Fig. 1.

(A) Scanning electron microscopy (SEM) image of one hematite ellipsoid ($\alpha -{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3$). (Scale bar, $500\hspace{0.17em}\mathrm{n}\mathrm{m}$.) (B and C) Colloidal gel assembled from doping a bath of silica spheres (${\mathrm{S}\mathrm{i}\mathrm{O}}_2$) with a few hematite ellipsoids in a water solution containing ${\mathrm{H}}_2{\mathrm{O}}_2$ ($9\%$ vol). The time $t=0\hspace{0.17em}\mathrm{s}$ (C) corresponds to the application of blue light ($\lambda =450-490\hspace{0.17em}\mathrm{n}\mathrm{m}$, $P=1.04\hspace{0.17em}\mathrm{m}\mathrm{W}$) that triggers the phoretic attraction, and C shows the stationary structure observed after $t=2.7\hspace{0.17em}\mathrm{m}\mathrm{i}\mathrm{n}$. (D) Enlargement of the central region shown in the box in C. (Scale bars, $50\hspace{0.17em}\mathrm{\mu }\mathrm{m}$ for all images.) See corresponding video (Movie S1). (E) Schematic showing the assembly of ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ particles due to photoactivated dopants. (F) Formation of one colloidal cluster composed of $4-\mathrm{\mu }\mathrm{m}$ silica particles, when light is applied at $t=0\hspace{0.17em}\mathrm{s}$. (Scale bar, $10\hspace{0.17em}\mathrm{\mu }\mathrm{m}$.)