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. 5: (Color) ‘Dynamic phase diagram’ for 40 kbp fragments of λ -DNA with one free end, at salt concentration c =50 mM. The dynamic stretching protocol is as Fig. 3: note the smaller fraction of S-DNA seen here. At room temperature, S-DNA is short-lived. 12