A method is provided for forming structures upon a substrate. The method comprises: depositing fluid onto a substrate so as to define a wetted region, the fluid containing electrically polahzable nanoparticles; applying an alternating electric field to the fluid on the region, using a first electrode and a second electrode, so that a plurality of the nanoparticles are assembled to form an elongate structure extending from the first electrode towards the second electrode; and removing the fluid such that the elongate structure remains upon the substrate.