Dynamics of Crystal Structure Formation in Spin-Coated Colloidal Films

Abstract

The spin-coating of colloidal suspensions is an inherently nonequilibrium process that gives rise to highly reproducible, but polycrystalline, films with different symmetries depending on experimental parameters. In this study, we explore the transient dynamics of evaporative colloid spin-coating for the first time, via a combination of high-speed imaging, atomic force microscopy, static photography, and scanning electron microscopy. As the wet colloidal film thins and dries, we observe several symmetry transitions, while at the same time remarkably, the thinning rate (in nondimensional time units) collapses to one universal curve for all rotation rates. We correlate static and dynamic measures of crossovers in ordering regimes, and obtain an estimate of the evaporation rate in the late stage of drying. We conclude that the thinning dynamics controls the local volume fraction and stress profiles, which in turn drives the structural transitions. SECTION Macromolecules, Soft Matter C olloidal self-assembly is a facile and promising method for making photonic crystals, 1 but the control of defects is a challenge. Nonequilibrium approaches to colloidal self-assembly are likely crucial to the making of large-area colloidal crystals.2,3 Colloid spin-coating has recently emerged as a highly robust and reproducible nonequilibrium method to make multilayer colloidal crystalline films. [4][5][6] However, in spite of its robustness, the spin-coating route to colloidal crystals is fraught with challenges. The highly uniform structural colors exhibited by these films have been shown to arise from a polycrystal where different crystallites exhibit longrange orientational order. 7 In order to develop strategies to produce crystals with a greater degree of translational order, a deeper understanding of the dynamical mechanisms of structure formation is required.While the dynamics of spin-coating has been studied extensively and quantitatively in simple one-component fluids 8,9 as well as in polymer solutions, 10,11 the study of the dynamics of colloid spin-coating has been limited to numerical studies of thinning rates.12 A recent work demonstrates that spin-coating flows (in the absence of evaporation) control the local stress profiles, and drive crystallization when the Peclet number exceeds a critical value. 6 Two questions that have remained unaddressed;how the dynamics interlinks with the emergence of different symmetries, and the role played by evaporation;are the focus of the experiments reported here.We begin with our observations of the transient dynamics. Using high-speed imaging and the lighting configuration A (Figure 6), we are sensitive to changes in long-range orientational order via the appearance of symmetries in the optical reflections. In all experiments (at varying rotation speeds) common dynamical features are observed in the fluid phase. While the results presented here pertain to experiments using methyl ethyl ketone (MEK) as solvent, similar results were found with acetone as solvent. This phase...

Micromagnetic modeling of experimental hysteresis loops for heterogeneous electrodeposited cobalt films

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Weak ac field-induced patterns in vertical deposition of colloids at various evaporation rates

“…The main purpose is to bring these affected colloidal systems into nonequilibrium transitions in a controlled way [1][2][3][4][5][6][7][8][9]. These transitions lead to macroscopic patterns, and are more complex in nature as their control parameters are influenced by external fields [10,11].…”

Impact of electric fields on the speed of contact line in vertical deposition of diluted colloids

“…We believe that this understanding is the key to make such applications possible. Recently, methods like Laser Confocal Microscopy and Atomic Force Microscopy are employed at colloidal length scales, allowing precise characterization of the structures [2][3][4]. Visualization at particle level can be achieved from these methods which increase the interest of the community and the number of contributions towards the field [5][6][7][8][9][10][11][12][13].…”

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