Drying suspensions of colloidal nanoparticles exhibit a variety of interesting strain release mechanisms during film formation. These result in the selection of characteristic length scales during failure processes such as cracking(1) and subsequent delamination. A wide range of materials (e.g., bulk metallic glasses) release strain through plastic deformations which occur in a narrow band of material known as a shear band. For the first time, we demonstrate that drying colloidal films also exhibit shear banding(2). Bands are observed to form a small distance behind the drying front and then to propagate rapidly at ∼45° to the direction of drying. The combined observations suggest that the critical shear rate (related to the film yield stress) controls the ratio of bandwidth to band spacing, which depends on particle size, salt concentration and the evaporation rate of the colloidal suspension. We also show that below the critical drying rate, the ratio obeys a simple Lever rule. To study the cause of shear band formation, we measured the local deformations in the early stages of drying using fluorescent tracer particles. The measurements were used to show that the existence of shear bands is linked to the compaction of particles perpendicular to the drying front. The spacing of shear bands was also found to be strongly correlated with the characteristic length scale of the compaction process. These combined studies elucidate the role of plastic deformation during pattern formation in drying films of colloidal nanoparticles. Reference 1. Singh KB & Tirumkudulu MS (2007) Cracking in drying colloidal films. Phys Rev Lett 98(21). 2. Yang B, Sharp JS, & Smith MI (2015) Shear Banding in Drying Films of Colloidal Nanoparticles. Acs Nano 9(4):4077-4084.

Shear banding in drying films of colloidal nanoparticles

251st National Meeting (2016)

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