40 50 60 70 80 $_{0s}$10s 1 0 ◦ C 0s 10s 40 50 60 70 80 0s 10s 1 5 ◦ C 0s 10s 40 50 60 70 80 0s 10s 2 0 ◦ C 0s 10s 40 50 60 70 80 0s 10s 2 2 . 5 ◦ C 0s 10s 40 50 60 70 80 0s 10s 2 5 ◦ C 0s 10s 40 50 60 70 80 0s 10s 3 5 ◦ C 0s 10s f$_{0}$ (pN) f$_{0}$ (pN) f$_{0}$ (pN) composition composition FIG. 3: (Color) ‘Dynamic phase diagram’ for 40 kbp fragments of λ -DNA with one free end, at salt concentration c =150 mM: compare the thermodynamic phase diagram of Fig 1. Color code: B-DNA (off-white), U-DNA (red), and S-DNA (blue). Here we use the ‘optical trap’ protocol described in Ref. [10] (trap stiffness 1 pN/nm) to stretch DNA at fixed temperature T at a rate of 1000 nm/s until a force f$_{0}$ is achieved. The loading rate is then set to zero, and the composition of the molecule is monitored (both molecular force and extension may vary, and force typically decreases slowly). We show molecular composition (fraction of B-DNA (off-white), U-DNA (red), and S-DNA (blue) denoted by bars on the vertical axis, where maximum height represents 40 kbp) for a molecule at specified temperature T for target force f$_{0}$ at times 0 and 10 seconds after setting the loading rate to zero. Two features are apparent. First, S-DNA is sufficiently long-lived to be seen after 10 seconds of waiting at low temperatures. Second, near and above room temperature, S-DNA is short-lived. 10