Bubbling and foaming assisted clearing of mucin plugs in microfluidic Y-junctions.

Microfluidic Y-junctions were used to study mechanical mechanisms involved in pig gastric mucin (PGM) plug removal from within one of two bifurcation branches with 2-phase air and liquid flow. Water control experiments showed moderate plug removal due to shear from vortex formation in the blockage branch and suggest a PGM yield stress of 35Pa, as determined by computational fluid dynamics. Addition of hexadecyltrimethylammonium bromide (CTAB) surfactant improved clearing effectiveness due to bubbling in 1mm diameter channels and foaming in 500µm diameter channels. Plug removal mechanisms have been identified as vortex shear, bubble scouring, and then foam scouring as air flow rate is increased with constant liquid flow. The onset of bubbling and foaming is attributed to a flow regime transition from slug to slug-annular. Flow rates explored for 1mm channels are typically experienced by bronchioles in generations 8 and 9 of lungs. Results have implications on treatment of cystic fibrosis and other lung diseases.

Temperature and gate voltage dependent Raman spectra of single-layer graphene.

Raman spectra of electrostatically gated single-layer graphene are measured from room temperature to 560 K to sort out doping and thermally induced effects. Repeated heating cycles under Ar led to convergent first-order temperature coefficients of the G-band (χ(G) = -0.03 cm(-1)/K) and the 2D-band (χ(2D) = -0.05 cm(-1)/K) frequencies, which are independent of doping level as long as the Fermi level does not shift with temperature. While the intrinsic behavior may be different (e.g., χ(G) ∼ -0.02 cm(-1)/K near room temperature), these values appear more appropriate in describing responses of most graphene samples on SiO(2) substrates. The more negative χ(G) value than theoretical expectations may be explained by interactions with the substrate reducing the lattice thermal expansion contribution to the temperature dependence of G-band frequency. Enhanced interactions with the substrate may also be responsible for zero-charge, room-temperature G-band line width increase and 2D-band frequency downshift.

Nonuniform compressive strain in horizontally aligned single-walled carbon nanotubes grown on single crystal quartz.

Interactions with the substrate that allow near perfect horizontal alignment in combination with large difference in the coefficient of thermal expansion are shown to lead to uniaxial compressive strain in as-grown single-walled carbon nanotubes on single crystal quartz. Temperature dependence of Raman G-band spectra along the length of individual nanotubes reveals that the compressive strain is nonuniform and can be larger than 1% locally at room temperature. A response of 27 cm(-1) upshift per % compressive strain is estimated for the G-band longitudinal optical phonon mode of semiconducting nanotubes. Comparison of Raman and atomic force microscope images suggests that the nonuniformity of the compression arises from the surface roughness induced by polishing. Effects of device fabrication steps on the nonuniform strain are also examined and implications on electrical performance are discussed.

Performance and photovoltaic response of polymer-doped carbon nanotube p-n diodes.

Two terminal abrupt junction diodes are fabricated from single semiconducting carbon nanotubes with simple photopatterned polymer layers defining air-stable p- and n-regions. These intratube diodes show nearly ideal behavior with relatively low series resistance and no sign of Zener breakdown at room temperature. Spatial doping profiles measured by micro-Raman spectroscopy and selective electrochemical gating of the n-region indicate that diode performance depends strongly on relative doping levels. A short circuit current of 1.4 nA with an open circuit voltage of 205 mV are measured when illuminated to saturation.