin film. By accurately measuring the mechanical response of the thin film, we obtain the elastic and loss moduli of the thin film using the linear response theory of elasto-hydrodynamics (EHD). In addition, we apply this new technique to obtain the viscoelastic property of live cells with dual frequency modulation, which are used to simultaneously measure the cell morphology and spatial distribution of the cell's elastic modulus and their temporal evolution during different stages of cell growth and division. The experiment verifies the theory and demonstrates the applications. This thesis work thus provides new techniques and perspectives of AFM and leads to better understanding of the interactions and dynamics at interfaces.