The direct current bias has been reported as a necessary condition for aligned growth of carbon nanotubes. To clarify the mechanisms of nanotube alignment we performed numerical calculations of the electrical field in the collisional sheath and the resulting electric force acting on the metallic droplet on the nanotube tip. Based on the model calculation, dependences of the force on various parameters, e.g. distance between nanotubes, height of nanotubes and form of their tips, is discussed in detail. The theoretical model also predicts how this force depends on the direct current bias and the resistivity of the biased substrate. (c) 2005 Elsevier B.V. All rights reserved.