Atomic force microscopy enabled analysis and modeling for micro bubble mechanical properties

Abstract

Since last century, human exploration of the world has turned from the macro scale into the micro scale, and nanotechnology has received a constant attention for its wide application fields. With nanotechnology, the micro-properties of the study objects can be obtained. Among them, in the past 50-60 years, the bubble has gained attention for its wide range of applications, especially in micro-scale and nano-scale. Research on the bubble’s mechanical properties is necessary for better manipulation and application of the bubble. Precise measurement of the microbubble’s mechanical properties (e.g., stiffness and surface tension) relies largely on delicate manipulation technology. As a result, atomic force microscopy (AFM) has been widely implemented. However, in previous AFM-scanned figures, microbubbles were usually captured with a relatively large distortion and the bubble morphology was not clear enough to locate precisely the AFM probe so that further nanomanipulation or measurements could not be performed accurately. In this research, AFM-based stiffness of a single micro-bubble has been studied. An electrochemistry method was applied to generate stable bubbles. By using force volume mode, a single bubble was captured. Besides, the bubble, which should be isotropic, showed instead location-dependent anisotropic characteristics. In this case, the shape of the bubble (which influences the angle between the bubble’s surface and the probe tip) was proposed as the key reason to explain the anisotropic phenomenon, and a corresponding model was established to verify the hypothesis. To achieve the overall aim of this research, several sections of work are performed as follows: Section Ⅰ: In this section, the experimental system and approach are introduced, including an overview of various bubble-generation methods, the principle of the current applied bubble-generation system, the establishment procedure of the bubble-generation system, as well as the introduction of the experimental set-up: Scanning electron microscope (SEM) and atomic force microscope (AFM). Section Ⅱ: In this section, the applied modelling approach is introduced. Bubbles are known as isotropic, whereas demonstrates anisotropic phenomenon, which induces the research interest. Here, the overview of previous modelling method – nonlinear least squares were introduced firstly. The principle and the establishment procedure of the applied modelling approach are described as well. The corresponding modelling results are displayed in the later section. Section Ⅲ: In this section, the experimental results are illustrated, including the measurement results of the single microbubble using AFM and the modelling results. In the former result, the single microbubble measurement is realized, which improves the accuracy of the measurement, performing more clear morphology of the single microbubble. In the latter result, the modelling results verify the proposed assumption that the shape of the bubble influences the angle between the bubble’s surface and the probe tip, further resulting in the anisotropy phenomenon. This study shall be valued for the measurement and evaluation of the single microbubble. In future studies, the proposed model can be applied further in the graphic correction (e.g., the correct stiffness image acquisition), and the calculation of Young’s modulus.