Model for water transport into powdered xanthan combining gel swelling and vapor diffusion

Water ingress into xanthan powder compressed to various packing densities has been studied using nuclear magnetic resonance stray field imaging (STRAFI). A foot is observed ahead of the main water ingress front which is attributed to vapor transport around the particles. The main development of the reported work is an analytical model which describes the coupling of vapor transport through the pore space and liquid transport through the progressively swelling gel which gradually occludes the vapor path. Good agreement between theory and experiment is found over a wide range of packing densities with the model requiring only one adjustable parameter, the water diffusivity in the gel measured in a constant polymer mass reference frame. It is suggested that the results are of considerable relevance to situations where the polymer is produced at low concentration by bacteria such as in the rhizosphere and aerial bio films.