The Investigation of Haliotis Rufescens (Red Abalone): Mesolayer Growth & Mechanical Behavior

Abstract

Self-assembly is a popular bottom-up process which results in highly-ordered and fine structures in micro- or nano-scale. The special structures produced through the self-assembly are always related to some unique and outstanding properties. Abundant research has been performed to aid in figuring out ideal ways to produce these materials for industrial applications. In the biological world, many creatures develop a self-assembly process in some of their physiological behaviors through millions of years of evolution which could inspire us with innovate ideas to produce the special and specific structure. The nacre of the abalone shell is on such example. Thus, studies of abalone nacre are helpful in learning about the design of such amazing structures, how they are formed, and the mechanical properties they possess. Nacre, which is also called “mother of pearl”, is an extremely important part of abalone’s shell. With the self-assembly produced structure, which is constructed from aragonite tablets connected by organic thin films, the abalone nacre demonstrates fascinating toughness. While the tidy “brick-wall” structure provides the main body of the nacre, there exists another structure in the nacre which shows obvious differences and appears throughout the nacre, especially in shells of abalone that grow in the seas (the wild abalone). It is called the mesolayer and it can be divided into three separate layers: prismatic layer, organic layer, and columnar layer. Each of them shows their own individual elastic properties. Though there has been much research performed which focuses on the tablets which make up the nacre, very limited attention has been paid to the mesolayer. The abalone creates a confined space for itself with its shell which it is attached to by a portion of its tissue known as the mantle. Abalone’s nacre grows in the colloidal mucus secreted by the abalone itself in that confined space. That colloidal mucus provides the self-assembly nacre growth with an isolated and complicated environment and will ultimately act as the organic matrix in the nacre. As it is hard to observe the in-situ nacre growth, some methods were developed to assist with investigating this biomineralization process such as the Flat Pearl Method. This is the method applied in this research to determine how the seawater environment effects the mesolayer growth. The California red abalones were purchased from an abalone farm and cultured in artificial seawater. The temperature, water quality, and food were well controlled to guarantee the abalones’ health and that the nacre and mesolayer could grow normally. Temperature, pH, Mg2+/Ca2+ ratio of the artificial seawater were parameters in need of control, based on literatures describing sea environment and sea creatures. Among these parameters, temperature and pH were controlled to allow the mesolayer to form. The mucus that the nacre grows from has also been observed out of abalone’s body and in a mimicked confined space, and shows some interesting dendrite formation. Elastic modulus is the main mechanical property of concern in this research. Nacre samples were tested with a nanoindenter. The main composition of nacre is calcium carbonate, which can be treated as a ceramic material, and shows excellent high toughness. Both the mesolayer and nacre were tested by the nanoindenter. For the mesolayer, each sublayer was tested to see the individual behaviors. For the nacre, when the test was applied on the nacre sample directly, crack formation began around the nanoindentation sites, which will influence the test result. Thus, the well developed Chen-Prorok method, which introduces a ductile metal film to the sample surface in order to avoid the substrate cracking, was applied in this research. As the nacre has a nano-scale architecture, a distinctive anisotropic modulus is present. The nacre samples tested in different architecture orientations were included in thois research. The data was collected, analyzed, and presented, here in. Nanoindentations on several nacre samples with different architecture orientations were milled using focused ion beam in order to show the behavior beneath the surface. In addition, a bending experiment was applied on the nacre sample to see its behavior. With the combination of this information, reasonable assumptions were developed to explain the anisotropic modulus of the nacre structure. The nacre was also prepared into cantilever beams for additional testing. The results were compared with the nanoindentation test results. The cantilever project has recently just begun, and future progress is to be expected. Although the nacre structure has been under investigation for quite some time, debates still exist as to how the nacre growth process functions and how the mechanical properties influence the structure. The work stated in this dissertation seeks to advance the knowledge of how the isolated biomineralization is effected by the outside environment and why this special architecture introduces a material with highly anisotropic mechanical properties.