Optical tweezers can measure forces like a dynamometer. If the trap is well calibrated, the optical force as a function of the bead's position is known. When an external force is applied to the trapped bead, it is possible to measure the external force by looking at the bead move towards a new equilibrium position, where the optical force and external force compensate.
Position detection techniques:
Using this micro-dynamometer, an accurate measurement of the bead's position relative to the trap center will tell us which is the applied force. That's why position detection is such an important issue.
The implemented position detection techniques are: back-focal-plane interferometry, and video analysis with image processing techniques.
Trap calibration methods:
The trap potential is usually assumed to be harmonic (F=-k·x), and there are several precise and reliable calibration methods to characterize it. Trap stiffness (k) must be calibrated in order to know the force as a function of the bead's position. Three calibration methods have been implemented in our lab:
- Equipartition theorem: it can give a fast estimation on the trap stiffness
- Power Spectrum analysis: accurate measurements of trap stiffness can be obtained, but fast detectors are needed.
- Drag forces: changes on the beads position are measured as a function of the (known) applied viscous force by moving the sample at a controlled speed.