Wicking Apart: Understanding the Separation Dynamics of Binary Mixtures

When multicomponent liquid mixture wicks into a porous medium, its individual components often travel through the porous network at different rates, resulting in distinct separation of the wetting fronts. Although many practical wicking applications--including wicking fabrics designed for effective sweat removal and evaporative cooling--involve complex multicomponent liquids, the detailed dynamics governing their separation remain relatively understudied compared to those of single-component wicking. Differences in both mechanical properties (e.g., surface tension and viscosity) and chemical interactions (e.g., affinity between liquid components and the porous medium) influence the spreading rates of individual components, ultimately leading to their separation. Inspired by the composition of human sweat, we use droplets of lactic acid solution and pH-sensitive sheets to investigate this separation phenomenon. By varying the concentration and the volume of the lactic acid mixture, we characterize how the overall size of the wetted area and the relative distribution of lactic acid within the wetting region evolve.