Waterborne colloidal films are used in a wide range of everyday applications, such as adhesives, inks, pharmaceutical coatings, agricultural treatments, paints and varnishes. For some applications, it is advantageous for a film’s top surface to have a structure and properties that differ from the underlayers. Properties such as wetting, hardness, adhesion, and gloss can be tailored. We have created self-stratifying layers of particles during the film formation of mixtures of colloids with two different particle sizes. When the evaporation rate of the continuous water phase is sufficiently high in relation to the self-diffusion of the colloids, as described by a Péclet number (Pe), the small particles accumulate in a layer near the surface of the film. In one example, we have stratified gold nanoparticles in a polymer colloid film to create a nanogrid structure that imparts photonic effects.<br/><br/>According to our model of diffusiophoresis and experiments using Raman mapping, self-stratification is prevented when the volume fraction of the small particles in the original suspension is greater than 0.2. We have recently discovered that the stratification of small colloids on top of large is also prevented when the viscosity of the continuous aqueous phase is increase with a thickener, and hence Pe is too high. We use our diffusiophoretic model to show that when the width of the gradient in concentration of small colloids is too narrow, stratification is suppressed. We have investigated via experiments, guided by models, how the relative particle sizes, viscosity of the continuous phase, drying conditions, and wet film thickness affect the stratification. With greater understanding of the process, we can now design nanomaterials with targeted surface structures and properties.

Self-stratification of binary colloids in waterborne films for designed surface properties

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