Tan Xinfeng, Guo Dan, Luo Jianbin
State Key Laboratory of Tribology, Tsinghua University Beijing 100084 China
RSC Adv. 2019 Sep 2;9(47):27464-27474. doi: 10.1039/c9ra03995c. eCollection 2019 Aug 29.
Dynamic force microscopy (DFM) has become a multifunctional and powerful technique for the study of the micro-nanoscale imaging and force detection, especially in the compositional and nanomechanical properties of polymers. The energy dissipation between the tip and sample is a hot topic in current materials science research. The out-of-plane interaction can be measured by the most commonly used tapping mode DFM, which exploits the flexural eigenmodes of the cantilever and a sharp tip vibrating perpendicular to the sample surface. However, the in-plane interaction cannot be detected by the tapping mode. Here a bimodal approach, where the first order flexural and torsional eigenmodes of the cantilever are simultaneously excited, was developed to detect the out-of-plane and in-plane dissipation between the tip and the polymer blend of polystyrene (PS) and low-density polyethylene (LDPE). The vibration amplitudes and phases have been recorded to obtain the contrast, energy dissipation and virial the setpoint ratio of the first order vibration amplitude. The pull-in phenomenon caused by a strong attractive force can occur near the transitional setpoint ratio value, the amplitude setpoint at which the mean force changes from overall attractive to overall repulsive. The in-plane dissipation is much lower than out-of-plane dissipation, but the torsional amplitude image contrast is higher when the tip vibrates near the sample surface. The average tip-sample distance can be controlled by the setpoint ratio to study the in-plane dissipation. Both flexural and torsional phase contrasts and torsional amplitude contrast can also be significantly enhanced in the intermediate setpoint ratio range, in which compliant heterogeneous materials can be distinguished. The experiment results are of great importance to optimize the operating parameters of image contrast and reveal the mechanism of friction dissipation from the perspective of in- and out-of-plane energy dissipation at different height levels, which adds valuable ideas for the future applications, such as compliant materials detection, energy dissipation and the lateral micro-friction measurement and so on.
动态力显微镜（DFM）已成为用于微纳尺度成像和力检测研究的多功能强大技术，尤其适用于聚合物的成分和纳米力学性能研究。针尖与样品之间的能量耗散是当前材料科学研究中的一个热门话题。面外相互作用可以通过最常用的轻敲模式DFM来测量，该模式利用悬臂梁的弯曲本征模和垂直于样品表面振动的尖锐针尖。然而，轻敲模式无法检测面内相互作用。在此，开发了一种双峰方法，其中悬臂梁的一阶弯曲和扭转本征模同时被激发，以检测针尖与聚苯乙烯（PS）和低密度聚乙烯（LDPE）的聚合物共混物之间的面外和面内耗散。记录了振动幅度和相位，以获得对比度、能量耗散以及一阶振动幅度的设定点比率。在过渡设定点比率值附近可能会出现由强吸引力引起的拉入现象，即平均力从整体吸引力变为整体排斥力时的幅度设定点。面内耗散远低于面外耗散，但当针尖在样品表面附近振动时，扭转幅度图像对比度更高。可以通过设定点比率来控制平均针尖 - 样品距离，以研究面内耗散。在中间设定点比率范围内，弯曲和扭转相位对比度以及扭转幅度对比度也都可以显著增强，在该范围内可以区分柔顺的异质材料。实验结果对于优化图像对比度的操作参数以及从不同高度水平的面内和面外能量耗散角度揭示摩擦耗散机制具有重要意义，这为未来的应用，如柔顺材料检测、能量耗散以及横向微摩擦测量等增加了有价值的思路。
