Micro and nanoparticle adsorption to and assembly by capillarity at fluid-fluid interfaces are intriguing aspects of soft matter science with broad potential in the directed assembly of anisotropic media. The importance of the field stems from the ubiquitous presence of multiphase systems, the malleability of fluid interfaces, and the ability to tune the interactions of the particles adsorbed on them. While homogeneous spherical particles at interfaces have been well studied, the behavior of anisotropic particles -whether the anisotropy originates from shape or chemical heterogeneity -has been considered only very recently. We review recent advances in the field of anisotropic particles at fluid interfaces, by focusing on particles in the micron and submicron range. We discuss capillary adsorption, orientation, migration, and self-assembly, on planar and curved interfaces, and the rheology of particle-laden interfaces. Prospects for future work and outstanding challenges are also discussed.

Capillarity can be used to direct anisotropic colloidal particles to precise locations and to orient them by using interface curvature as an applied field. We show this in experiments in which the shape of the interface is molded by pinning to vertical pillars of different cross-sections. These interfaces present well-defined curvature fields that orient and steer particles along complex trajectories. Trajectories and orientations are predicted by a theoretical model in which capillary forces and torques are related to Gaussian curvature gradients and angular deviations from principal directions of curvature. Interface curvature diverges near sharp boundaries, similar to an electric field near a pointed conductor. We exploit this feature to induce migration and assembly at preferred locations, and to create complex structures. We also report a repulsive interaction, in which microparticles move away from planar bounding walls along curvature gradient contours. These phenomena should be widely useful in the directed assembly of micro-and nanoparticles with potential application in the fabrication of materials with tunable mechanical or electronic properties, in emulsion production, and in encapsulation.anisotropic particles | colloidal interactions | colloidosome | Pickering | interfacial assemblies F luid interfaces are remarkable sites for directed migration and assembly of particles (1-6). When particles distort an interface, spontaneous, long-range interactions occur owing to capillary energy, given by the product of the surface tension and the area of the distortion. When distortions induced by neighboring particles overlap, the interfacial area decreases, resulting in capillary interactions that cause particles to attract and assemble. This effect, responsible for clustering of cereal in a bowl of milk (7), is now an important means for microparticle assembly at otherwise planar fluid interfaces, in particular for anisotropically shaped objects, which assemble with preferred orientations (4, 6, 8-13). At planar interfaces, the magnitude of the interaction is determined by the particle geometry, size, and surface energies. Hence, once the particles are placed at the interface, the strength of resulting capillary interactions is fixed. Assembly occurs at random locations on the interface determined by sites of initial encounter between the particles. In this work, we show that interface curvature can be employed as an external field to direct the location at which particles assemble. The phenomenon is entirely controlled by the coupling between geometry and capillarity. Therefore, it can be applied to colloids made of any material.When an anisotropic particle is placed on a curved fluid interface, capillary interactions arise, as the area of the interface then depends on the particle's orientation with respect to the principal axes and its location in a curvature gradient, resulting in torques (5, 14) and forces (5) on the particle. Because fluid interfaces can be molded and reconfigured using pinning sites, ...

In this research, we study cylindrical microparticles at fluid interfaces. Cylinders orient and assemble with high reliability to form end-to-end chains in dilute surfaces or dense rectangular lattices in crowded surfaces owing to capillary interactions. In isolation, a cylinder assumes one of two possible equilibrium states, the end-on state, in which the cylinder axis is perpendicular to the interface, or the side-on state, in which the cylinder axis is parallel to the interface. A phase diagram relating aspect ratio and contact angle is constructed to predict the preferred state and verified in experiment. Cylinders in the side-on state create distortions that result in capillary interactions. Overlapping deformations by neighboring particles drive oriented capillary assembly. Interferometry, electron microscopy, and numerical simulations are used to characterize the interface shape around isolated particles. Experiments and numerics show that "side-on" cylinders have concentrated excess area near the end faces, and that the interface distortion resembles an elliptical quadrupole a few radii away from the particle surface. To model the cylinder interactions for separations greater than a few radii, an anisotropic potential is derived based on elliptical quadrupoles. This potential predicts an attractive force and a torque, both of which depend strongly on aspect ratio, in keeping with experiment. Particle trajectories and angular orientations recorded by video microscopy agree with the predicted potential. In particular, the analysis predicts the rate of rotation, a feature lacking in prior analyses. To understand interactions near contact, the concentrated excess area near the cylinder ends is quantified and its role in creating stable end-to-end assemblies is discussed. When a pair of cylinders is near contact, these high excess area regions overlap to form a capillary bridge between the particles. This capillary bridge may stabilize the end-to-end chains. Finally, on densely packed surfaces, cylinder-covered colloidosomes form with particles arranged in regular, rectangular lattices in the interface; this densely packed structure differs significantly from assemblies reported for colloidosomes or particle-stabilized droplets in the literature.