Supporting Information

Files in this Data Supplement:

Movie 1. This movie shows the beginning of an experiment from below the dish; the shadow of the source electrode is visible behind the particles. The particles originally are placed in a circular arrangement with the power supply initially turned off. After approximately 2 sec, the power is turned on and ramped to 20 kV. At that time, particles at the perimeter begin to move and line up to form the strands that reach toward the boundary electrode that lines the circular dish of diameter 12 cm; the formation of strands is stage I described in the text. The number of particles for this experiment was 1,045. The movie is in real time.

Movie 2. This movie, played in real time, shows the particles entering stage II; it takes place after » 2 min from the first movie. At the beginning of the movie, several strands have grown significantly toward the boundary, and one strand, toward the bottom, is ready to make contact with it. When it does reach the boundary, stage II initiates, and the particles connect to one another and are electrically connected to the boundary. At that point, the remaining strands quit reaching for the boundary and instead join the network as long termini.

Movie 3. Now that most particles are connected to one another, they expand into the remaining space as a network until they reach the steady-state network; this is stage III. The movie is shown at 30´ speed and would otherwise have a duration of » 10 min in real time. The expansion is rapid at first but slows as time proceeds. Notice that even in the steady-state the network wiggles quite a bit but remains topologically intact.