Abstract
An automated data acquisition and processing system is established to measure the force applied by an optical trap to an object of unknown composition in real time. Optical traps have been in use for the past 40 years to manipulate microscopic particles, but the magnitude of applied force is often unknown and requires extensive instrument characterization. Measuring or calculating the force applied by an optical trap to nonspherical particles presents additional difficulties which are also overcome with our system. Extensive experiments and measurements using well-characterized objects were performed to verify the system performance.
Highlights
Our laboratory is interested in measuring the mechanical properties of a biological object, the primary cilium.[1]
Use of optical traps[2] to apply forces to spherical and near-spherical objects is well understood in the context of Mie scattering and its generalizations,[3,4,5,6,7,8,9] trapping cylindrical objects such as bacteria and certain virus particles has largely consisted of qualitative experiments,[10,11,12] and use of scattering models to quantify the optical trapping of slender, cylindrical objects is greatly complicated by geometry.[13,14,15,16,17]
We first track the position of a trapped object with a quadrant photodiode (QPD), calculate the mean-squared displacement (MSD) of the particle’s trajectory, and use a fitting algorithm in which the spring constant and viscous drag are free parameters to calculate the stiffness of the optical trap
Summary
Our laboratory is interested in measuring the mechanical properties of a biological object, the primary cilium.[1] Optical traps provide a localized noncontact method to apply a well-controlled force while simultaneously permitting the observation of the deformation (bending) of this organelle. In contrast to measurements that apply a known disturbance with fluid flow,[55,56] optical trapping of cilia and flagella could provide improved information about the mechanical properties of these important organelles. We have constructed a calibrated optical trap apparatus that provides near real-time measurements of the transverse spring constant without requiring knowledge of optical properties of the trapped object, suspending solvent, and trapping beam geometry. A more complicated system than a free particle, we demonstrate here a first step—measuring the trapping force applied to an E. coli bacterium, a rod-shaped bacterium 1 μm in diameter and 3 μm long
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