Abstract
Continuous convective-sedimentation assembly (CCSA) is a deposition method that constantly supplies the coating suspension to the meniscus behind the coating knife by inline injection, allowing for steady-state deposition of ordered colloids (which may include particles or cells or live cell-particle blends) by water evaporation. The constant inflow of suspended particles available for transport to the drying front yields colloidal arrays with significantly larger surface areas than previously described and thus expands the ability of convective assembly to deposit monolayers or very thin films of multiple sizes of particles on large surfaces. Using sulfated polystyrene microspheres as a model system, this study shows how tunable process parameters, namely particle concentration, fluid sonication, and fluid density, influence coating homogeneity when the meniscus is continuously supplied. Fluid density and fluid flow-path sonication affect particle sedimentation and distribution. Coating microstructure, analyzed in terms of void space, does not vary significantly with relative humidity or suspended particle concentration. This study evaluated two configurations of the continuous convective assembly method in terms of ability to control coating microstructure by varying the number of suspended polymer particles available for transport to the coating drying front through variations in the meniscus volume.
Highlights
The results demonstrate how continuous convective assembly can be optimized for continuous deposition of polymer particle coatings with larger surface areas than previously described
We investigated how two variations of the convective-sedimentation assembly (CCSA) apparatus perform in generating larger surface area polymer particle arrays between 1 and 3 particle diameters thick by continuously delivering particles to the drying front for coating assembly
Changing the suspension delivery mode yields disparate meniscus volumes and uneven particle delivery to the drying front, which alters the coating microstructure by varying the total number of particles available for deposition. For any of these convective assembly deposition modes and the same initial meniscus volume, coating quality was found to be independent of suspension delivery rate and meniscus volume during coating fabrication, at least for the coating apparatus employed in this study
Summary
Convective assembly is a commonly employed colloidal structuring technique for depositing self-assembled, ordered, thin crystalline coatings (polymer particle arrays) over large areas [1,2,3]Techniques for ordering and assembling colloidal particles into closely packed arrays by solvent evaporation have been thoroughly investigated, including ring formation in drying droplets [4,5,6,7] and colloidal coating formation in thin wetting films [8,9,10,11] some of which in water borne latex systems contain reactive live cells (bacteria, yeast, cyanobacteria or algae) for future biotechnology applications [9,10]. Reactive microorganisms behave as charged particles in aqueous deposition systems and we have found that net charge leading to repulsion between particles or between particles and cells is an important factor in coating assembly [12] These findings have resulted in emerging methods to generate colloidal arrays with varying thicknesses, particle sizes, and types ranging from charged latex particles [8,11,12,13,14,15,16] to live cells [9] to composite charged particle plus live cell mixtures [9,12,17]. A particle mass balance relates the coating growth rate, vc, to the fluid evaporation rate and particle volume fraction: vc je l h 1 1
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