Nikolaides M G, Bausch A R, Hsu M F, Dinsmore A D, Brenner M P, Gay C, Weitz D A
Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.
Nature. 2002 Nov 21;420(6913):299-301. doi: 10.1038/nature01113.
Nanometre- and micrometre-sized charged particles at aqueous interfaces are typically stabilized by a repulsive Coulomb interaction. If one of the phases forming the interface is a nonpolar substance (such as air or oil) that cannot sustain a charge, the particles will exhibit long-ranged dipolar repulsion; if the interface area is confined, mutual repulsion between the particles can induce ordering and even crystallization. However, particle ordering has also been observed in the absence of area confinement, suggesting that like-charged particles at interfaces can also experience attractive interactions. Interface deformations are known to cause capillary forces that attract neighbouring particles to each other, but a satisfying explanation for the origin of such distortions remains outstanding. Here we present quantitative measurements of attractive interactions between colloidal particles at an oil-water interface and show that the attraction can be explained by capillary forces that arise from a distortion of the interface shape that is due to electrostatic stresses caused by the particles' dipolar field. This explanation, which is consistent with all reports on interfacial particle ordering so far, also suggests that the attractive interactions might be controllable: by tuning the polarity of one of the interfacial fluids, it should be possible to adjust the electrostatic stresses of the system and hence the interparticle attractions.
水相界面处的纳米级和微米级带电粒子通常通过排斥性库仑相互作用实现稳定。如果形成界面的其中一相是无法维持电荷的非极性物质（如空气或油），粒子将表现出长程偶极排斥；如果界面面积受限，粒子之间的相互排斥会导致有序排列甚至结晶。然而，在不存在面积限制的情况下也观察到了粒子有序排列，这表明界面处的同种电荷粒子也会经历吸引相互作用。已知界面变形会产生毛细力，使相邻粒子相互吸引，但对于这种变形的起源，尚未有令人满意的解释。在此，我们给出了油水界面处胶体粒子间吸引相互作用的定量测量结果，并表明这种吸引力可由界面形状畸变产生的毛细力来解释，而这种畸变是由粒子偶极场引起的静电应力导致的。这种解释与目前所有关于界面粒子有序排列的报道一致，同时也表明吸引力相互作用可能是可控的：通过调节其中一种界面流体的极性，应该能够调整系统的静电应力，从而调整粒子间的吸引力。
