Cracking of drying films

这项提议的目的是研究一个尚未解决但极其普遍的、概念上令人愉快的、与工业相关的问题：为什么胶体膜在干燥时会破裂。有明确的实验证据表明，毛细压力与薄膜在干燥过程中破裂的倾向有关，并表明观察到的破裂模式与拉伸薄膜的预期模式相同。这导致了一些想法，预测裂缝会扩展到材料中，在材料仍然是流体的地方，或者在毛孔已经开始排空液体的地方。两者都没有被观察到。相反，裂纹在具有一定机械强度的充满液体的颗粒阵列中生长，毛细压力应在其上施加压应力。使用氧化铝悬浮液，我们最近表明，充满液体的颗粒阵列存在收缩，并且这种收缩与裂纹的开始有关。利用现有的流体流动分析和裂纹扩展的位置，我们预测了收缩所需的压力是毛细压力的量级，远远大于其他胶体力。由于收缩与这些氧化铝悬浮液中的裂纹明显相关，我们希望了解导致收缩的过程，并学习控制它。为了实现这一理解，我们将研究收缩事件在不同系统中如何变化；充满液体的颗粒阵列的性质；阵列如何随着其收缩而变化，假设是在压力下，并在建立了发生的潜在机制后，对裂缝进行定量分析。最近在干燥、破裂和胶体中极短距离作用力的性质方面都有相当大的科学活动，这主要是由涂料工业推动的，两名研究人员有幸参与了传播过程。正如下面讨论的那样，干燥薄膜中的应力分布变得清晰起来，并且现在有可能进行必要的实验来阐明薄膜破裂的物理机制。这项工作的目的是开发我们在这一领域的新想法，利用AFR的胶体专业知识和WJC的骨折专业知识。

This aim of this proposal is to study an unsolved but incredibly common, conceptually pleasing and industrially relevant problem: Why colloidal films crack when they dry. There is clear experimental evidence that associates the capillary pressure with the tendency of a film to crack during drying and shows that the observed cracking patterns are those expected for a film in tension. This has led to ideas which predict cracks that grow into material where it was either still fluid or where the pores have begun to empty of fluid. Neither is observed. Instead the cracks grow in a liquid-filled particle array with some mechanical strength and on which the capillary pressure should impose a compressive stress.Using alumina suspensions we have recently shown that there was a shrinkage of the liquid-filled particle array and that this shrinkage was associated with the onset of cracking. Using existing fluid flow analyses and the position of the growing cracks we have predicted the pressure required for this shrinkage to be of the order of the capillary pressure, much larger than other colloidal forces.Because the shrinkage is so clearly associated with cracking in these alumina suspensions, we wish to understand the processes that lead to the shrinkage, and learn to control it. To achieve this understanding, we will study how the shrinkage events vary in different systems; the nature of the liquid-filled particle array; how the array changes as it shrinks, presumably under stress and, having established the underlying mechanisms occurring, to analyse the cracking quantitatively. There has recently been a considerable scientific activity in both drying, cracking and the nature of very short range forces in colloids, driven primarily from the coatings industries and the two investigators have been fortunate to be involved in the dissemination process. As discussed below the stress profile in the drying film is becoming clear and the experiments necessary to elucidate the physics of film cracking are now possible. It is the aim of this work to develop our new ideas in this area, taking advantage of the colloid expertise of AFR and the fracture expertise of WJC.