| AVS 62nd International Symposium & Exhibition | |
| Scanning Probe Microscopy Focus Topic | Friday Sessions |
| Session SP+AS+MI+NS+SS-FrM |
| Session: | Probe-Sample Interactions |
| Presenter: | Nancy Burnham, Worcester Polytechnic Institute |
| Authors: | x. Yu, Worcester Polytechnic Institute M. Tao, Worcester Polytechnic Institute N.A. Burnham, Worcester Polytechnic Institute |
| Correspondent: | Click to Email |
By oscillating a micro-sized cantilever beam at a certain frequency and observing its interaction with the sample surface, dynamic mode atomic force microscopy (AFM) has gained attention for characterizing mechanical properties of a variety of materials at the micro and nano scales. The thin air film, confined between the oscillating cantilever beam and the stationary sample surface, causes the so-called “squeeze-film effect” when the gap between the two boundaries is less than a hundred microns. Although studies have shown that the squeeze film can act as a spring and a damper in accelerometers and microelecromechanical systems [1], the influence of the squeeze-film effect on the dynamics of an AFM cantilever has not been previously explored, to the authors’ knowledge. In this project, the stiffness and damping properties of the squeeze film between an oscillating AFM cantilever and a glass slide were calculated from the cantilevers’ amplitude and phase responses as recorded by the AFM digital system. The smaller the cantilever-sample gap, the larger the absolute values of the stiffness and the damping of the squeeze film. Results from different cantilevers (consequently having different spring constants and resonant frequencies) indicated that the air film exhibited negative stiffness and positive damping, with normalized changes from free values of up to 40%. Theoretical analysis was conducted using an equivalent-circuit model [2] along with the phasor diagram, and the derived stiffness and damping values were in excellent agreement with the experimental ones. Interestingly, a rotation angle between 20o and 30o in the fit of the data to the model reveals a phase lead of the squeeze-film damping before the usual air damping when the cantilever is far from a surface: the maximum squeeze-film damping occurs before the maximum velocity of the cantilever because air becomes less dense as it rushes out of the tip-sample gap. The surprising sign of the stiffness is thus explained by the phase lead. Future work includes incorporating the squeeze-film effect into more accurate measurements of a material’s stiffness and damping properties using dynamic AFM.
References:
1. Starr, James B. "Squeeze-film damping in solid-state accelerometers." Solid-State Sensor and Actuator Workshop, 1990. 4th Technical Digest., IEEE. IEEE, 1990.
2. Veijola, Timo. "Compact models for squeezed-film dampers with inertial and rarefied gas effects." Journal of Micromechanics and Microengineering 14.7 (2004): 1109.