Deposits of drying drops of a nanotube suspension

We have made observations of the pattern formed by deposits of an evaporating sessile drop of a carbon nanotube suspension. The nanotubes are chains of carbon molecules, 2 nm diameter and approximately 15 micrometers long.The suspension concentration is 0.25 mg/ml and initially, the drops volume is 2 $\mu$l. Nanotubes are transported by the flow generated by evaporation at the surface of the drops and the resulting patterns are the result of the drag of the filaments by the fluid motion. The pattern observed is composed of a circular band with several (order ten) spots with higher concentration of nanotubes. Also, the inner rim of the band displays a higher concentration of nanotubes. In contrast to similar observations where the suspensions are prepared with microspheres, no ring formation at the outer edge of the initial footprint of the drop (coffee effect) is clearly identified. Our observations are interpreted in terms of existing theories.