The stretching of individual polymers in a spatially homogeneous velocity gradient was observed through use of fluorescently labeled DNA molecules. The probability distribution of molecular extension was determined as a function of time and strain rate. Although some molecules reached steady state, the average extension did not, even after a approximately 300-fold distortion of the underlying fluid element. At the highest strain rates, distinct conformational shapes with differing dynamics were observed. There was considerable variation in the onset of stretching, and chains with a dumbbell shape stretched more rapidly than folded ones. As the strain rate was increased, chains did not deform with the fluid element. The steady-state extension can be described by a model consisting of two beads connected by a spring representing the entropic elasticity of a worm-like chain, but the average dynamics cannot.

Single molecules of DNA, visualized in video fluorescence microscopy, were stretched to full extension in a flow, and their relaxation was measured when the flow stopped. The molecules, attached by one end to a 1-micrometer bead, were manipulated in an aqueous solution with optical tweezers. Inverse Laplace transformations of the relaxation data yielded spectra of decaying exponentials with distinct peaks, and the longest time component (tau) increased with length (L) as tau approximately L 1.68 +/- 0.10. A rescaling analysis showed that most of the relaxation curves had a universal shape and their characteristic times (lambda t) increased as lambda t approximately L 1.65 +/- 0.13. These results are in qualitative agreement with the theoretical prediction of dynamical scaling.

The stretching of single, tethered DNA molecules by a flow was directly visualized with fluorescence microscopy. Molecules ranging in length (L) from 22 to 84 micrometers were held stationary against the flow by the optical trapping of a latex microsphere attached to one end. The fractional extension x/L is a universal function of eta vL 0.54 +/- 0.05, where eta and v are the viscosity and velocity of the flow, respectively. This relation shows that the DNA is not "free-draining" (that is, hydrodynamic coupling within the chain is not negligible) even near full extension (approximately 80 percent). This function has the same form over a long range as the fractional extension versus force applied at the ends of a worm-like chain. For small deformations (< 30 percent of full extension), the extension increases with velocity as x approximately v0.70 +/- 0.08. The relative size of fluctuations in extension decreases as sigma x/x approximately equal to 0.42 exp (-4.9 x/L). Video images of the fluctuating chain have a cone-like envelope and show a sharp increase in intensity at the free end.

Tube-like motion of a single, fluorescently labeled molecule of DNA in an entangled solution of unlabeled lambda-phage DNA molecules was observed by fluorescence microscopy. One end of a 16- to 100-micrometer-long DNA was attached to a 1-micrometer bead and moved with optical tweezers. The molecule was stretched into various conformations having bends, kinks, and loops. As the polymer relaxed, it closely followed a path defined by its initial contour. The relaxation time of the disturbance caused by the bead was roughly 1 second, whereas tube-like motion in small loops persisted for longer than 2 minutes. Tube deformation, constraint release, and excess chain segment diffusion were also observed. These observations provide direct evidence for several key assumptions in the reptation model developed by de Gennes, Edwards, and Doi.

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