Curvature capillary repulsion

Abstract

Directed assembly of colloids is an exciting field in materials science to form structures with new symmetries and responses. Fluid interfaces have been widely exploited to make densely packed ordered structures. We have been studying how interface curvature can be used in new ways to guide structure formation. On a fluid interface, the area of the deformation field around adsorbed microparticles depends on interface curvature; particles move to minimize the excess area of the distortions that they make in the interface. For particles that are sufficiently small, this area decreases as particles move along principle axes to sites of high deviatoric curvature. We have studied this migration for microparticles on a curved host interface with zero mean curvature created by pinning an oil-water interface around a micropost. Here, on a similar interface, we demonstrate capillary curvature repulsion, that is, we identify conditions in which microparticles migrate away from high curvature sites. Using theory and experiment, we discuss the origin of these interactions and their relationship to the particle's undulated contact line. We discuss the implications of this new type of interaction in various contexts from materials science to microrobotics.

Capillary force on an ‘inert’ colloid: a physical analogy to dielectrophoresis

“…The effect of a static contact-line undulation, e.g., due to an irregular particle shape 14 or surface roughness, 23,24,30 is not our primary focus (although it is briefly considered in Appendix E). As was discussed in Section 2, such static undulations can produce a permanent capillary quadrupole (as well as higher multipoles 31,56 ) in the interface shape. Below, we focus only on the induced quadrupole.…”

“…Colloids may create their own quadrupoles by virtue of their anisotropic shape, [12][13][14][15]52,53 surface chemistry, 3,4,54 or surface roughness. 23,24,30,31,[55][56][57] We refer to such distortions as permanent quadrupoles, following the terminology of Domínguez et al, 29 because they persist even in the absence of a background curvature. In the presence of a curvature gradient, an embedded particle with a permanent quadrupole moment experiences a lateral force F and transverse torque T. 58 Hu and Bush 59 famously showed that this coupling enables the larva of the waterlily leaf beetle to climb water menisci by simply adopting an arched body posture, which creates a quadrupolar deformation.…”

Motion of micro- and nano- particles interacting with a fluid interface

“…Surface heterogeneities and roughness distort the contact line at the nanoscale, but this is sufficient to induce capillary interactions between particles [76][77] [78], in the same way as curved menisci around anisotropic particles do [69].…”

Nanoparticle‐Shelled Catalytic Bubble Micromotor

“…The bubble raft is formed by the buoyancy of the bubbles and macroscopic deformation of the meniscus. The phenomenon of capillary curvature attraction is now well established, in prior work it was reported how interface curvature influence an attraction of microparticles . An assembly of unmodified SiO 2 and amphiphilic Janus microbubbles showed strikingly different behaviors at a convex air–water interface .…”