Highly ordered 2D microgel arrays: compression versus self-assembly

Monolayers of micro- and nanoparticles at fluid interfaces are a key component in a variety of applications, ranging from particle lithography to stabilizers in foams or emulsions. In addition to commonly used "hard" colloids, soft polymeric particles like microgels are attracting increasing attention due to their potential in the fabrication of tailored and responsive assemblies. In particular, regular hexagonal arrays of microgels have been previously deposited after assembly at a fluid interface. While the arrangement cannot be easily controlled after adsorption and self-assembly from the bulk phase, specific structures can be achieved by compressing an interfacial microgel monolayer spread in a Langmuir trough and by transferring it onto substrates at distinct compression states. The degree of ordering after compression surpasses the one that is reached after self-assembly from the bulk and is, in general, independent from the presence of charges and different microgel morphologies. As a consequence, by monitoring the surface pressure during compression it is possible to produce highly ordered microgel arrays where the interparticle distance can be systematically and externally controlled.