López-Guerra Enrique A, Solares Santiago D
The George Washington University, Department of Mechanical and Aerospace Engineering, Washington, DC 20052, USA.
Park Systems Inc., Santa Clara, CA, 95054, USA.
Beilstein J Nanotechnol. 2020 Sep 15;11:1409-1418. doi: 10.3762/bjnano.11.125. eCollection 2020.
Atomic force microscopy (AFM) is a widely use technique to acquire topographical, mechanical, or electromagnetic properties of surfaces, as well as to induce surface modifications at the micrometer and nanometer scale. Viscoelastic materials, examples of which include many polymers and biological materials, are an important class of systems, the mechanical response of which depends on the rate of application of the stresses imparted by the AFM tip. The mechanical response of these materials thus depends strongly on the frequency at which the characterization is performed, so much so that important aspects of behavior may be missed if one chooses an arbitrary characterization frequency regardless of the materials properties. In this paper we present a linear viscoelastic analysis of intermittent-contact, nearly resonant dynamic AFM characterization of such materials, considering the possibility of multiple characteristic times. We describe some of the intricacies observed in their mechanical response and alert the reader about situations where mischaracterization may occur as a result of probing the material at frequency ranges or with probes that preclude observation of its viscoelastic behavior. While we do not offer a solution to the formidable problem of inverting the frequency-dependent viscoelastic behavior of a material from dynamic AFM observables, we suggest that a partial solution is offered by recently developed quasi-static force-distance characterization techniques, which incorporate viscoelastic models with multiple characteristic times and can help inform dynamic AFM characterization.
原子力显微镜（AFM）是一种广泛应用的技术，用于获取表面的形貌、力学或电磁特性，以及在微米和纳米尺度上诱导表面改性。粘弹性材料，包括许多聚合物和生物材料，是一类重要的系统，其力学响应取决于AFM针尖施加应力的速率。因此，这些材料的力学响应强烈依赖于进行表征的频率，如果不考虑材料特性而任意选择表征频率，可能会错过行为的重要方面。在本文中，我们考虑了多个特征时间的可能性，对这类材料的间歇接触、近共振动态AFM表征进行了线性粘弹性分析。我们描述了在其力学响应中观察到的一些复杂性，并提醒读者注意由于在频率范围或使用排除观察其粘弹性行为的探针探测材料而可能出现错误表征的情况。虽然我们没有提供从动态AFM可观测量反演材料频率依赖粘弹性行为这一艰巨问题的解决方案，但我们建议最近开发的准静态力-距离表征技术提供了部分解决方案，这些技术结合了具有多个特征时间的粘弹性模型，并有助于指导动态AFM表征。
