扩散表面活性剂的荧光可视化
Fluorescent visualization of a spreading surfactant
摘要：
表面活性剂在薄膜上的铺展是一种重要的工业和医学现象，但其动力学是高度非线性的，表面活性剂动力学的可视化一直是一个长期存在的实验挑战。我们执行的第一个定量的，时空分辨测量的不溶性表面活性剂在薄流体层上的扩散。在扩散过程中，我们直接观察扩散液滴的径向高度分布和荧光标记的表面活性剂的空间分布。我们发现，表面活性剂扩散的圆形层的前缘在下伏流体中形成Marangoni脊，脊的后面是预期的波谷。然而，使用荧光技术观察到几个新的功能，包括表面活性剂浓度的峰值，其尾随前缘，和平坦的，单层规模的扩展膜，其不同于当前模型预测的浓度分布。马兰戈尼脊和表面活性剂前缘都可以描述为以$R \propto t^δ$展开。我们发现的扩展指数，$δ_H \approx 0.30$和$δ_Γ\approx 0.22$的脊峰和表面活性剂的前缘，分别与理论预测的$δ= 1/4$。此外，我们观察到，表面活性剂的前缘最初导致的马兰戈尼脊的峰值，峰值后来赶上的前缘。
英文摘要：
The spreading of surfactants on thin films is an industrially and medically important phenomenon, but the dynamics are highly nonlinear and visualization of the surfactant dynamics has been a long-standing experimental challenge. We perform the first quantitative, spatiotemporally-resolved measurements of the spreading of an insoluble surfactant on a thin fluid layer. During the spreading process, we directly observe both the radial height profile of the spreading droplet and the spatial distribution of the fluorescently-tagged surfactant. We find that the leading edge of spreading circular layer of surfactant forms a Marangoni ridge in the underlying fluid, with a trough trailing the ridge as expected. However, several novel features are observed using the fluorescence technique, including a peak in the surfactant concentration which trails the leading edge, and a flat, monolayer-scale spreading film which differs from concentration profiles predicted by current models. Both the Marangoni ridge and surfactant leading edge can be described to spread as $R \propto t^δ$. We find spreading exponents, $δ_H \approx 0.30$ and $δ_Γ\approx 0.22$ for the ridge peak and surfactant leading edge, respectively, which are in good agreement with theoretical predictions of $δ= 1/4$. In addition, we observe that the surfactant leading edge initially leads the peak of the Marangoni ridge, with the peak later catching up to the leading edge.
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