†School of Physics and Astronomy and ‡Nottingham Nanotechnology and Nanoscience Centre, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom.
ACS Nano. 2015 Apr 28;9(4):4077-84. doi: 10.1021/acsnano.5b00127. Epub 2015 Apr 2.
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 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. Here we show that drying colloidal films also exhibit shear banding. Bands are observed to form a small distance behind the drying front and then to propagate rapidly at ∼45° to the direction of drying. It is shown that the spacing of the bands depends on salt concentration and the evaporation rate of the colloidal suspension. These combined observations suggest that there is a critical shear rate (related to the film yield stress) which controls the ratio of bandwidth to band spacing. Local deformations were measured 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.
干燥的胶体纳米粒子悬浮液在成膜过程中表现出多种有趣的应变速率释放机制。这些机制导致在失效过程（如开裂和随后的分层）中选择特征长度尺度。广泛的材料（例如，块状金属玻璃）通过塑性变形释放应变，这种塑性变形发生在称为剪切带的材料窄带中。在这里，我们表明干燥的胶体膜也表现出剪切带。在干燥前沿后一小段距离观察到带的形成，然后以约 45°的角度快速向干燥方向传播。结果表明，带的间距取决于盐浓度和胶体悬浮液的蒸发速率。这些综合观察表明，存在一个临界剪切速率（与膜屈服应力有关），它控制着带宽与带间距的比值。使用荧光示踪粒子在干燥的早期阶段测量局部变形。测量结果表明，剪切带的存在与垂直于干燥前沿的颗粒的压实有关。还发现，剪切带的间距与压实过程的特征长度尺度强烈相关。这些综合研究阐明了胶体纳米粒子干燥膜中塑性变形在图案形成过程中的作用。
