Abstract

Control over the assembly of colloidal particles has important applications for the design of structures at micrometer and nanometer length scales. While typically, colloidal assembly is governed by geometric exclusion and charge interactions that are fixed and cannot be adjusted with external control, the increasing need for ever more complex nanoscale structures calls for new techniques that allow precise control over the assembly process. In this talk, I will present a new technique to control the assembly of particles with temperature using the Critical Casimir effect. This effect allows direct control over particle interactions via temperature-dependent solvent fluctutions: In analogy to the confinement of fluctuations of the electromagnetic field between two conducting plates (quantum mechanical Casimir effect), the confinement of fluctuations of a critical solvent leads to an attraction between surfaces that are immersed in this solvent. This allows exquisite temperature control over the interactions of colloidal particles. We show that this temperature control allows us to “freeze” a dilute colloidal gas into a dense colloidal liquid, and a crystalline solid. By using confocal microscopy, we follow these phase transitions directly in real space, and we measure the particle pair potential directly. We find that the gas-liquid condensation is accurately described by the Van der Waals theory used for molecular gases. We also observe that these phase transitions are reversible: the colloidal solid “melts”, and the colloidal liquid “evaporates”, when the temperature is changed back to reduce the attractive force. This reversible control opens new routes for the growth and annealing of perfect nanoscale structures.