previous work by the authors [1-3] DESIGN AND EXPERIMENTAL METHODS: Numerical simulations were carried out using commercially available software Ansys®. Specifically, the simulations involved modal analysis on an undamped, freestanding beam with oscillations in the longitudinal mode both with and without attached mass. The structural physical (engineering) discipline is preferred for the Modal analysis of Magnetostrictive sensors. The selected element type was SOLID186. The sensor size employed here was 250 x 50 x 4 µm and the attached mass were representative of an actual E. Coli O157:H7 cell, size of 1.43 x 0.73 x 0.73 microns and weight of 1 pictogram. The boundary conditions were set to obtain longitudinal vibration mode of the sensor platform. Prediction model was developed involving the factors influencing the resonance behavior such as (a) Mass distribution, (b) Position of the mass distributed and (c) Physical dimension of the sensor platform in compliance with the theoretical equations and simulation results. Mass of E. coli cells were distributed as a single layer for uniform distribution. The later was glued to the sensor platform and subjected to numerical simulation with the application of boundary condition over the whole setup. The density of the layer was modified each time for different amount of mass in uniform distribution case and the corresponding variation in frequency shift was observed. In case of the non uniform distribution the layer was split and concentrated along the free ends gradually moving towards the central nodal line and vice versa as shown in Figure 3. The darker layer in the figure is considered as the mass of E. coli cell distributed over the sensor platform. Glass beads attachment : Sensors with dimensions of 5 mm length and 1 mm width were cut from a 28 μm thick commercially available MetglasTM 2826MB strip. These specimens were prepared, by cleaning and drying, using the identical procedures described by the authors elsewhere. Glass beads with a diameter about 425 μm were employed to simulate the concentrated mass and were carefully loaded on to the sensor surface at prescribed locations and secured with adhesive. The average mass of a sensor and glass bead were 1066 μg and 181.5 μg, respectively. It should be noted that these experiments are aimed as assessing the position of the mass concentrations and not focused on demonstrating minimum sensitivity. Thus, significantly sized beads were employed. The amount of glue employed to affix each bead as well as its position were well controlled to minimize any errors. After a glass bead was loaded on the sensor surface, it was immobilized by drying at Figure 1: Non-Uniform distribution of E. coli cells as layer over the sensor Figure 2: Simulation result of resonant frequency shift due to E. coli cells distribution 11
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