Credit: © 2008 APS

Charged particles dispersed in a fluid move under the action of an external electric field. An understanding of this electrophoretic behaviour in rod-like semiflexible particles is important in the understanding of biomolecules, such as short DNA fragments, and colloidal rods, such as micelles. However, despite the long history of electrophoresis, reliable imaging methods for this behaviour are still required. Now, Martin G. L. van den Heuvel and colleagues1 at Delft University of Technology and the Universita degli Studi di Milano report using fluorescence microscopy to monitor the orientation and shape of microtubules — which act as a model system for semiflexible rods — in an electric field.

The researchers used microtubules of charged, cylindrical biopolymers with diameters of 25 nm and lengths of several micrometres, and their electrophoretic motion was imaged inside microfluidic channels. It was found that the microtubules could not only orient parallel to the electric field, as would be expected from a polarization mechanism, but also perpendicular to it. Furthermore, the average microtubule orientation exhibited a scaling behaviour that suggests a balance between driving and elastic deformation forces. Surprisingly, about 10% of the microtubules could also be bent into a U-shape by the electric field.

The authors expect that this interaction between electrohydrodynamics and electric polarization will be useful for a detailed understanding of the eletrokinetics of rods.