Ag colloids attract each other by ionic diffusiophoresis. (A) Schematic diagram of a SiO₂-Ag microsphere attracting an inert microsphere by ionic diffusiophoresis in H₂O₂. The Ag cap reacts with H₂O₂ and releases Ag⁺ and OH⁻, which leads to a self-generated electric field that points away from the Ag colloid. A negatively charged tracer nearby reacts to this electric field by electrophoretically migrating toward the Ag colloid under the influence of an electrophoretic force F_EP. In addition, the electric field causes an electroosmotic flow from the negatively charged bottom substrate that advects the tracer via an electroosmotic draf force F_EO. F_EP dominates over F_EO, so that tracers are attracted to Ag colloids. (B) Fluorescence photograph of SiO₂-Ag in 0.01 wt% H₂O₂. Stronger green fluorescence indicates higher pH values. A pH-sensitive fluorescent dye Solvent Green 7 was used at a concentration of 100 μmol L⁻¹. Blue light of 475 nm and ~75 mW cm⁻² was used to excite fluorescence.