pH-Switchable Stratification of Colloidal Coatings: Surfaces “On Demand”

Abstract

Stratified coatings are used to provide properties at a surface, such as hardness or refractive index, which are different from underlying layers. Although time-savings are offered by self-assembly approaches, there have been no methods yet reported to offer stratification on demand. Here, we demonstrate a strategy to create self-assembled stratified coatings, which can be switched to homogenous structures when required. We use blends of large and small colloidal polymer particle dispersions in water that self-assemble during drying because of an osmotic pressure gradient that leads to a downward velocity of larger particles. Our confocal fluorescent microscopy images reveal a distinct surface layer created by the small particles. When the pH of the initial dispersion is raised, the hydrophilic shells of the small particles swell substantially, and the stratification is switched off. Brownian dynamics simulations explain the suppression of stratifi-cation when the small particles are swollen as a result of reduced particle mobility, a drop in the pressure gradient, and less time available before particle jamming. Our strategy paves the way for applications in antireflection films and pro-tective coatings in which the required surface composition can be achieved on demand, simply by adjusting the pH prior to deposition

Influence of hydrodynamic interactions on stratification in drying mixtures

“…While there are currently no experiments available for stratification in polymer mixtures, colloid mixtures have been shown to stratify in experiments. [12][13][14] The extent of colloid stratification in the experiments appears weaker than predicted by models lacking hydrodynamic interactions, 12 consistent with our analysis. Larger chemical potential gradients on the larger component would be required to increase the stratification, which could be induced by, for example, larger size ratios or additional cross-interactions between the components.…”

“…17 The stratification was more pronounced for larger size ratios and faster drying speeds, although it was found to persist even at moderate drying rates. 13,15,17 Stratification has been modeled theoretically as a diffusion process 7,18 qualitatively predicted a maximum in the number of large colloids on top of the film when Pe 1 < 1 and Pe 2 > 1 (here 1 is the smaller colloid and 2 is the larger colloid). 20 For computational and theoretical convenience, many previous studies have employed a free-draining approximation for hydrodynamic interactions on the colloids or polymers, incorporating the Stokes drag on each particle but neglecting other interactions.…”

Control of Stratification in Drying Particle Suspensions via Temperature Gradients

“…Martín-Fabiani et al studied a system with the smaller particles coated with hydrophilic shells and explored the effect of changing the pH of the initial dispersion. [19] In a dispersion with low pH, α is large enough to lead to "small-ontop" stratification. When the pH is raised, α is reduced as the hydrophilic shells swell substantially, and stratification is suppressed.…”

Structure Formation in Soft‐Matter Solutions Induced by Solvent Evaporation

“…Reproduced with permission. [46] Copyright 2016, ACS. in the opposite particle-on-top structure ( Figure 4B).…”

“…Martin-Fabiani et al [46] synthesized nanoparticles with a hairy shell composed of hydrophilic poly(methyl acrylate) (PMAA) chains. By increasing the pH, the polymer chains change from a collapse state to an extended state, effectively increasing the particle size.…”