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'.

In directionally dried colloidal dispersions regular bands can appear behind the drying front, inclined at AE45°to the drying line. Although these features have been noted to share visual similarities with shear bands in metal, no physical mechanism for their formation has ever been suggested, until very recently. Here, through microscopy of silica and polystyrene dispersions, dried in Hele-Shaw cells, we demonstrate that the bands are indeed associated with local shear strains. We further show how the bands form, that they scale with the thickness of the drying layer, and that they are eliminated by the addition of salt to the drying dispersions. Finally, we reveal the origins of these bands in the compressive forces associated with drying, and show how they affect the optical properties (birefringence) of colloidal films and coatings. [7,8]. This abundance of patterns, recently reviewed in [9,10], suggests diverse means for the directed self-assembly of microstructured materials, if the underlying dynamics can be understood and controlled.This Letter focuses on establishing and manipulating the driving forces behind a banded (or striped) structure that is commonly found in drying dispersions, and shown in Fig. 1. Such bands were first noticed in sol-gels by Hull and Caddock [11], who studied desiccation as an analog for thermal contraction in geophysical settings. They found that regular bands always preceded fracture, and that the bands were oriented at AE45°to the cracks. Similar bands can be seen in the figures of many studies of drying colloids (e.g., [5,[12][13][14]), but passed without further discussion until the work of Berteloot et al., who described their appearance on the surface of drying colloidal droplets [15]. They suggested that the bands might be either shear bands, based on a visual similarity to these features in metals [16], or the surface buckling of a colloidal skin.Recently, Yang et al. have shown that these bands are indeed linked to the shear response of a drying film [17]. They studied freestanding films of colloidal polystyrene, and effectively controlled the yield stress and strain rate (drying rate) of the films. In particular, they showed how the spacing and relative widths of the bands-i.e., the fraction of sheared material-agree with the lever rule of shear localization [17]. This rule, derived from the physics of two-phase coexistence, describes the instability by which shear localizes into finite bands in a variety of complex fluids [18].Here we give a complementary view of these shear bands, identifying the forces behind their formation. We show by direct means that they are the result of shear deformations, and further demonstrate how their pattern scales, how it can be suppressed, and discuss its origins in the compressive forces that accompany the directional drying of colloidal dispersions. In contrast to [15,17], we study slow drying in Hele-Shaw cells, to eliminate any possible influence of a free surface or skin.To prepare banded films, we used a variety of chargestab...

Monte-Carlo simulations and small-angle x-ray scattering experiments were used to determine the phase diagram of aqueous dispersions of titratable nano-colloids with a moderate size polydispersity over a broad range of monovalent salt concentrations, 0.5 mM ≤ cs ≤ 50 mM and volume fractions, φ. Under slow and progressive increase in φ, the dispersions freeze into a face-centered-cubic (fcc) solid followed unexpectedly by the formation of a body centered cubic (bcc) phase before to melt in a glass forming liquid. The simulations are found to predict very well these observations. They suggest that the stabilization of the bcc solid at the expense of the fcc phase at high φ and cs results from the interaction (charge) polydispersity and vibrational entropy. arXiv:1909.02774v1 [cond-mat.soft]

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