Time-Resolved Rheometry of Polymer Melts at High Shear Rates
Nonlinear Aspects of Hertzian Contact Vibrations in Atomic Force Microscopy
Vibrations of Atomic Force Microscope Cantilevers
Date: Tuesday, September 19, 2000
Time: 3:30 p.m.
Place: W128 Nebraska Hall
Time-Resolved Rheometry of Polymer Melts at High Shear Rates
Yungui Hu
Department of Engineering Mechanics
University of Nebraska, Lincoln, NE 68588-0526
Advisor: Dr. Ruqiang Feng
Characterizing the behavior of polymer melts at high shear rates is an important need in developing computerized process design and optimization for injection molding and extrusion of polymer products. A new polymer melt rheometer utilizing the Kolsky torsion bar technique has been developed. This technique enables time-resolved measurements of the transient, large-deformation response of polymer melts for shear rates up to 104 s-1 and for temperatures up to 300°C. In this presentation, results of a series of Kolsky bar rheometer experiments on a low-density polyethylene melt will be presented. The transient shear response of the material at 190°C and 150°C were measured at various shear rates and for large shear strains. The results show that during large shear deformation, the material undergoes significant elastic strain hardening. The material response is strongly viscoelastic in contrast to the solely viscous treatment that is commonly used in the process simulations. The significance of this finding as well as the future work will be discussed.
Nonlinear Aspects of Hertzian Contact Vibrations in Atomic Force Microscopy
Baowei Wei
Department of Engineering Mechanics
University of Nebraska, Lincoln, NE 68588-0526
Advisor: Dr. Joseph Turner
Atomic force acoustic microscopy (AFAM), recently developed by Rabe and colleagues at the IzfP, can be used to make quantitative measurements of surface stiffness with high spatial resolution. The technique utilizes the dynamic response of the AFM cantilever, specifically in terms of the higher-order cantilever modes. The AFM is modeled as a cantilever, its tip in contact with the sample surface. If sufficient force is applied to the cantilever, the contact force between the tip and sample may be modeled as Hertzian. Nonlinear contact forces of this sort can induce significant nonlinear features into the vibration. The nonlinearities of the motion are characteristic of the material properties of the sample. The nonlinear vibrations of the AFM are studied using the method of harmonic balance. These analyses show the anticipated frequency shift of the natural frequencies as a function of contact force and excitation amplitude. The dependence on the nonlinear parameters of the problem is clearly defined.
Vibrations of Atomic Force Microscope Cantilevers
Joshua S. Wiehn
Department of Engineering Mechanics
University of Nebraska, Lincoln, NE 68588-0526
Advisor: Dr. Joshua Turner
Recent advances in atomic force microscopy (AFM) have led to new measurement techniques, which have enabled the quantitative evaluation of material properties with nanoscale resolution. Quantitative measurements of material properties are important for research of the physical properties of matter, measurement of microelectromechanical systems (MEMS), as well as for quality control. Some of these new techniques rely on indentation of the AFM cantilever tip into the sample surface. Others use dynamic techniques in which the cantilever or sample are excited. For dynamic techniques the vibrating cantilever interacts with the sample surface through the tip of the cantilever. The tip-sample interaction forces and the surface properties will affect each vibration mode differently. In order to gain insight into the tip-sample interaction and the sample material properties the vibration spectra and mode shapes of an AFM cantilever will be measured. These measurements will be made for the cantilever in contact and out of contact with the sample surface. The mode shapes will be measured using a heterodyne laser interferometer that will be scanned along the length of the cantilever in order to accurately measure the displacement of the cantilever excited at a resonant frequency.
