Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
Nature. 2011 Aug 17;476(7360):308-11. doi: 10.1038/nature10344.
When a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in ring-like fashion. This phenomenon, known as the coffee-ring effect, is familiar to anyone who has observed a drop of coffee dry. During the drying process, drop edges become pinned to the substrate, and capillary flow outward from the centre of the drop brings suspended particles to the edge as evaporation proceeds. After evaporation, suspended particles are left highly concentrated along the original drop edge. The coffee-ring effect is manifested in systems with diverse constituents, ranging from large colloids to nanoparticles and individual molecules. In fact--despite the many practical applications for uniform coatings in printing, biology and complex assembly-the ubiquitous nature of the effect has made it difficult to avoid. Here we show experimentally that the shape of the suspended particles is important and can be used to eliminate the coffee-ring effect: ellipsoidal particles are deposited uniformly during evaporation. The anisotropic shape of the particles significantly deforms interfaces, producing strong interparticle capillary interactions. Thus, after the ellipsoids are carried to the air-water interface by the same outward flow that causes the coffee-ring effect for spheres, strong long-ranged interparticle attractions between ellipsoids lead to the formation of loosely packed or arrested structures on the air-water interface. These structures prevent the suspended particles from reaching the drop edge and ensure uniform deposition. Interestingly, under appropriate conditions, suspensions of spheres mixed with a small number of ellipsoids also produce uniform deposition. Thus, particle shape provides a convenient parameter to control the deposition of particles, without modification of particle or solvent chemistry.
当一滴液体在固体表面干燥时，其悬浮的颗粒物会以环状的方式沉积。任何观察过一滴咖啡干燥的人都对这种现象很熟悉，这种现象被称为咖啡环效应。在干燥过程中，液滴边缘会被固定在基底上，并且从液滴中心向外的毛细流动会将悬浮颗粒带到边缘，随着蒸发的进行。蒸发后，悬浮颗粒在原来的液滴边缘高度集中。咖啡环效应在具有不同成分的系统中表现出来，从大胶体到纳米粒子和单个分子。事实上，尽管在印刷、生物学和复杂组装等领域有许多均匀涂层的实际应用，但该效应的普遍存在使其难以避免。在这里，我们通过实验表明，悬浮颗粒的形状很重要，可以用来消除咖啡环效应：在蒸发过程中，椭球体均匀沉积。颗粒的各向异性形状显著变形界面，产生强烈的颗粒间毛细相互作用。因此，在球体引起的咖啡环效应将椭球体带到气液界面之后，椭球体之间强烈的长程相互吸引导致在气液界面上形成松散或停滞的结构。这些结构阻止了悬浮颗粒到达液滴边缘，从而确保了均匀的沉积。有趣的是，在适当的条件下，混合了少量椭球体的球体悬浮液也会产生均匀的沉积。因此，颗粒形状为控制颗粒沉积提供了一个方便的参数，而无需修改颗粒或溶剂化学。
