Directional wetting properties of topographically micro-patterned surfaces

具有微观结构的表面通常表现出不同寻常的润湿特性。与微结构接触的液滴通常表现出其流动性的方向依赖性。在许多情况下，人们可以观察到液体向结构的某些方向流动的增强。本项目的目标是全面研究具有均匀横截面的微柱装饰的规则图案表面上的定向润湿现象。首先，我们将考虑部分湿润液体的情况，即在微图案表面形成液滴的液体，与结构的典型尺寸相比，液滴较大。液气界面刚开始运动时的视接触角是由界面相对于微图样的相对方向决定的。这些“前进”和“后退”的接触角应该用与微图案表面接触的界面的数值模型来计算。典型的界面不稳定推进可以用发生在液体、空气和衬底材料之间三相接触线附近的特征界面不稳定谱来描述。在高度湿润的液体中，人们可以观察到液体膜的形成，它正在吸干微观结构。同样在后一种情况下，观察到液体运动的强烈方向依赖性，这可能导致传播膜边缘的切面。类似于不形成薄膜的液体界面的推进模式，我们将在数值模型中研究面化的机制。在接触角向不同方向移动的大对比度限制下，我们提出了观察生长液滴不同特征形状的可能性。这些形状应计算为任意依赖于推进接触角对界面的方向。对于大反差，我们期望观察到一个单向的，即丝状的液滴生长，而对于小反差，生长的液体将形成变形的球形帽。为了进一步得出液膜在微结构中传播速度的方向依赖性的结论，我们将计算不同结构和填充程度下液膜的粘性流动阻力。该流动阻力将进一步用于制定液膜动力学的粗粒度模型。

Surfaces equipped with microscopic structures often display unusual wetting properties. Liquids droplets brought into contact with the microstructures typically exhibit a directional depedence of their mobility. In many cases, one can observe an enhanced mobility of the liquid into certain directions of the structures. Goal of the present project is a comprehensive study of directional wetting phenomena on surfaces that are decorated with a regular pattern of micro-posts with uniform cross-section. First, we will consider the case of partially wetting liquids, i.e. liquids which form droplets on the micropatterned surface that are large compared to the typical dimensions of the structures. The apparent contact angle at which the liquid-air interface just starts to move is determined by the relative orientation of the interface with respect to the micro-pattern. These "advancing" and "receding" contact angles shall be computed with numerical models of the interface in contact to the micro-patterned surface. The typically unsteady advancement of the interface can be described by a spectrum of characteristic interfacial instabilities that occur in the vicintiy of the three-phase contact line between the liquid, air, and the substrate material. In case of highly wetting liquids, one may observe the formation of a liquid film that is wicking the micro-structures. Also in the latter case, a strong directional dependence of the liquid motion is observed which can lead to a facetting of the propagating film edge. Similar to the advancing modes of an inteface for liquid that do not form a film, we will investigate the mechanisms underlying the facetting in numerical models. In the limit of large constrasts of the advancing contact angle into different directions, we propose the possibilty to observe distinct characteristic shapes of growing droplets. These shapes shall be computed for a arbitrary dependence of the advancing contact angle on the orientation of the interface. For lage contrasts, we expect to observe a unidirectional, i.e. filamenteous droplet growth, while for small constrats, the growing liquid will form deformed spherical caps. To reach further conclusions about the directional dependence of the propagation velocity of a liquid film in the microstructures, we will compute the viscous flow resistance of the film for various structures and filling degress. This flow resistance will be further employed to fomulate a coarse grained model for the liquid film dynamics.