Motion of a colloidal particle in an optical trap
Motion of a colloidal particle in an optical trap
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DOI：
10.1103/physreve.76.011112
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发表时间：
2007-07-01
影响因子：
2.4
通讯作者：
Forro, Laszlo
中科院分区：
文献类型：
--
作者：
Lukic, Branimir;Jeney, Sylvia;Forro, Laszlo
利用后焦平面干涉法测量了胶体粒子在光阱中的热位置涨落，测量精度为微秒级。均方位移和功率谱密度与考虑流体力学记忆效应的谐波势中布朗粒子的理论非常一致。粒子的运动在短时间内由记忆效应支配，在长时间内由势支配。我们确定了粒子运动不受势影响的时间，并发现它近似为tau（k）/20，其中tau（k）是势恢复力的弛豫时间。这使我们能够排除存在的自由扩散运动，成比例的t，即使是一个球体的半径小到0.27 μ m的潜力弱到1.5 μ N/m。由于布朗运动的物理学可以用来校准光阱，我们表明，忽略记忆效应导致低估超过10%的探测器的灵敏度和陷阱刚度的实验与微米大小的粒子和采样频率高于200 kHz。此外，这些校准误差增加了一个非平凡的方式与颗粒大小，陷阱刚度，和采样频率。最后，我们提出了一种方法来评估典型的光阱实验中的记忆效应所造成的校准误差。
Thermal position fluctuations of a colloidal particle in an optical trap are measured with microsecond resolution using back-focal-plane interferometry. The mean-square displacement and power spectral density are in excellent agreement with the theory for a Brownian particle in a harmonic potential that accounts for hydrodynamic memory effects. The motion of a particle is dominated at short times by memory effects and at longer times by the potential. We identify the time below which the particle's motion is not influenced by the potential, and find it to be approximately tau(k)/20, where tau(k) is the relaxation time of the restoring force of the potential. This allows us to exclude the existence of free diffusive motion, proportional to t, even for a sphere with a radius as small as 0.27 mu m in a potential as weak as 1.5 mu N/m. As the physics of Brownian motion can be used to calibrate an optical trap, we show that neglecting memory effects leads to an underestimation of more than 10% in the detector sensitivity and the trap stiffness for an experiment with a micrometer-sized particle and a sampling frequency above 200 kHz. Furthermore, these calibration errors increase in a nontrivial fashion with particle size, trap stiffness, and sampling frequency. Finally, we present a method to evaluate calibration errors caused by memory effects for typical optical trapping experiments.