Institute for Complex Molecular Systems, Eindhoven University of Technology, Post Office Box 513, 5600 MB Eindhoven, The Netherlands.
Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog Weg 5, 8093 Zürich, Switzerland.
Nanoscale. 2019 Apr 4;11(14):6654-6661. doi: 10.1039/c8nr08633h.
Solid particles adsorbed at fluid interfaces are crucial for the mechanical stability of Pickering emulsions. The key parameter which determines the kinetic and thermodynamic properties of these colloids is the particle contact angle, θ. Several methods have recently been developed to measure the contact angle of individual particles adsorbed at liquid-liquid interfaces, as morphological and chemical heterogeneities at the particle surface can significantly affect θ. However, none of these techniques enables the simultaneous visualization of the nanoparticles and the reconstruction of the fluid interface to which they are adsorbed, in situ. To tackle this challenge, we utilize a newly developed super-resolution microscopy method, called iPAINT, which exploits non-covalent and continuous labelling of interfaces with photo-activatable fluorescent probes. Herewith, we resolve with nanometer accuracy both the position of individual nanoparticles at a water-octanol interface and the location of the interface itself. First, we determine single particle contact angles for both hydrophobic and hydrophilic spherical colloids. These experiments reveal a non-negligible dependence of θ on particle size, from which we infer an effective line tension, τ. Next, we image elliptical particles at a water-decane interface, showing that the corresponding interfacial deformations can be clearly captured by iPAINT microscopy.
固体质点在流体界面上的吸附对于 Pickering 乳液的机械稳定性至关重要。决定这些胶体动力学和热力学性质的关键参数是颗粒接触角θ。最近已经开发出几种方法来测量在液-液界面上吸附的单个颗粒的接触角，因为颗粒表面的形貌和化学不均匀性会显著影响θ。然而，这些技术都无法同时对吸附在界面上的纳米粒子进行可视化和重建，以实现原位观察。为了解决这个挑战，我们利用了一种新开发的超分辨率显微镜方法，称为 iPAINT，它利用光活化荧光探针对界面进行非共价和连续标记。通过这种方法，我们可以以纳米级精度确定单个纳米粒子在水-辛醇界面上的位置和界面本身的位置。首先，我们确定了疏水和亲水球形胶体的单个颗粒接触角。这些实验表明，θ与颗粒尺寸有不可忽略的依赖性，从中我们推断出有效的线张力τ。接下来，我们在水-癸烷界面上对椭圆形颗粒进行成像，结果表明 iPAINT 显微镜可以清晰地捕捉到相应的界面变形。
