Two orthogonal standing acoustic waves, generated by piezoelectric excitation, can form a two-dimensional pressure field in microfluidic devices. A phase difference of the excitation waves can be employed to rotate spherical m-sized silica particles by a torque mediated through the viscous boundary δ around the particle. The measurement of the rotational rate is, so far, limited to high-speed cameras and their frame rate, and gets increasingly difficult when the sphere gets smaller. We report here a new method for measuring the rotational rate of m sized spherical particles. We utilize an optical trap with high-speed position detection to overcome the frame rate limitation of wide field image recording. The power spectrum of an optically trapped, rotating particle reveals additional peaks corresponding to the rotational frequencies—compared to a non-rotating particle. We validate our method at low rotational rates against high-speed video observation. To demonstrate the potential of this method we addressed a recent controversy about the rotation of particles with a relatively large viscous boundary layer δ. We measured steady-state rotational rates up to 229 Hz (13.8 10³ rpm) for a particle with a radius R ≈ δ. Recent numerical research suggests that in this regime the existing theoretical approach (valid for ) overpredicts the steady-state rotational rate by a factor of 10. With our new method we also confirm the numerical results experimentally.

压电激励产生的两个正交的驻声波可以在微流体装置中形成二维压力场。激发波的相位差可用于旋转球形尺寸为m的二氧化硅颗粒通过扭矩传递，该扭矩通过颗粒周围的粘性边界δ传递。到目前为止，旋转速率的测量仅限于高速相机及其帧速率，并且当球体变小时，难度越来越大。我们在这里报告一种新的方法来测量转速m大小的球形颗粒。我们利用具有高速位置检测功能的光阱来克服广角图像记录的帧速率限制。与非旋转粒子相比，被光学捕获的旋转粒子的功率谱显示出与旋转频率相对应的其他峰值。我们针对高速视频观察以低旋转速率验证了我们的方法。为了证明这种方法的潜力，我们解决了有关具有较大粘性边界层δ的粒子旋转的最新争议。我们测量稳态旋转速率高达229赫兹（13.8 10 ³的转速），用于与半径的粒子- [R 听，说：  δ。最近的数值研究表明，在这种情况下，现有的理论方法（适用于）对稳态旋转速率的预测过高了10倍。使用我们的新方法，我们还通过实验证实了数值结果。