Cracking of drying films

这项提案的目的是研究一个尚未解决但非常普遍的、概念上令人愉悦的、与工业相关的问题：为什么胶体膜在干燥时破裂。有明确的实验证据表明，毛细管压力与薄膜在干燥过程中开裂的趋势有关，并表明观察到的开裂模式是拉伸薄膜所期望的。这导致了一些想法，这些想法预测裂缝会在仍然是流体的材料中生长，或者在孔隙开始清空流体的地方生长。两者都没有被观察到。相反，裂纹在充满液体的颗粒阵列中生长，具有一定的机械强度，毛细压力应该对其施加压应力。使用氧化铝悬浮液，我们最近表明，有一个收缩的液体填充粒子阵列，这种收缩与开裂的开始有关。利用现有的流体流动分析和裂缝生长的位置，我们预测了这种收缩所需的压力是毛细管压力的数量级，比其他胶体力大得多。因为在这些氧化铝悬浮液中，收缩是如此明显地与开裂有关，我们希望了解导致收缩的过程，并学会控制它。为了达到这种理解，我们将研究收缩事件在不同系统中的变化；充液粒子阵列的性质；假设阵列在压力下收缩时是如何变化的，并且已经确定了发生的潜在机制，可以定量分析开裂。最近有相当多的科学活动，主要是由涂料工业推动的，在干燥、开裂和胶体中非常短的距离力的性质方面，两位研究人员很幸运地参与了传播过程。正如下面所讨论的，干燥膜中的应力分布正变得越来越清晰，阐明膜开裂物理学所必需的实验现在是可能的。这项工作的目的是利用AFR的胶体专业知识和WJC的压裂专业知识，在这一领域发展我们的新想法。

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.