Interplay of micro-scale flow and fluid micro/nanostructure: Solutions of DNA and suspensions of single walled carbon nanotubes
Doctor of Philosophy
The dynamics of dilute solutions of DNA flowing in a scaled clown roll-knife free surface coating flow are investigated on multiple scales. The flow is generated between a rotating roll and a stationary glass knife. Extension of fluorescently stained DNA molecules is measured at the minimum gap at low roll speeds. The macroscopic flow is computed and microscopic predictions are obtained by simulating the DNA by Brownian dynamics combined with successive fine-graining (Sunthar and Ravi Prakash 2005). 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 of linear velocity across the chains fails near free surfaces and is the likely cause of the discrepancy. At high roll speed two separation surfaces arise in the coating bead. The distribution of DNA extension is measured at the separation surface upstream of minimum gap. Slow nodular recirculations are present under the upstream and downstream free surfaces; unexpectedly, DNA molecules are stretched axially in these regions. Individual single-walled carbon nanotubes (SWNTs) in aqueous suspension are visualized directly by fluorescence video-microscopy. The fluorescent tagging is simple, biocompatible, and allows observation of the dynamics of SWNTs in water. The rotational diffusion coefficient in confinement is measured and the critical concentration at which SWNTs in suspensions start interacting is determined. By analyzing the fluctuating shape of SWNTs, the persistence length of SWNTs is found to range between 32 and 174 mum, in agreement with theoretical estimates; thus, common SWNTs in liquids can be considered as rigid Brownian rods in the absence of imposed external fields. Drying microscopic drops of a suspension of individual SWNTs in aqueous solution of F68 pluronic surfactant exhibit complex dynamics. The drops dry on glass substrates forming a "crust" at the free surface. The crust is thin (∼ 100 nm) and consists of an entangled mesh of nanotubes and pluronic. The self-assembled crust envelopes the drying drop and leads to a free surface inversion as evaporation proceeds. The convective flow associated with the drying preferentially assembles the micelles into hexagonal arrangement. This technique is promising for developing thin, optically transparent, coatings and films consisting of SWNT's.
Chemical engineering; Engineering; Materials science