Dependence on sphere composition

Figure 7: Labyrinthine electrohydrodynamic convection of 0.5  $\unit{\mu m}$ diameter PS spheres in a sample with $H = 390~\ensuremath{\unit{\mu m}}\xspace$ . (a) $-6.0~\unit{V}$ : labyrinth in the bulk. (b) Effect of reducing the bias to $-4.0~\unit{V}$ . Scale bar indicates 1 mm.

To avoid particle deposition under negative bias conditions, we repeated these experiments with 0.5  $\unit{\mu m}$ diameter PS spheres, whose density of $1.05~\unit{g/cm^3}$ is more closely matched to that of water. These particles form labyrinthine patterns comparable to those in Fig. 2 under comparable positive biases. Figure 7(a) shows these particles also form steady-state labyrinths in the bulk at $-6.0~\unit{V}$ . A similar trend of domain coarsening with increasing driving also is observed with negative biases. These observations suggest that gravity plays no significant role in establishing the observed electrohydrodynamic convection patterns. They also confirm that the patterns are sensitive to neither the spheres' density nor their diameter.

Labyrinth coarsening appears not to be reversible. Reducing the bias to $-4.0~\unit{V}$ might be expected to select a finer-grained pattern. With the coarser pattern already established, however, the result is to coalesce the colloid-rich convection rolls into thinner domains, separated by wider margins, as shown in Fig. 7(b).