Battery electrode manufacturing has a drying problem. The slurry — active material, conductive additive, polymer binder in solvent — is coated onto a metal foil and dried. During drying, the solvent evaporates from the top surface, creating a capillary flow that drags dissolved binder upward. The finished electrode has too much binder at the top and not enough at the bottom, right where the binder needs to anchor the active material to the current collector.
Kumberg and collaborators built a pore-scale model that explicitly tracks this capillary transport through the microstructure. The results confirm the migration mechanism but reveal an unexpected variable: particle size. Smaller particles create a more homogeneous binder distribution. The finer pore network slows the capillary flow and distributes it more evenly, counteracting the surface-driven gradient.
The counterintuitive finding is about viscosity. Solvent viscosity has only a minor influence on binder migration, as long as no hydrophilic or hydrophobic wetting behavior emerges. Viscosity seems like it should be a dominant parameter — a more viscous solvent should slow the capillary flow and reduce migration. But in the pore-scale model, the driving force (capillary pressure) and the resistance (viscous drag) both scale with viscosity, largely canceling each other. The gradient depends on the geometry of the pore network, not the speed of flow through it.
The fix for binder migration isn't in the solvent chemistry. It's in the particle size distribution — the architecture of the pores, not the fluid filling them.