Figure 5: Electrophoretic origin of the photo-induced assembly mechanism.

(a) Zeta potential values of PHSA-stabilized PMMA colloids undergo a change in sign at about 20 μM added TBAB. The dotted curve is a line to guide the eye. The error bars indicate s.d. (b) When the photo-induced effect is generated in a cell with electrodes, the measured current exhibits a rapid change as a direct response of the UV light being turned on and off. This experiment was carried out with PHSA-stabilized PMMA colloids prepared at 5% volume fraction and 1,000 μM concentration of TBAB. (c–e) CLSM images corresponding to the times marked in b. Images taken perpendicular to the coverslip show that the effect of light on the particle assembly is temporally correlated with changes in current density due to the illumination. Scale bar, 15 μm. (f–i) Because the current density is produced only in the illuminated region, the photo-induced assembly and depletion effects can be achieved when complex shapes are illuminated by the light, as is possible by, for example, using the optics of the scanning microscope. (f,g) Before and after images of colloidal assembly show accumulation of particles in the illuminated region of complex shape. The colloidal solution was prepared at 5% initial volume fraction with 1,000 μM TBAB in CHB/decalin. (h,i) Before and after images of colloidal depletion show depletion of particles within the illuminated region of complex shape. The initial colloidal volume fraction was 10%, and the solvent was pure CHB/decalin. Scale bars, 20 μm.