Predictive model for evolving density and viscosity gradients in band-forming ultracentrifugation

Band-forming experiments allow the study of a wide variety of systems by overlaying two solutions with different densities in an analytical ultracentrifuge. Despite their potential benefits over other methods, these experiments are rarely used because all available fitting software encounters systematic errors, failing to account for the evolving gradient in density and viscosity due to diffusive mixing between the two layers. We develop and experimentally validate a predictive model for the purely diffusive mixing of two solutions in a cylindrical system. Capturing the space- and time-dependent evolution of density and viscosity in band-forming experiments, the model enhances their interpretation and underscores the need for analysis software to account for these dynamic changes.