Manica Rogério, Connor Jason N, Clasohm Lucy Y, Carnie Steven L, Horn Roger G, Chan Derek Y C
Particulate Fluids Processing Centre, Department of Mathematics and Statistics, The University of Melbourne, Parkville 3010, Australia.
Langmuir. 2008 Feb 19;24(4):1381-90. doi: 10.1021/la701562q. Epub 2007 Jul 27.
This article reports real-time observations and detailed modeling of the transient response of thin aqueous films bounded by a deformable surface to external mechanical and electrical perturbations. Such films, tens to hundreds of nanometers thick, are confined between a molecularly smooth mica plate and a deformable mercury/electrolyte interface on a protuberant drop at a sealed capillary tube. When the mercury is negatively charged, the water forms a wetting film on mica, stabilized by electrical double layer forces. Mechanical perturbations are produced by driving the mica plate toward or by retracting the mica plate from the mercury surface. Electrical perturbations are applied to change the electrical double layer interaction between the mica and the mercury by imposing a step change of the bias voltage between the mercury and the bulk electrolyte. A theoretical model has been developed that can account for these observations quantitatively. Comparison between experiments and theory indicates that a no-slip hydrodynamic boundary condition holds at the molecularly smooth mica/electrolyte surface and at the deformable mercury/electrolyte interface. An analysis of the transient response based on the model elucidates the complex interplay between disjoining pressure, hydrodynamic forces, and surface deformations. This study also provides insight into the mechanism and process of droplet coalescence and reveals a novel, counterintuitive mechanism that can lead to film instability and collapse when an attempt is made to thicken the film by pulling the bounding mercury and mica phases apart.
本文报道了由可变形表面界定的薄水膜对外部机械和电扰动的瞬态响应的实时观测和详细建模。这种厚度为几十到几百纳米的薄膜，被限制在一个分子光滑的云母板和一个密封毛细管上突出液滴上的可变形汞/电解质界面之间。当汞带负电时，水在云母上形成一个润湿膜，由双电层力稳定。机械扰动是通过将云母板推向汞表面或从汞表面缩回云母板产生的。电扰动是通过在汞和本体电解质之间施加偏置电压的阶跃变化来改变云母和汞之间的双电层相互作用。已经开发了一个理论模型，可以定量地解释这些观测结果。实验与理论之间的比较表明，在分子光滑的云母/电解质表面和可变形的汞/电解质界面处存在无滑移流体动力学边界条件。基于该模型对瞬态响应的分析阐明了分离压力、流体动力和表面变形之间的复杂相互作用。这项研究还深入了解了液滴聚结的机制和过程，并揭示了一种新颖的、违反直觉的机制，当试图通过拉开边界汞相和云母相来加厚薄膜时，这种机制会导致薄膜不稳定和坍塌。
