Interplay of Binary Solvent-Driven Flows and Solvent-Dependent Polymer Phase Behavior in Drying Colloidal Droplets

Abstract

The drying behavior of poly(N-isopropylacrylamide)-grafted silica (SiO2–PNIPAM) particle suspensions in binary ethanol/water (EtOH/H2O) mixtures was investigated as a function of solvent composition and temperature. In pure H2O, the particles undergo a hydrophilic-to-hydrophobic transition near the lower critical solution temperature (LCST, ∼33 °C), producing coffee-ring patterns at 25 °C and more uniform deposits at 40 °C. The addition of EtOH enhances droplet wetting and accelerates evaporation while also inducing binary-solvent-driven internal flows and altering PNIPAM phase behavior. At an ethanol mole fraction (XEtOH) of 0.2, co-non-solvency inhibits the LCST transition, leading to aggregation and irregular deposits at both temperatures. In contrast, at XEtOH = 0.4, drying becomes strongly temperature-dependent: at 25 °C, capillary flow after particle redistribution by vortical motions induces coffee-ring formation, whereas at 40 °C, rapid entry into the two-phase regime suppresses particle transport, yielding relatively uniform deposits. Collectively, these results reveal that solvent-responsive phase behavior can be exploited to tailor drying behavior in colloidal systems with polymer brushes, enabling the simultaneous control of deposition morphology, wetting behavior, and drying time.