Role of Geometry and Amphiphilicity on Capillary-Induced Interactions between Anisotropic Janus Particles

Abstract

We study the capillary interactions between ellipsoidal Janus particles adsorbed at flat liquid-fluid interfaces. In contrast to spherical particles, Janus ellipsoids with a large aspect ratio or a small difference in the wettability of the two regions tend to tilt at equilibrium. The interface deforms around ellipsoids with tilted orientations and thus results in energetic interactions between neighboring particles. We quantify these interactions through evaluation of capillary energy variation as a function of the spacing and angle between the particles. The complex meniscus shape results in a pair interaction potential which cannot be expressed in terms of capillary quadrupoles as in homogeneous ellipsoids. Moreover, Janus ellipsoids in contact exhibit a larger capillary force at side-by-side alignment compared to the tip-to-tip configuration, while these two are of comparable magnitude for their homogeneous counterparts. We evaluate the role of particles aspect ratio and the degree of amphiphilicity on the interparticle force and the capillary torque. The energy landscapes enable prediction of micromechanics of particle chains, which has implications in predicting the interfacial rheology of such particles at fluid interfaces.

Controllable Capillary Assembly of Magnetic Ellipsoidal Janus Particles into Tunable Rings, Chains and Hexagonal Lattices

“…In the absence of an external magnetic field, an isolated ellipsoidal Janus particle adsorbed at an interface takes its equilibrium orientation to minimize the total adsorption free energy. [22,23] The total adsorption free energy is written as E = γ 12 A 12 + γ a1 A a1 + γ p2 A p2 + γ a2 A a2 + γ p1 A p1 , where γ ij are the interface tensions between phases i and j and A ij are the contact surface areas between phases i and j, where i, j = {1: fluid, 2: fluid, a: apolar, p: polar}. There is no exact analytical expression for the free energy of a tilted Janus ellipsoid at an interface, due to the difficulty in modeling the shape of the deformed interface and the segment area of the ellipsoid.…”

“…A pair of Janus ellipsoids interacting through hexapolar or dipolar capillary interactions prefers to align in a side-side configuration ( Figure S3, Supporting Information), corresponding to a capillary energy minimum configuration. [17,23] However, many-body effects in the capillary assembling of multiple Janus ellipsoids under external magnetic fields remain to be explored. In Figure 3 we show the assembled structures for particle surface fractions Φ = 0.16, 0.62 that form as we vary the dipole-field strengths B x and B z .…”

Equilibrium Orientation and Adsorption of an Ellipsoidal Janus Particle at a Fluid–Fluid Interface

“…For homogeneous ellisoidal particles and rough particles, the leading term is the quadrupole term (m = 2) [37]. However, the final state of a symmetric Janus particle at a flat fluid interface is found to have a leading order of a hexapolar symmetry of the interface deformation around an ellipsoidal particle in a tilted orientation [15,38]. In this case, Equation 21withm = 3 and B 3 = ∆φm(m = 3) = 0 reduces to…”

Effect of Geometric and Chemical Anisotropy of Janus Ellipsoids on Janus Boundary Mismatch at the Fluid–Fluid Interface

“…Particularly, particles adopted tilted configurations when the geometric effect (e.g., aspect ratio, AR ) is stronger than the chemical effect (e.g., wettability), whereas particles under the opposite conditions adopted upright configurations. Janus particles with tilted configurations likely deformed their surrounding fluid interface due to unpreferred wetting (e.g., apolar surfaces exposed to water, and vice versa), and the resulting interface deformation led to lateral capillary interactions between particles to minimize the surface areas of the deformed interfaces [33,34,35,36,37,38,39]. Capillary interactions between particles at the interface directly affected their microstructure, and therefore the rheological properties of the particle-laden interface.…”