Force-clamp feedback for optical tweezers

A ~16 um long DNA-molecule is tethered between optically trapped plastic beads. Beads are held by a stationary trap (lower blue cross-hairs) and a steerable trap (upper green cross-hairs). The graphs on the right show the measured force (red) and the force set-point (blue) (top), the distance between the traps (middle), and the force-extension curve with a green cross indicating the current value (bottom). A force-extension curve of the tether is first obtained manually, before force-clamp feedback is switched on at t=24 s. The force set-point is first at 5.5 pN, then increased to 11.4 pN at 30 s and finally increased to 17.4 pN at 35 s. Scale-bar 5 µm. Anders Wallin et al. University of Helsinki, Finland, 2011.