RT - Journal Article T1 - Effect of Fluid and Size on the Nonlinear Dynamic of Atomic Force Microscope Based on Modified Couple Stress Theory JF - mdrsjrns YR - 2019 JO - mdrsjrns VO - 19 IS - 4 UR - http://mme.modares.ac.ir/article-15-24394-en.html SP - 1029 EP - 1038 K1 - Atomic Force Microscope K1 - Size Effect K1 - Modified Couple Stress Theory K1 - Stability Analyses K1 - Nonlinear Dynamic AB - The atomic force microscope (AFM) determines the topography of surfaces in nano scale based on the changes in the exited micro-cantilever’s dynamic characteristics. Therefore, it is essential to simulate and predict more accurately the dynamic behavior of cantilever beams for use in design and fabrication of AFM. Based on the experimental observations, in contrast to the classic theory, the normalized stiffness of structures is not constant with the reduction of dimensions in micro and nano scales. This change, which can be either softness or stiffness, results in size-dependent behavior, non-classic continuum theories. This paper studies the effect of size on the dynamic behavior of AFM based on modified couple stress theory, and compares the results with those obtained from classic theory. The nonlinear partial differential governing equation of the system is derived, considering intermolecular and hydrodynamic forces, based on the modified couple stress theory. By applying Galerkin projection method, partial differential equations are transformed into ordinary equations and the discrete system is extracted. It is shown that considering size effect leads to enlargement of expected working domain of AFM, and also predicted amplitude and frequency of oscillations decreases and increases, respectively. Moreover, two theories predict different start point of bi-stability region. Solution approach is verified by comparing the results with two degrees-of-freedom model and analogue equations method. Furthermore, effect of hydrodynamic forces of fluid on dynamic behaviour of AFM is investigated. LA eng UL http://mme.modares.ac.ir/article-15-24394-en.html M3 ER -