Microstructure of Film Growth from Filtrating Mono-dispersed Particle Suspension
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The deposition mechanism that determines the microstructure of colloidal films was investigated using a constant-filtration-rate procedure. In particular, we studied the dependence of the grain size on the filtration rate and the volume fraction of the particles in liquid. We correlated the surface diffusion distance to the final grain size and identified a processing window for large-grain colloidal films. The necessary conditions for homoepitaxy of colloidal crystals were successfully identified. In particular, the surface diffusion distance of deposited particles before embedment emerged as an important parameter for film microstructural control. Sufficiently long the diffusion distances correlated well with the slow deposition rates often required for crystal growth. Low deposition rates allowed more time for depositing particles to sample surface sites for low-energy configurations. This surface diffusion model for microstructure control has universal application to other film deposition methods for mono-dispersed particles.
I. INTRODUCTION
II. EXPERIMENTAL
Recently there has been renewed interest in colloidal films because of new applications as photonic crystals for optical switches and filters as well as intermediate forms for new metals and ceramics with controlled porosity.1–3 There has been a keen interest in understanding the microstructure and processing conditions for colloidal films, since this intermediate form can determine the properties and performance of the final materials.4–6 Earlier studies of the filtration behavior of colloidal suspensions found that there was a universal transition at 1000-particle-layer thickness to a denser (64% to 74%) and more structured film.7,8 In these studies the filtration rates were usually not constant throughout the depositing film thickness, making it difficult to interpret the microstructure of these films as a result of the processing conditions. Here we present a constant-filtration-rate experiment to isolate the effects of deposition mechanics of these films. Our objectives are to identify processing conditions for desired microstructures and to provide control of properties through microstructural control and engineering.
We used aqueous suspensions of mono-dispersed polystyrene (PS) particles (Bangs Labs, Fishers, IN) with diameters of 100, 490, and 930 nm. Films of PS particles assemblies were prepared by filtering the aqueous suspension with a controlled flow rate. The experimental setup is shown in Fig. 1. The filtration system was placed on a heavy bench, while the mechanical pump was on the floor and connected to the filtration system by heavyduty rubber tubing that was clamped down. The suspension was added in such a way as to cause no convection in the solution. The regular colloidal arrays that we often obtained with slow filtration confirmed that we successfully avoided convection. The filtration rates varied from 1 to 150 ml/h. Reynold’s number (Re) differentiates between laminar flow and turbulence. Re ⳱ d/, where is the density of th
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