Laboratory of Physical Chemistry and Colloid Science, Wageningen University, 6703 HB, Wageningen, The Netherlands.
Proc Natl Acad Sci U S A. 2013 Jun 4;110(23):9220-4. doi: 10.1073/pnas.1222196110. Epub 2013 May 20.
Objects floating at a liquid interface, such as breakfast cereals floating in a bowl of milk or bubbles at the surface of a soft drink, clump together as a result of capillary attraction. This attraction arises from deformation of the liquid interface due to gravitational forces; these deformations cause excess surface area that can be reduced if the particles move closer together. For micrometer-sized colloids, however, the gravitational force is too small to produce significant interfacial deformations, so capillary forces between spherical colloids at a flat interface are negligible. Here, we show that this is different when the confining liquid interface has a finite curvature that is also anisotropic. In that case, the condition of constant contact angle along the three-phase contact line can only be satisfied when the interface is deformed. We present experiments and numerical calculations that demonstrate how this leads to quadrupolar capillary interactions between the particles, giving rise to organization into regular square lattices. We demonstrate that the strength of the governing anisotropic interactions can be rescaled with the deviatoric curvature alone, irrespective of the exact shape of the liquid interface. Our results suggest that anisotropic interactions can easily be induced between isotropic colloids through tailoring of the interfacial curvature.
处于液体界面上的物体，如漂浮在一碗牛奶中的早餐麦片或软饮料表面的气泡，由于毛细吸引力而聚集在一起。这种吸引力源于液体界面因重力而产生的变形；这些变形会导致表面积增加，如果颗粒相互靠近，表面积就可以减少。然而，对于微米大小的胶体，重力太小，无法产生显著的界面变形，因此在平坦界面处球形胶体之间的毛细力可以忽略不计。在这里，我们表明，当限制液体界面具有有限的各向异性曲率时，情况就不同了。在这种情况下，只有当界面变形时，三相接触线沿线的恒接触角条件才能得到满足。我们提出了实验和数值计算，证明了这如何导致颗粒之间的四极毛细相互作用，从而导致规则的正方形晶格的形成。我们证明了，无论液体界面的确切形状如何，各向异性相互作用的强度都可以单独用偏差曲率来缩放。我们的结果表明，通过调整界面曲率，各向同性胶体之间很容易产生各向异性相互作用。
