In situ scanning electron microscopy observations of filler material transport in branched carbon microtubes by Joule heating.

: Y-branched or side-by-side-branched carbon microtubes with metal filler material were fabricated, and material transport in the branched microtubes with Joule heating was investigated using in situ scanning electron microscopy with micro-electrode probes. When a voltage and electric current were applied, the material enclosed in the microtubes moved from its original position. The movement was not related to the direction of the electric current; therefore, it is concluded that the movement was not due to electromigration, but rather a temperature gradient, volume expansion and increased vapor pressure by Joule heating. In Y-branched microtubes, a part of the metal filler material moved from one branch to another branch, which would be useful for microfluidic flow switching. A cylindrical filler material was also observed to be expelled from a branch while its shape was maintained, and this phenomenon is presumably caused by vaporization-induced high pressure and could find application in micro-mechanical manipulators such as punching needles. In side-by-side-branched carbon microtubes, Joule heating caused thermal volume expansion to fill the spaces in the branches that were initially empty. The microtubes then reverted to a state almost identical to the initial state with empty spaces when the electric current was turned off. These results suggest that thermal volume expansion could be employed for flow switching.