School of Physics and Astronomy and Nottingham Nanotechnology and Nanoscience Centre, University of Nottingham, University Park, Nottingham, NG7 2RD, UK.
Langmuir. 2011 Jul 5;27(13):8009-17. doi: 10.1021/la2000624. Epub 2011 Jun 8.
Crack formation and the evolution of stress in drying films of colloidal particles were studied using optical microscopy and a modified cantilever deflection technique, respectively. Drying experiments were performed using polystyrene particles with diameters of 47 ± 10 nm, 100 ± 16 nm, and 274 ± 44 nm that were suspended in water. As the films dried, cracks with a well-defined spacing were observed to form. The crack spacing was found to be independent of the particle size used, but to increase with the film thickness. The characteristic crack spacing was found to vary between 20 and 300 μm for films with thickness values in the range 3-70 μm. Cantilever deflection measurements revealed that the stresses that develop in the film increase with decreasing film thickness (increasing surface-to-volume ratio). The latter observation was interpreted in terms of the effects of a substrate constraint which causes the build up of stresses in the films. This interpretation was confirmed by crack formation experiments that were performed on liquid mercury surfaces in which removal of the substrate constraint prevented crack formation. Experiments were also performed on compliant elastomer surfaces in which the level of constraint was varied by changing the substrate modulus. The cracking length scale was found to increase with decreasing substrate modulus. A simple theory was also developed to describe the substrate modulus dependence of the cracking length scale. These combined experiments and theory provide convincing evidence that substrate constraints are an important factor in driving crack formation in thin colloidal films.
使用光学显微镜和改进的悬臂梁挠度技术分别研究了胶体颗粒干燥膜中的裂纹形成和应力演变。使用直径为 47 ± 10nm、100 ± 16nm 和 274 ± 44nm 的聚苯乙烯颗粒在水中悬浮进行干燥实验。随着膜的干燥，观察到形成具有明确定义间隔的裂纹。发现裂纹间距与使用的颗粒尺寸无关，但随膜厚度增加而增加。对于厚度值在 3-70μm 范围内的膜，特征裂纹间距在 20 和 300μm 之间变化。悬臂梁挠度测量显示，膜中产生的应力随膜厚度减小（表面积与体积比增大）而增大。后一种观察结果可以根据基底约束的影响来解释，这会导致膜中应力的积累。通过在液态汞表面上进行的裂纹形成实验证实了这一解释，在这些实验中，去除基底约束可以防止裂纹形成。还在顺应性弹性体表面上进行了实验，其中通过改变基底模量来改变约束水平。发现裂纹长度尺度随基底模量的降低而增加。还开发了一个简单的理论来描述裂纹长度尺度对基底模量的依赖性。这些综合实验和理论提供了令人信服的证据，表明基底约束是驱动薄胶体膜中裂纹形成的重要因素。
