Experiments on the spreading of droplets on rotating plates and their stability

一方面，液体在旋转基材上的扩散是旋转涂布工艺的基础。如果需要平面基材的固体涂层，则通常采用此工艺。例子包括半导体、太阳能电池板或液晶显示器（lcd），在所有情况下，都必须添加许多具有特定电学或光学特性的固体层。在大多数情况下，这种涂层最初应用均匀的液体层，由于溶剂的凝固或蒸发而转变为固体层。另一方面，这种涂层流动及其稳定性的建模提出了一个科学挑战，因为至少涉及一个移动接触线，一个流体层厚度变得非常小的区域。因此，连续介质力学有望在这些区域内触及其有效性的极限。如果液体层在旋转中心破裂并产生脱湿锋，甚至可能出现二次移动接触线。在过去的几十年里，这一过程的理论模型有了显著的进步。特别是，有足够的模型可以捕捉旋转对称的扩散基流及其（线性）稳定性，以抵抗周向周期性的小扰动。然而，关于这一过程的仔细实验仍然很少，这使得验证理论模型变得困难。少数已发表的实验集中在旋转中心连续浇注液体时的液体轮廓，在非平面基板上扩散时的液体轮廓，以及接触线的手指不稳定性。我们提出的实验旨在为完全润湿和部分润湿的牛顿液体在平面旋转基底上的扩散提供广泛的数据基础。我们将捕捉（平均）液滴半径的动态，接触线的速度，特别是液滴随时间扩散的轮廓。这也允许推断接触角的历史。此外，旋转中心液膜的破裂和相应的脱湿锋的动态行为是我们研究的一个重要方面。最后，对润湿锋的稳定性进行了系统的研究。

The spreading of liquids on rotating substrates, on the one hand, is the basis of the spin-coating process. This process is often engaged, if a solid coating of plane substrates is needed. Examples include semi-conductors, solar panels, or liquid-crystal displays (LCDs), where in all cases numerous solid layers of defined electrical or optical properties have to be added. In most cases, this coating applies initially a homogeneous liquid layer, which transform due to solidification or evaporation of the solvent into a solid layer. On the other hand, the modelling of such coating flows and their stability presents a scientific challenge, since at least one moving contact line is involved, a region where the fluid-layer thickness becomes very small. Hence, continuum mechanics is expected to touch its limit of validity within those regions. Even a secondary moving contact line may occur, if the liquid layer ruptures at the centre of rotation and creates a dewetting front.During the last decades, the theoretical modelling of this process has significantly advanced. Particularly, adequate models are available which capture the rotationally-symmetric spreading base flow and its (linear) stability against small disturbances which are periodic in the circumferential direction. Nevertheless, careful experiments on this process remain rare, which makes it hard to validate the theoretical models. The few published experiments focus on liquid contours during continuous liquid pouring at the rotation centre, on liquid contours during spreading across non-plane substrates, and on the finger instability of the contact line. Our proposed experiments aim to provide a broad data basis for the spreading of perfectly- and partially-wetting, Newtonian liquids on plane rotating substrates. We shall capture the dynamics of the (mean) drop radius, the speed of the contact line, and particularly contours of spreading droplets in time. This allows also to infer the history of the contact angle. Moreover, the rupture of the liquid film at the centre of rotation and the dynamic behavior of the corresponding dewetting front is an important aspect of our investigations. Finally, the systematic investigation of the stability of the wetting front is within our focus.