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The working principle of lateral flow assays, such as the widely used COVID-19 rapid tests, is based on the capillary-driven liquid transport of a sample fluid to a test line using porous polymeric membranes as the conductive medium. In order to predict this wicking process by simplified analytical models, it is essential to determine an effective capillary radius for the highly porous and open-pored membranes. In this work, a parametric study is performed with selected simplified structures, representing the complex microstructure of the membrane. For this, a phase-field approach with a special wetting boundary condition to describe the meniscus formation and the corresponding mean surface curvature for each structure setup is used. As a main result, an analytical correlation between geometric structure parameters and an effective capillary radius, based on a correction factor, are obtained. The resulting correlation is verified by applying image analysis methods on reconstructed computer tomography scans of two different porous polymeric membranes and thus determining the geometric structure parameters. Subsequently, a macroscale flow model that includes the correlated effective pore size and geometrical capillary radius is applied, and the results are compared with wicking experiments. Based on the derived correction function, it is shown that the analytical prediction of the wicking process in highly porous polymeric membranes is possible without the fitting of experimental wicking data. Furthermore, it can be seen that the estimated effective pore radius of the two membranes is 8 to 10 times higher than their geometric mean pore radii.

To describe the dynamics of fluid flow in Lateral Flow Assays (LFAs) and to understand the effect of geometry on the propagation speed of the fluid front, a single-phase model is developed. The model can predict wicking time for different geometries. Axisymmetric geometries with changes in their cross sections are studied to understand the wicking behavior. To validate the modeling results, imaging experiments that capture the fluid front are conducted on all geometries. In all cases, convincing agreement between modeling results and experimental data has been observed. Using data-driven information and knowledge about structure–property correlations, it is possible to control wicking processes to establish a desired velocity at a specific position in LFAs. The proposed approach serves as a basis for the creation of a design tool for application-oriented membranes.

Directional solidification is a favored process to manufacture homogeneous microstructures which lead to macroscopically unique properties for a material. The dependence of the spacing and type of the arising microstructure from the solidification velocity for constant velocities is extensively investigated. However the effect of changes in the solidification velocity on the resulting microstructure adjustment processes is still unclear. Therefore large-scale (3D+t) simulations of the ternary eutectic system Ag-Al-Cu with changing solidification velocities are conducted with a phase-field model based on the grand potential approach. To study the spatially complex rearrangement process during velocity changes in statistical representative volume elements, simulations with different velocity profiles are calculated in large-scale domains. The results show, that the evolving microstructure continuously rearranges by splitting and merging of the rods despite constant growth conditions. By increasing the velocity, the microstructure refines by splitting of the Al2Cu phase. Whereas by decreasing the velocity, the microstructure coarsens by overgrowing events of both intermetallic phases.

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