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

초록

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, similar to 33 degrees C), producing coffee-ring patterns at 25 degrees C and more uniform deposits at 40 degrees 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 (X EtOH) of 0.2, co-non-solvency inhibits the LCST transition, leading to aggregation and irregular deposits at both temperatures. In contrast, at X EtOH = 0.4, drying becomes strongly temperature-dependent: at 25 degrees C, capillary flow after particle redistribution by vortical motions induces coffee-ring formation, whereas at 40 degrees 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.

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