| The goal of this research was to measure, analyze, and simulate the macro- and nanomechanical properties of the bio-composite structure of plant cell walls using nondestructive evaluation methods. Dimension ICON Atomic Force Microscopy (AFM) and Dynamic Mechanical Analysis (DMA) were utilized as measurement instruments. Onion and celery epidermal peels were used as test samples. High resolution images were taken of microfibril structure of cell walls. This research uncovered, for the first time, an unexpected architecture of intact plant cell wall utilizing extremely low force of the AFM tip in a liquid environment. The newly discovered features include [1] the presence of single fibers, strands of several fibers and large bundles of fibers; [2] the layering of the fiber bundles; [3] the changes in angle of orientation of the fibers between layers, for example, in celery the top layer has an angle of 42 to 50 degrees and the second layer has an angle of 111 to 117 degrees; [4] possible evidence of slight twisting of the bundles. These features are significant because they give insight into the architecture of cell wall structure. The statistical analyses were able to provide a detailed description and understanding of both mechanical properties and experimental data. The results showed that: 1) AFM was determined as an effective method for nondestructive evaluation of materials in nano scale level; 2) an innovative Peak Force Tapping technique in AFM was used to scan the cell walls using extreme low forces (.200 picoN); 3) the AFM images showed the orientation of the microfibrils in the same cell were consistent, regardless of position of the cell; 4) it was determined that enzyme and buffer solutions had a negative influence on the stiffness and the strength of the cells' epidermis; 5) celery epidermis had comparable behavior to both viscoelastic and elastoplastic modeling in the linear region; 6) a simulation Model showed the force change as a function of calculated displacement in the same direction for plant cell wall microfibrils, primarily in the first layer; 7) the changes in microfibrillar orientation had an significant role in the change of elastic properties; 8) in modulus of elasticity analysis, onion had the least coefficient of variation (23% or 77% equality), representing the highest consistency during the data collection ; 9) the Young's modulus of onion and celery ranged from 0.2-4.9 MPa and 5-13 MPa, respectively; and 10) the stiffness of onion and celery ranged from 91 to 197 N/m and from 98 to 525 N/m, respectively. |
