Individual single-walled carbon nanotubes (SWNTs) in aqueous suspension are visualized directly by fluorescence video microscopy. The fluorescent tagging is simple, biocompatible, and does not modify the SWNTs. The dynamics of individual SWNTs in water are observed and quantified for the first time. We measure the confined rotational diffusion coefficient and find it in reasonable agreement with predictions based on confined diffusion of dilute Brownian rods. We determine the critical concentration at which SWNTs in suspensions start interacting. By analyzing the fluctuating shape of SWNTs in the 3 to 5 microm range, we determine that their persistence length ranges between 32 and 174 microm, in agreement with theoretical estimates; thus, commonly available SWNTs in liquids can be considered as rigid Brownian rods in the absence of imposed external fields or self-attractive forces.

Single-walled carbon nanotubes (SWNTs) are currently the focus of extensive interdisciplinary studies because of their unique physical and chemical properties and potential electronic applications, for example, in making sensors and fieldemission devices.[1] Processing of SWNT-based materials into engineered macroscopic materials is still in its infancy; the most successful methods so far have been based on adapting techniques that had been developed in other areas of material science such as colloids and polymers. Recent successes include preparing fibers and ribbons of SWNTs; [2] films of pure SWNTs, [3,4] polymers doped with SWNTs, [5,6] and growth in situ of SWNT arrays. [7] Evaporation of drops on substrates has been used for patterned deposition of solutes onto non-porous substrates, such as in DNA microarrays, [8] nanolithography, [9] protein crystallization, [10] and stretching DNA for hybridization studies. [11,12] Shimoda et al. [13] prepared continuous selfassembled films of SWNT bundles on glass near a receding contact line by solvent evaporation. The moving contact line of a drying drop could be similarly used to form aligned patterns of SWNTs on substrates for making films or for nanofabrication. Drops of a solution on a substrate follow one of two drying mechanisms: either the drop maintains a constant contact angle by de-pinning the contact line (e.g., water on non-wetting substrates [14] ), or the contact line gets pinned and the drop maintains a fixed contact area (e.g., colloidal dispersions [15] ).Deegan and co-workers [15][16][17] have studied the drying of drops of colloidal dispersions and found that the particles deposit in a ring at the periphery of the drop due to capillary flow in which the pinned contact line causes the solvent to flow towards the edge. Recent investigations have also shown the formation of a skin or crust at the free surface of drops of polymers and colloidal suspensions. [18,19] Pauchard and Allain [19] found that the crust may collapse and evolve into different shapes as the surface area remains constant while the drop volume decreases due to solvent evaporation. "Crusting" on the surface of spin-cast films [20,21] is a well-known phenomenon.De Gennes [22] suggested a transport model for crust formation in spin-cast films. Because the glass-transition or gelation temperature of a pure polymer/colloid is higher than that in solution, [19] at any temperature below the glass transition there is a critical particle concentration at which the system transitions from fluid to glassy or gel-like. Evaporation of solvent from the free surface leads to a local increase in concentration of the polymer/suspension at the free surface, and a very thin glassy or gelled crust is formed at the free surface. Here, we investigated drying of a sessile drop of individually suspended SWNTs in an aqueous solution of F68 Pluronic. We found that, instead of assembling on the substrate, the SWNTs selfassemble into a crust at the free surface. This entangled meshlike crust was characterized b...

Individual DNA molecules in an ultradilute solution were observed with a fluorescence microscope as they flow between a scaled-down rotating roll and a stationary glass knife. The roll picks up a thin layer of liquid from a pool and drags it to the knife, establishing a bead delineated by two menisci. At low roll speed the flow is premetered and there is a large recirculation. The DNA experiences nearly rectilinear shear flow at the minimum gap position where there is a zero velocity surface. We report the mean and the distribution of fractional extension of DNA molecules and show that the mean fractional extension asymptotes to 0.5, in agreement with the results of Smith et al. ͓D. E. Smith et al., Science 283, 1724 ͑1999͔͒. Interestingly, no polymer distortion is observed at the two menisci. At high roll speed, capillarity is not strong enough to drive backflow; the big recirculation under the coverslip breaks into two smaller recirculations and two separation surfaces arise upstream and downstream of the location of the minimum gap. At the upstream separation surface, most DNA molecules are extended parallel to the knife as they traverse the field of view. We report the distribution of DNA extension and shape in this flow region. Slow, nodular recirculations are present under the upstream and downstream free surfaces. Notably, most DNA molecules stretch axially as they move in these slow recirculating regions.

SynopsisThe dynamics of dilute solutions of DNA flowing in a scaled down roll-knife coating flow are investigated on multiple scales. The flow is generated between a rotating roll and a stationary glass knife, and extension of fluorescently stained DNA molecules is measured at the minimum gap at low roll speeds. The macroscopic flow is computed by solving the continuum equations of motion with the finite element method; the microscale behavior of DNA molecules is predicted by Brownian dynamics combined with successive fine-graining. The simulations predict that the DNA should stretch almost to full extension near the roll surface in the region of minimum gap; this does not agree with experimental measurements. The assumption that the flow is nearly homogeneous on the length scale of the polymer molecules, commonly used in processing flows as well as Brownian dynamics simulations of simple flows, fails near free surfaces, and is the likely cause of the discrepancy. Evidence from the literature suggests that similar nonlocal effects may be present in coating and ink-jet printing flows of high molecular weight polymer solutions.