Physics of Spin Casting Dilute Solutions

MRSEC Seminar series presents Prof. Hans Riegler (Technische Universität Berlin). The spin casting of dilute (ideal) binary mixtures of non-volatile solutes in volatile solvents is analyzed. It is the prototype for widely used evaporation-controlled processes to cast thin films and/or to deposit specifically structured (sub)monolayers (¿evaporation-induced self-assembly¿).As basic process first the analytical description of the thinning of a volatile liquid film simultaneously subject to spinning and evaporation is presented. It reveals that the duration of a spin casting process is linked to the process parameters via power laws.Then to address the solute aggregation and deposition, a diffusion-advection model is used to reveal and to analyze the equations that govern the spatio-temporal evolution of the internal film composition i.e., the solute concentration enrichment, its gradient, and its time evolution. This evolution is of key importance because it determines the nucleation and growth behavior of the solute aggregates and thus of the structure of the finally deposited film. It is found that this evolution is related to the process parameters via power laws. The physics behind these power laws is uncovered and discussed. The findings offer palpable guidelines to better understand and control the spin casting of ideal solutions but they are also useful and offer new quantitative insights into the spin casting of non-ideal solutions.