Viscosity-controlled particle distribution in convective drying of polydisperse colloids

In convective drying of polydisperse colloids, capillary-driven liquid flow carries small particles toward the evaporating interface, often resulting in non-uniform particle distributions. Because this flow is opposed by viscous resistance, colloidal viscosity plays a key role in determining particle transport and deposition patterns. This study examines how liquid viscosity governs particle distribution during convective drying. Drying experiments were performed on binary suspensions composed of micrometer- and nanometer-sized particles dispersed in aqueous polymer solutions with tunable viscosity. Increasing viscosity shortened the constant drying rate period, during which capillary-driven flow actively redistributes particles, thereby limiting particle migration and producing more uniform deposition. A theoretical model based on the balance between capillary pressure and viscous resistance captures the observed viscosity-dependent drying behavior and agrees well with the experimental results. These findings demonstrate that controlling viscosity provides an effective route to achieve uniform particle organization in practical coating and drying processes.