In Situ Monitoring of Non-Fickian Liquid–Liquid Diffusion with a High Space-Time Resolution Fiber-Integrated Plasmonic Sensor

Abstract

Liquid−liquid diffusion is a complex but important process, for which there is still no perfect theory. Many experimental methods have been proposed to measure the liquid−liquid diffusion coefficient, but none of them can accurately monitor the diffusion process with a large space-time range and high spatial resolution. Here, we propose a method of point-topoint concentration detection by scanning an optical fiber microscale plasmonic sensing probe in liquid to in situ monitor the diffusion process of water and 50% glycerol solution. By this method, we have measured the concentration space-time diagram in the time range of 24 h and spatial range of ∼14.3 mm with a resolution of 15.2 μm. From this, we have observed a non-Fickian phenomenon where the diffusion process appears as a timedependent shift of the diffusion layer superimposed on its Fickian diffusive broadening and explain it by a non-Fickian diffusion model constructed by introducing the equivalent surface tension effect into the Fickian diffusion model. It has demonstrated the practicability and superiority of the high space-time resolution probe scanning method for in situ monitoring of various natural diffusion processes and disclosing new diffusion laws.