An exclusion distance controls the efflorescence pattern distribution on porous media during salt solution evaporation

Understanding salt crystallization due to evaporation of a salt solution from a porous medium is critical in applications ranging from saltwater distillation to preservation of historical monuments. Efflorescence is the crystallization of salt on the exposed surface of the porous medium. In this work, efflorescence patterns are visually observed and characterized on sintered copper particle wicks with spatially uni-modal and bimodal compositions of different particle sizes. Efflorescence is found to form earlier and spread readily over a wick made from smaller particles, owing to their lower porosity, while it is lim-ited to certain areas of the surface for wicks composed of the larger particles. A scaling analysis explains the observed efflorescence patterns in the bimodal wicks caused by particle size-induced nonuniform porosity and permeability. The non-uniformity reduces the advective flux in a high-permeability region by diverting flow towards a low-permeability region resulting from different pressure drops along the regions. This reduction in advective flux manifests as an exclusion distance surrounding a crystalliza-tion site where efflorescence is not expected to occur. The dependence of this exclusion distance on the porosity and permeability of the porous medium and the operating conditions is investigated. A large exclusion distance associated with the regions with bigger particles in the bimodal wicks explains pref-erential efflorescence over the regions with smaller particles. Our novel scaling analysis coupled with the introduction of the exclusion distance provides guidelines for designing heterogeneous porous media that can localize efflorescence.& COPY; 2023 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the efflorescence patterns on sintered copper particle wicks with different particle sizes. It is found that smaller particles lead to earlier and wider spread of efflorescence due to their lower porosity, while larger particles limit efflorescence to certain areas. A scaling analysis explains the observed patterns by considering the nonuniform porosity and permeability induced by particle size, which reduces the advective flux and results in an exclusion distance where efflorescence does not occur.