In-situ measurement of the internal compaction of a soft material caused by permeation flow.

Hypothesis: The compaction of hydrogel films under permeation flow can be measured, in-situ, by tracking the internal displacements of their structure, thereby revealing the internal deformation profile. Additionally, monitoring the permeation flow rate and applied pressure over time enables determination of variations in the hydrogel's permeability due to flow-induced compaction. Hydrogels are soft porous materials capable of containing high amounts of water within their polymeric matrix. Flow-induced internal deformation can modify the hydrogel's permeability and selectivity, which are important attributes in separation processes, both industrial (e.g., membrane-based water purification) and natural (mucous filters in suspension feeders and intestinal lining) systems. Measuring the flow-induced compaction in thin hydrogels films can reveal the interplay between flow and permeability. However, the micro-scale internal compaction remains uncharted for due to experimental challenges.

Experiments: A technique is demonstrated for analyzing the compaction and stratification of permeable soft materials, in-situ, created by a pressure-driven permeation flow. To this end, the internal deformations within a soft material layer are calculated, based on tracking the positions of fluorescent micro-tracers that are embedded within the soft material. We showcase the capabilities of this technique by examining a hundred-micron-thick calcium-alginate cake deposited on a nanofiltration membrane, emphasizing the achieved micro-scale resolution of the local compaction measurements.

Findings: The results highlight the possibility to examine thin hydrogel films and their internal deformation produced by flow-induced stresses when varying the flow conditions. The method enables the simultaneous calculation of the soft material's permeance, as the pressure-driven flow conditions are continuously monitored. In summary, the proposed method provides a powerful tool for characterizing the behaviour of permeable soft materials under permeation conditions, with potential applications in engineering, biophysics and material science.