Formation of Shear Bands in Drying Colloidal Dispersions

Abstract

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...

Drying-induced back flow of colloidal suspensions confined in thin unidirectional drying cells

“…[9][10][11] A suspension is introduced into a narrow space between parallel glass plates. By using this drying cell, quantitative measurements on drying-induced phenomena, such as lm formation, 12 crack formation 13 and shear-band formation, 14 have been demonstrated. Horizontal solvent ow is essential for structural anisotropy of particulate lms, 15 the formation of shear bands 16 and for the packing ordering of weakly Brownian particles.…”

Drying dynamics of a charged colloidal dispersion in a confined drop

“…Moreover, a transition from a liquid to a poro-elastic gel often occurs for these highly charged colloids at concentrations well below the close-packing [7,8,11,30]. Such a transition was shown to play a crucial role for the formation of shear bands in unidirectional drying [11,13,14]. Despite the numerous works employing these charged nanoparticles to investigate the drying of dispersions, a quantitative description of the concentration process taking into account these phenomena is still missing.…”

The interplay of crack hopping, delamination and interface failure in drying nanoparticle films

“…Shear bands are known to form as a result of plastic rearrangements in the film that shear the original close packed particle structures formed during drying. These shear bands also influence the local strain in the colloidal films16. Such a variation in the local strain (or packing of particles) might be expected to modify the local structure in the films and hence influence crack propagation.…”

“…Immediately behind the compaction front, lines known as shear bands are often observed to form at an orientation of 45 degrees to the direction of drying616. These plastic deformations occur in a relatively narrow band in the film, within which particle rearrangements take place616.…”

“…These plastic deformations occur in a relatively narrow band in the film, within which particle rearrangements take place616. A little further behind the compaction front, the film is observed to form cracks that propagate parallel to the drying direction.…”