Drying paint: from micro-scale dynamics to mechanical instabilities

Abstract

One contribution of 13 to a theme issue 'Patterning through instabilities in complex media: theory and applications' . LG, 0000-0002-3858-7295Charged colloidal dispersions make up the basis of a broad range of industrial and commercial products, from paints to coatings and additives in cosmetics. During drying, an initially liquid dispersion of such particles is slowly concentrated into a solid, displaying a range of mechanical instabilities in response to highly variable internal pressures. Here we summarize the current appreciation of this process by pairing an advection-diffusion model of particle motion with a Poisson-Boltzmann cell model of inter-particle interactions, to predict the concentration gradients in a drying colloidal film. We then test these predictions with osmotic compression experiments on colloidal silica, and small-angle X-ray scattering experiments on silica dispersions drying in Hele-Shaw cells. Finally, we use the details of the microscopic physics at play in these dispersions to explore how two macroscopic mechanical instabilities-shear-banding and fracture-can be controlled.This article is part of the themed issue 'Patterning through instabilities in complex media: theory and applications'.

Buoyancy-driven dispersion in confined drying of liquid binary mixtures

“…More specifically, experiments exploiting drying to induce concentration gradients, similarly to the experiment shown in Fig. 1, were even recently reported to measure mutual diffusion coefficients of various complex fluids, namely copolymer solutions [1] and charged colloidal dispersions [4,16]. For the latter case, the rheological properties of the colloidal dispersions strongly evolve with the colloid (solute) concentration up to reaching the formation of colloidal glasses at a concentration below the colloid closepacking.…”

“…[11][12][13]. We will assume an initial concentration Φ i = 0.001 and radius a = 5 nm for the silica nanoparticles, leading to a diffusivity D 4.37 × 10 −11 m 2 /s according to the Stokes-Einstein relation for a temperature of 25 • C (we do not consider here enhanced values due to colloidal interactions occurring for such systems at high concentrations [4,16]). For such a very dilute silica dispersion, the solutal expansion coefficient at the reference concentration Φ i is well approximated by β s ρ s /ρ w − 1 1.2 where ρ s 2200 kg/m 3 is the density of silica, and ρ w 1000 kg/m 3 that of water.…”

“…This geometry is not only prone to a simple modeling, but also commonly used to probe mass transport and uni-directional drying in complex fluids ranging from colloidal dispersions to surfactant mixtures, see e.g. [11][12][13][14][15][16]. In the following, we consider for simplicity a dilute binary mixture, solvent + non-volatile solute at concentration Φ, for which both interdiffusion coefficient D and kinematic viscosity ν are constant.…”

Packing Polydisperse Colloids into Crystals: When Charge-Dispersity Matters

“…We used industrially produced, nanometric and highly charged silica particles, dispersed in water (Ludox TM50, Sigma-Aldrich). These were cleaned and concentrated as detailed elsewhere [8,[18][19][20]. Briefly, dispersions were filtered and dialysed against aqueous NaCl solutions of concentration c s at pH 9 ± 0.5 (by addition of NaOH).…”

Jellium and cell model for titratable colloids with continuous size distribution

“…Fig. 10 compares the experimental equations of state of the HS40 and TM50 silica dispersions 66 with the micro-ion pressure calculated with the polydisperse cell model at various bulk concentrations of monovalent salt and pH 9. The osmotic pressure is seen to increase when the ionic strength of the bulk and the mean particle radius ( R p (HS40) < R p (TM50) ) decreases, in good agreement with the polydisperse cell model.…”