Mechanical forces play an important role in the behaviour of cells, from differentiation to migration and the development of diseases. Optical tweezers provide a quantitative tool to study these forces and must be combined with other tools, such as phase contrast and fluorescence microscopy. Detecting the retro-reflected trap beam is a convenient way to monitor the force applied by optical tweezers, while freeing top access to the sample. Accurate in situ calibration is required especially for single cells close to a surface where viscosity varies rapidly with height. Here, we take advantage of the well contrasted interference rings in the back focal plane of the objective to find the height of a trapped bead above a cover slip. We thus map the viscous drag dependence close to the surface and find agreement between four different measurement techniques for the trap stiffness down to 2 μm above the surface. Combining this detection scheme with phase contrast microscopy, we show that the phase ring in the back focal plane of the objective must be deported in a conjugate plane on the imaging path. This simplifies implementation of optical tweezers in combination with other techniques for biomechanical studies.

MEMS摩阻传感器是一种专门为测量高超音速飞行器模型表面摩擦阻力设计的立体式MEMS传感器，为了实现其可靠组装，设计了MEMS摩阻传感器微组装系统，对包括双显微视觉系统、高精度治具设计、精密操作工具、精密定位平台、高精度视觉识别算法以及点胶工艺等进行了研究。首先，分析确定了传感器性能与传感器浮动杆与芯片组装的形位公差、总成后的浮动杆上端圆面与传感器管壳上组件圆孔的同心度以及端面平齐度与高度差相关。然后，针对影响因素设计了高精度治具保证组装前传感器各个部件的形位公差在理论范围内，设计了精密操作工具确保精确吸取部件并在组装过程中稳定搬运部件，利用双显微视觉精确识别装配的轴孔位置，驱动精密定位平台将各部件搬运到对应装配位置。最后，研究点胶工艺对于传感器装配性能的影响。实验结果表明：传感器微组装完成后，其浮动头与管壳上组件圆孔同心度偏差平均值约4.90 μm；浮动头与上端盖端面高度差跳动值为1 μm。MEMS摩阻传感器微组装系统完全满足传感器装配要求。