Tunable coffee-ring patterns of sessile suspension droplets through silica particle encapsulation with thermo-responsive block copolymers

초록

Silica (Si) particles coated with poly(methyl methacrylate-b-N-isopropylacrylamide) (PMMA-b-PNIPAM) block copolymers (Si-PM-PN particles) were engineered to mitigate the coffee-ring effect (CRE) frequently encountered during the drying process of particulate suspensions. Leveraging the thermo-responsive transition between the hydrophilic and hydrophobic states of PNIPAM, with its lower critical solution temperature (LCST) around 32 degrees C, enabled precise adjustment of the CRE. Through microrheological analysis using multi-speckle diffusing wave spectroscopy and observation under an optical microscope, the dynamic behaviors of Si-PM-PN particles in sessile suspension droplets during drying were scrutinized both below and above the LCST. Above the LCST, the Si-PM-PN particles exhibited hydrophobic characteristics, with some migrating toward the center along the droplet interface, resulting in a more uniform particle distribution post-drying. Conversely, below the LCST, the homogeneously dispersed Si-PM-PN particles displayed rapid Brownian motion, contributing to a pronounced CRE. This approach, involving the surface treatment of particles with thermo-responsive polymers, effectively altered the droplet pattern by modulating particle interactions with the air-water interface.

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