Substrate-Temperature-Driven Microstructural Transitions in Dried Patterns of Microgel Particles

PNIPAM microgels, a model soft colloid known to respond to external stimuli such as temperature or pH, show significantly different swelling kinetics at the fluid-fluid interface compared to bulk. When an aqueous dispersion of these microgels dries on a solid substrate, they adsorb onto the air-water interface and undergo significant flattening along the interface. This interfacial rearrangement enables the formation of ordered monolayers, offering a bottom-up route for fabricating colloidal films. Leveraging on their temperature-responsive swelling and surface activity, we investigate how poly(N-isopropylacrylamide) (PNIPAM) microgels can be used to obtain complex deposition patterns by drying sessile aqueous drops containing microgels at different concentrations on hydrophilic substrates maintained at temperatures, below and above the volume phase transition temperature (VPTT) of the microgels. The evaporation dynamics of the drop is monitored using in situ video microscopy and a contact angle goniometer, while the resulting microstructures are characterized using atomic force microscopy (AFM). Our observations reveal a rich diversity in final deposit morphologies, ranging from uniform films to classic coffee-ring and multiring patterns. These deposition outcomes depend on both the initial particle concentration and the substrate temperature. Notably, we find that the average height of the dried microgels increases with the substrate temperature. Moreover, spatial heterogeneity in the height profiles of the microgels across different regions of the deposits suggests that thermally modulated interfacial behavior of the microgels plays a critical role in directing their self-assembly during evaporation.