In situ monitoring of the microstructure of detergent drops during drying using a rapid nuclear magnetic resonance diffusion measurement

The application of a nuclear magnetic resonance (NMR) diffusion measurement suitable for monitoring, in situ, the evolution of microstructure in commercial surfactant-based detergent drops as a response to drying is demonstrated for the first time. We utilise pulsed field gradient (PFG) NMR diffusion techniques to observe variations in self-diffusion coefficient, surface-to-volume ratio and characteristic pore size by probing the water content of the drops. Previously, we quantified the evolution of microstructure in vials of detergent paste using conventional PFG techniques. Now we apply a rapid PFG technique to enable this measurement during the relatively fast in situ drying of a drop with spherical geometry, relevant to spray drying processes. A finer structure is seen to form in the drops compared to the paste in the vials.