Electroosmotic flow (EOF), a consequence of an imposed electric field onto an electrolyte solution in the tangential direction of a charged surface, has emerged as an important phenomenon in electrokinetic transport at the micro/nano‐scale. Because of their ability to efficiently pump liquids in miniaturized systems without incorporating any mechanical parts, electroosmotic methods for fluid pumping have been adopted in versatile applications—from biotechnology to environmental science. To understand the electrokinetic pumping mechanism, it is crucial to identify the role of an ionically polarized layer, the so‐called electrical double layer (EDL), which forms in the vicinity of a charged solid–liquid interface, as well as the characteristic length scale of the conducting media. Therefore, in this tutorial review, we summarize the development of EDL models from a historical point of view to elucidate the interplay and configuration of water molecules and ions in the vicinity of a solid–liquid interface. Moreover, we discuss the physico‐chemical phenomena owing to the interaction of EDL when the characteristic length of the conducting media is decreased from the micro‐scale to the nano‐scale. Finally, we highlight the pioneering studies and the most recent works on EOF devoted to both theoretical and experimental aspects. This article is protected by copyright. All rights reserved

The thermal dependence of salinity-gradient-driven energy conversion by reverse electrodialysis using a mesoporous silica thin film with pores ca. 2–3 nm in diameter was studied in a temperature range of 293–333 K. As the temperature increases, the surface charge density of mesopores increases owing to an increase in the zeta potential of the pore walls, which in turn increases the concentration of counter-ions in the electrical double layer. The ion mobility also increases with increasing temperature owing to a decrease in the liquid viscosity. As a result, the temperature increase improves the ion conductance of mesopores both in the surface-charge-governed regime at low ion concentrations and in the bulk regime at high ion concentrations. However, further increases in temperature induce bubble nucleation. In particular, in highly concentrated salt solutions, hydrophobic patches appear on the pore surfaces because of the salting-out effect and mask the surface charge. The weakened polarity in mesopores allows more co-ions to enter them, decreasing the potential difference across the film, resulting in a serious deterioration of the energy conversion efficiency. The thermal dependence of the performance characteristics of mesoporous-silica-based nanofluidic devices was also evaluated.