When fluid molecules are confined within the three-dimensional porous network of an aerogel—characterized by scales of only a few nanometers—their physicochemical properties (such as melting point, boiling point, vapor pressure, and phase transition kinetics) undergo radical changes; this phenomenon is known in physical chemistry as the "Confined Fluid Effect." Thanks to their ultra-high nanoporosity and extremely narrow pore size distribution, aerogels serve as the "ultimate physical laboratory" for studying phase transition behavior in nanoconfined spaces. A classic example of this is Kelvin capillary condensation occurring within the nanopores.
Under ambient macroscopic conditions, the transition from a gaseous to a liquid state requires the substance to reach its normal saturation vapor pressure. However, when gas comes into contact with the walls of aerogel nanopores—which are typically smaller than 10 nanometers—the strong van der Waals adsorption potential between the pore walls and the gas molecules triggers condensation and liquefaction within the nanopores at pressures far below the saturation vapor pressure.
Beyond capillary condensation, the solid-liquid phase transition kinetics of confined fluids within aerogel nanopores—such as freezing point depression—also exhibit anomalous behavior. Due to geometric spatial constraints imposed by the nanopore walls on molecular motion and the influence of interfacial free energy, water molecules cannot form regular macroscopic nuclei within the pores; consequently, the freezing point can drop drastically to tens of degrees below zero Celsius while the water remains in a supercooled liquid state. This profound understanding of phase transition kinetics in confined fluids not only explains the unique physical stability of aerogels in environments characterized by extreme fluctuations in heat, humidity, or cold, but also lays a solid physicochemical foundation for developing technologies such as aerogel-based phase-change thermal energy storage, high-efficiency gas separation and purification, and nanofluidic control devices.