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Critical Casimir Interactions : New temperature control of nanoparticle assembly
Control over the assembly of colloidal particles has important
applications for the design of structures at micrometer and nanometer length scales. 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 the particle pair potential via temperature-dependent solvent fluctuations : 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 the temperature control allows us to "freeze" a dilute colloidal gas into a dense colloidal liquid, and a
crystalline solid. These phase transitions are reversible : the
colloidal solid "melts", and the liquid "evaporates", when the
temperature is changed back to reduce the attractive force. We follow these phase transitions directly in real space and time using confocal microscopy. In this talk, I will focus on the nucleation of the liquid : using three-dimensional particle scale imaging, we elucidate the structure of liquid nuclei as a function of their size and the interparticle potential. This allows us to - for the first time - connect the surface tension and amount of supercooling, important parameters of nucleation theory, directly to the size and structure of critical nuclei.