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Wetting of soluble polymers
The wetting of a soluble substrate is a situation commonly encountered in day-to-day life. For instance, the practical motivation of this study concerns the preparation of beverages using dehydrated powders, consisting of water-soluble substances such as carbohydrates. Hydrodynamical theories describing wetting onto insoluble substrates cannot account for experimental observations in the case of a liquid spreading onto a soluble substrate. Tay et al. made the hypothesis that the water content at the contact line controls the contact angle value and they showed the importance of the evaporation/condensation of the solvent during wetting. In this study, we demonstrate that other mass transfers have to be considered to improve the understanding on wetting dynamics on soluble surfaces ; thus diffusion within the polymer of the condensed water or directly from the droplet are processes that contribute to the hydration of the substrate and the modification of the contact angle of the drop. Therefore, we use the following approach to carry out this study : (i) finite elements simulations are performed in order to take into account diffusion effects in the layer and validate our theoretical arguments, (ii) spreading experiments of water onto maltodextrin thin layers are performed in order to study the wetting and the hydration ahead the contact line. This work allows us to highlight the influence of the diffusion in the polymer that makes the hydration profiles ahead the contact line more complex with a diffusion region where evaporation occurs. A wetting diagram thickness-velocity (e-U) with different regimes is established. Those regimes are validated with the experimental data. In particular, we evidence a regime where the contact angle is a function of the quantity eU. Furthermore, we evidence the effect of glass transition in the polymer on the contact angle and the hydration of the substrate. Finally, a preliminary study is performed to understand the influence of another transfer at stake in the wetting process : the dissolution of the polymer in the solvent.