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

一方面，液体在旋转基板上的铺展是旋涂工艺的基础。如果需要对平面基材进行固体涂层，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，都必须添加大量具有定义的电或光学特性的固体层。在大多数情况下，这种涂层最初应用的是均匀的液层，由于溶剂的固化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模拟是一项科学挑战，因为至少涉及一条移动的接触线，这是一个流体层厚度变得非常小的区域。因此，连续介质力学预计将触及其在这些区域内的有效性极限。如果液体层在旋转中心破裂并产生去湿前沿，甚至可能出现二次移动的接触线。在过去的几十年中，对这一过程的理论模型有了显著的进步。特别是，有足够的模型可用来捕捉旋转对称的扩展基流及其(线性)稳定性，以对抗沿周向周期性的小扰动。然而，对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，在非平面衬底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湿润和部分润湿的牛顿液体在平面旋转衬底上的传播提供广泛的数据基础。我们将及时捕捉(平均)液滴半径、接触线速度的动态变化，特别是液滴扩散的轮廓。这还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去湿锋的动力学行为也是我们研究的一个重要方面。最后，湿润锋稳定性的系统研究也是我们关注的重点。

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.