In-situ characterization of microgel monolayers: Controlling isostructural phase transitions for homogeneous crystal...

The self-assembly of microgels at fluid interfaces and transfer to solid substrates has proven valuable in fields like photonics, plasmonics, and nanofabrication. However, this process is constrained by the isostructural phase transition (IPT) that occurs under sufficiently high compression, disrupting the monolayer order. Understanding the mechanisms driving IPT is crucial to extend their applicability to a wider range of interparticle distances. We tackle this problem by studying the monolayer conformation via in-situ microscopy at the interface. We monitored the microgel monolayer throughout the different stages of the deposition onto a solid substrate. We found that neither the compression at the interface nor the capillary forces arising from the receding meniscus during the deposition triggered the IPT. In fact, the still wet deposited monolayers do not exhibit IPT regardless of the compression of the monolayer. Instead, the IPT occurs during the drying of the wet deposited monolayers, particularly when the capillary force overcomes the adhesion force. Additionally, we found a new mechanism to modulate the interparticle distance by light-induced Marangoni forces. Instead, IPT arises from capillary forces generated during the drying of the water film after the monolayer is transferred. We propose a theoretical model to estimate the adhesion force between the microgels and the substrate based on the compression curve of the monolayer. Furthermore, we suggest a novel method combining a Langmuir-Schaefer deposition with supercritical drying to fully prevent the IPT, resulting also in a new tool to study an otherwise inaccessible regime with highly compressed monolayers. Our findings advance the understanding of soft colloidal self-assembly at fluid interfaces and expand their applications, enabling the creation of larger substrates with highly ordered self-assembled microgel monolayers with tunable interparticle distance.