- The spreading of a liquid drop on a solid surface is a simple everyday phenomenon, yet much of the process is complex and remains under investigation. Hydrodynamic analysis of the spreading leads to a non-physical singularity at the contact line, or the triple point where air, solid, and liquid meet. In 1919, Sir William Bates Hardy discovered that the edge of a spreading drop emits a microscopically thin layer of fluid, invisible to the naked eye. The existence of this "precursor film" relieves the singularity issue. In the mid-1980s, Pierre-Gilles de Gennes and coworkers developed a theoretical model for the precursor film, considering intermolecular forces close to the contact line. Since then, physicists have striven to capture experimental evidence of its behavior and characteristics. However, due to the film's nano-scale features, it has been a challenge to overcome the limitations of many detection techniques. Researchers have recently measured the dynamic evolution of the precursor film using fluorescence microscopy. This work is the first to provide experimental support for the theory governing the precursor film's behavior with respect to time and space.
