Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, USA.
Nature. 2012 Feb 23;482(7386):510-3. doi: 10.1038/nature10779.
Fibrous media are functional and versatile materials, as demonstrated by their ubiquity both in natural systems such as feathers and adhesive pads and in engineered systems from nanotextured surfaces to textile products, where they offer benefits in filtration, insulation, wetting and colouring. The elasticity and high aspect ratios of the fibres allow deformation under capillary forces, which cause mechanical damage, matting self-assembly or colour changes, with many industrial and ecological consequences. Attempts to understand these systems have mostly focused on the wetting of rigid fibres or on elastocapillary effects in planar geometries and on a fibre brush withdrawn from an infinite bath. Here we consider the frequently encountered case of a liquid drop deposited on a flexible fibre array and show that flexibility, fibre geometry and drop volume are the crucial parameters that are necessary to understand the various observations referred to above. We identify the conditions required for a drop to remain compact with minimal spreading or to cause a pair of elastic fibres to coalesce. We find that there is a critical volume of liquid, and, hence, a critical drop size, above which this coalescence does not occur. We also identify a drop size that maximizes liquid capture. For both wetting and deformation of the substrates, we present rules that are deduced from the geometric and material properties of the fibres and the volume of the drop. These ideas are applicable to a wide range of fibrous materials, as we illustrate with examples for feathers, beetle tarsi, sprays and microfabricated systems.
纤维介质是功能多样的材料，这一点在它们在自然系统（如羽毛和粘性垫）和工程系统（从纳米纹理表面到纺织品）中的普遍存在中得到了证明，在这些系统中，它们在过滤、绝缘、润湿和着色方面提供了益处。纤维的弹性和高纵横比允许在毛细作用力下变形，这种变形会导致机械损伤、自组装的缠结或颜色变化，这对许多工业和生态都有影响。为了理解这些系统，人们大多集中在刚性纤维的润湿或平面几何中的弹性毛细效应以及从无限浴中抽出的纤维刷上。在这里，我们考虑了一种常见的情况，即液滴沉积在柔性纤维阵列上，结果表明，灵活性、纤维几何形状和液滴体积是理解上述各种观察结果所必需的关键参数。我们确定了使液滴保持紧凑状态而最小化扩展或导致一对弹性纤维合并所需的条件。我们发现存在一个临界液体体积，因此，存在一个临界液滴尺寸，超过该尺寸则不会发生这种合并。我们还确定了最大液体捕获量的液滴尺寸。对于基底的润湿和变形，我们提出了基于纤维的几何和材料特性以及液滴体积的规则。这些想法适用于广泛的纤维材料，我们用羽毛、甲虫跗节、喷雾和微加工系统的示例来说明了这一点。
