We exploit the aggregation between oppositely charged particles to visualize and quantify the equilibrium position of charged colloidal particles at the fluid–water interface. A dispersion of commercially available charge-stabilized nanoparticles was used as the aqueous phase to create oil–water and air–water interfaces. The colloidal particles whose charge was opposite that of the nanoparticles in the aqueous phase were deposited at the chosen fluid–water interface. Heteroaggregation, i.e., aggregation between oppositely charged particles, leads to the deposition of nanoparticles onto the larger particle located at the interface; however, this only occurs on the surface of the particle in contact with the aqueous phase. This selective deposition of nanoparticles on the surfaces of the particles exposed to water enables the distinct visualization of the circular three-phase contact line around the particles positioned at the fluid–water interface. Since the electrostatic association between the nanoparticles and the colloids at interfaces is strong, the nanoparticle assembly on the larger particles is preserved even after being transferred to solid substrates via dip-coating. This facilitates the easy visualization of the contact line by electron microscopy and the determination of the equilibrium contact angle of colloidal particles (θ) at the fluid–water interface. The suitability of the method is demonstrated by the measurement of the three-phase contact angle of positively and negatively charged polystyrene particles located at fluid–water interfaces by considering particles with sizes varying from 220 nm to 8.71 μm. The study highlights the effect of the size ratio between the nanoparticles in the aqueous phase and the colloidal particles on the accuracy of the measurement of θ.

中文翻译：

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利用异质聚集来量化界面处带电胶体的接触角

我们利用带相反电荷的粒子之间的聚集来可视化和量化带电胶体粒子在流体-水界面处的平衡位置。市售的电荷稳定纳米粒子的分散体被用作水相以产生油-水和空气-水界面。其电荷与水相中的纳米颗粒相反的胶体颗粒沉积在选定的流体-水界面处。异质聚集，即带相反电荷的粒子之间的聚集，导致纳米粒子沉积到位于界面处的较大粒子上；然而，这仅发生在与水相接触的颗粒表面上。纳米颗粒在暴露于水的颗粒表面上的这种选择性沉积使得围绕位于流体-水界面的颗粒的圆形三相接触线清晰可见。由于纳米粒子与界面处的胶体之间的静电结合很强，即使在通过浸涂转移到固体基材后，较大粒子上的纳米粒子组装也得以保留。这有助于通过电子显微镜轻松观察接触线并确定流体 - 水界面处胶体颗粒的平衡接触角（θ）。该方法的适用性通过测量位于流体-水界面处的带正电和带负电的聚苯乙烯颗粒的三相接触角来证明，考虑尺寸从 220 nm 到 8.71 μm 的颗粒。该研究强调了水相中的纳米颗粒与胶体颗粒之间的尺寸比对 θ 测量精度的影响。