Dynamic light scattering (DLS) is a well-known technique for measuring the size of nanoparticles from the diffusion coefficient. We developed a new DLS technique with high sensitivity and used it as a detection tool in microchemical applications. We found a step-like downward change in the diffusion coefficient of nanoparticles just by starting a slight flow in the fluid channel. Because the diffusion coefficient is defined by the ratio of the force due to the chemical potential gradient to the frictional coefficient for the particle by a solvent, we propose that the frictional coefficient increased significantly. This result suggests that nanoscale friction under liquid motion differs from that under a steady state.
