Department of Physics, Univ. of Massachusetts, Amherst, MA 01003, USA.
Soft Matter. 2017 Nov 15;13(44):8234-8239. doi: 10.1039/c7sm01548h.
When a particle adsorbs at a liquid interface, the 3-phase contact line geometry depends on the shape of the particle and of the liquid interface. The shape of the contact line is the key to controlling capillary forces among particles, and is therefore a useful means to direct assembly of interfacial particles. We measured the shape of the contact line around millimeter-sized PDMS-coated glass spheres at water/air interfaces with anisotropic shapes. We studied the advancing and receding conditions separately. We focused on interfaces with a cylindrical shape, where the predominant deformation of the meniscus and the contact line both have quadrupolar cos(2ϕ) symmetry. We related the measured magnitude of the quadrupolar deformation to the applied vertical force on the sphere and the interface's deviatoric curvature, D. For modest curvature (D < 0.1 × sphere radius), our results agree with the theoretical prediction for free particles. At higher curvature, the measurements exceed the theory. The theory appears to apply even when there is contact-angle hysteresis, as long as the measured contact angle is used rather than the equilibrium (Young-Dupré) angle. The magnitude of the quadrupolar deformation depends on the applied force. Together, these results show the range of validity of the theory.
当颗粒在液体界面上吸附时，三相接触线几何形状取决于颗粒和液体界面的形状。接触线的形状是控制颗粒间毛细力的关键，因此是指导界面颗粒组装的有用手段。我们用各向异性形状的水/空气界面上涂有 PDMS 的毫米大小的玻璃球测量了接触线的形状。我们分别研究了前进和后退条件。我们关注的是具有圆柱形形状的界面，其中，弯月面和接触线的主要变形都具有四极 cos(2ϕ)对称。我们将测量到的四极变形量与施加在球体和界面上的垂直力以及界面的偏曲率 D 联系起来。对于适度的曲率（D < 0.1 × 球体半径），我们的结果与自由颗粒的理论预测相符。在更高的曲率下，测量结果超过了理论值。只要使用测量得到的接触角而不是平衡（杨氏-杜普雷）角，该理论似乎适用于即使存在接触角滞后的情况。四极变形的大小取决于所施加的力。这些结果共同展示了该理论的适用范围。
