Optical Tweezers

Optical Tweezers and Polymer Force Extension

Pull on a DNA-like tether with an optical trap and compare the force-extension curves of two polymers with different persistence lengths. The bead displacement inside the trap reports force, while the polymer extension curve reports the molecule mechanics.

Contour length L_c in nm 1000

Polymer A persistence length in nm 20

Polymer B persistence length in nm 50

Maximum force in pN 20

Force ramp in pN per second 2.0

Current force

0.00 pN

Polymer A extension

0.0 nm

Polymer B extension

0.0 nm

Trap displacement Δx with k_trap = 0.08 pN per nm

0.0 nm

Experimental Setup and Spring Intuition

Force Extension Curves for Two Polymers

Main ideas The optical trap reads out force from the bead displacement. The polymer itself is an entropic spring, so its extension grows nonlinearly with force. Here the two polymers differ in persistence length. A larger persistence length means a less floppy polymer, which extends more at a given force.