Enhanced light scattering in coatings templated with interfacially stabilized colloids
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Controlling inter-pore distances enables tuning the color or whiteness of microvoid coatings. While pore spacings have been modified in limited area inverse opal films, little work has occurred studying the feasibility of controlling pore spacings and thus the appearance of scalable, spray-on, microvoid inorganic coatings. In this work we investigated using interfacially stabilized colloidal templates to increase pore spacing and thus enhance Mie scattering in porous silica films. Coatings were fabricated by forming monodisperse colloids with or without a polyvinylpyrrolidone (PVP) interfacial stabilizing layer, dispersing them in a silica precursor solution, and spraying this suspension on a substrate. The films were cured and the colloids subsequently solution extracted at mild temperatures to create porous surfaces. Coatings made with PVP coated colloids had thicker pore walls and scattered light approximately 3× more efficiently than coatings made with bare colloids. Furthermore, a viewing angle dependent color shift was observed in the PVP colloid templated coatings. Side illumination of the samples with white light causes an orange appearance under angles of specular reflection, while a light blue appearance is observed out of these angles because of strong Mie scattering of short-wavelength radiation in both situations. Lastly, modeling based on Mie scattering confirms that it is the dominant optical effect in these coatings and explains the appearance of these coatings. The approach of using interfacially stabilized colloids to improve pore separation applies to many porous films and should be considered when increased light scattering is desired.
KeywordsMie scattering Spray coating Porous silica Colloidal templating
The authors thank Robert E. Doty for collecting SEM images and John J. Vajo and William B. Carter for suggestions on improving this manuscript.
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