Developments for Soft-Matter Characterization in Atomic Force Microscopy
Open AccessCharacterization of soft materials utilizes Atomic Force Microcopy (AFM) frequently due to its versatility and ability to operate from micro to nanoscale. Properties like viscoelasticity, local electrical conductivity and charge carrier behavior is sought for polymeric, biological and biomimetic samples (for example, cells, tissues, hydrogels). This dissertation focuses on expanding AFM techniques to accurately and delicately accommodate soft material characterization. Namely, first, we focus on improving viscoelastic inversion from the force-distance experiment by proposing a method that directly obtains force spectroscopy from such an experiment. Finally, propose Fourier-based techniques to i) increase the accuracy of the force measurements during an intermittent contact experiment and ii) explore the feasibility of local conductivity measurements utilizing intermittent contact.The first chapter serves as an overall introduction, and the next three chapters of this dissertation provide background information on various subjects covered and utilized throughout the dissertation. In the second chapter, we discuss signals and integral transformations. In this work, experimental measurements and variables are widely treated as discrete signals, and Laplace and Fourier-based integral transformation techniques are extensively utilized. The third chapter provides an overview of linear viscoelasticity and rheological viscoelastic models. This chapter also includes a functional shape analysis of storage and loss modulus to provide insight into material behavior and experimental expectations. Finally, Chapter four introduces AFM and AFM techniques utilized in the text. It also includes a few shortcomings of the current AFM characterizations of soft materials we try to address. Chapters five and six discuss our proposed viscoelastic inversion method. The proposed technique's analytical developments are shown in chapter 5 and compared to numerical simulations and analytical material behavior. The method encompasses the advantages of Fourier techniques and can directly access viscoelastic spectrography information. Chapter 6 uses the method in actual AFM experimental data, and the process is demonstrated. Viscoelastic models are derived in Z and modified Fourier domains to be directly compatible with the proposed method. Hence, a model fit in the frequency domain is also performed. In chapters seven and eight, we investigate higher harmonics of the cantilever trajectory to extract more information from the intermittent contact experiments. In chapter 7, we propose a Gaussian-based frequency extrapolation - time interpolation method to recreate force-distance curves from intermittent contact experiments by prescribing decay of the higher harmonics of the motion. Finally, in chapter 8, we investigate the higher harmonics effect on tip-sample distance and contact area to foresee the harmonic behavior of the electrical-tip sample current and investigate the feasibility of using lock-in amplifiers to capture it.
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