A variable transmission medium comprises a fluid and a plurality of nanoparticles dispersed in the fluid, wherein addition of acid to the fluid causes the nanoparticles to flocculate and form aggregates of particles that scatter light. The nanoparticles may comprise at least one metal oxide, such as titanium dioxide, zinc oxide or zirconium dioxide. The fluid may have a dielectric constant less than about 10. The medium may be used in, for example, privacy glass for a conference room.