57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Janus particles are asymmetric particles that have two regions of opposite wettability. Because of their amphiphilicity, it is widely hypothesized that Janus particles would make an ideal solid surfactant for the stabilization of multiphasic fluid mixtures. We have studied the behaviors of Janus particles at fluid interfaces based on experimental and numerical approaches. We showed that 1) Janus spheres at an air-water interface interact with each other via attractive capillary interactions due to the deformation of the fluid interface caused by the undulating Janus boundary, 2) the equilibrium orientation of non-spherical Janus particles is significantly affected by the geometry and wettability of these particles, and 3) asymmetry in the wetting and geometric properties of non-spherical amphiphilic particles lead configurations that are not observed in Janus particles.

Janus Spheres at Fluid-Fluid Interfaces (Brugarolas et al., Adv. Func. Mater., 2011, 21, 3924)

In this study, we investigated the synthesis of Janus bubbles and their interfacial behavior at an air-water interface. Janus bubbles were generated by preparing monodisperse bubbles that are stabilized by a stiff nanoparticle shell. We quantitatively demonstrated that it is critical to control the dimension of the bubbles to produce nanoparticle-shelled bubbles that do not undergo elastic instability. Amphiphilic Janus bubbles were generated by selectively depositing gold onto one hemisphere of nanoparticle-shelled bubbles. The gold surface was subsequently modified with a self-assembled monolayer of a hydrophobic thiol, octadecanethiol, to impart hydrophobicity. These amphiphilic Janus bubbles were re-dispersed in water and their interfacial behavior was studied. Un-modified bubbles and amphiphilic Janus bubbles showed completely different assembly behaviors at a curved air-water interface. The unmodified nanoparticle-shelled bubbles did not exhibit any noticeable interaction forces, and more or less interacted with each other as "hard spheres". At the curved air-water interface, these un-modified bubbles eventually packed into close-packed hexagonal arrays as they floated to the top of the drop. In contrast, Janus bubbles with amphiphilicity assembled into fractal like structures, which strongly indicates the presence of long-range attractive interactions between the amphiphilic Janus bubbles.

We found that the observed lateral attraction between Janus bubbles at the air-water interface is due to the irregular contact line of water at the boundary between the hydrophobic and hydrophilic regions of Janus bubbles (around the Janus boundary). Because of the method used in the fabrication of Janus bubbles, the Janus boundary is not perfectly sharp and smooth but rather is intrinsically rough and undulating. When these Janus bubbles are at the fluid-fluid interface, the three phase contact line is pinned around the Janus boundary, which leads to the undulating three phase contact line. We measured the force between two interacting Janus bubbles via a particle tracking method and showed that these Janus spheres are indeed interacting with each other via quadrupolar capillary interactions due to the random undulation of fluid interface around the particles. We have also directly observed the random undulation using by optical profilometry.

Equilibrium orientation of non-spherical Janus particles at an oil-water interface (Park and Lee, ACS Nano, 2012, 6, 782)

We studied the equilibrium orientation of nonspherical Janus particles at an oil-water interface. Two types of nonspherical Janus particles were considered: Janus ellipsoids and Janus dumbbells. To find their equilibrium orientation, we calculated the attachment energy of each Janus particle as a function of its orientation angle with respect to the oil-water interface. We found that the equilibrium orientation of the interface trapped Janus particles strongly depends on the particle characteristics, such as their size, aspect ratio, and surface properties. In general, nonspherical Janus particles adopt the upright orientation (i.e., the long axis of the particle is perpendicular to the interface) if the difference in the wettability of the two sides is large or if the particle aspect ratio is close to 1. In contrast, Janus particles with a large aspect ratio or a small difference in the wettability of the two regions tend to have a tilted orientation at equilibrium. Interestingly, we found that Janus ellipsoids, under appropriate conditions, can be kinetically trapped in a metastable state due to the presence of a secondary energy minimum. In contrast, Janus dumbbells possess only a primary energy minimum, indicating that these particles prefer to be in a single orientation. The absence of a secondary minimum is potentially advantageous for obtaining particle layers at fluid-fluid interfaces with uniform orientation.

Configuration of non-spherical amphiphilic particles at an oil-water interface (Park and Lee, Soft Matter, 2012, 8, 7690)

In this work, we presented the equilibrium configuration of amphiphilic ellipsoids and amphiphilic dumbbells with asymmetric shape (i.e., unequal surface areas for apolar and polar sides) and surface wetting properties at an oil-water interface. We found that the orientation and vertical displacement of nonspherical amphiphilic particles are significantly influenced by their shape, aspect ratio, surface properties, and the location of the wettability separation line. In particular, due to the asymmetry in the particle geometry and wettability of these amphiphilic particles, we observed unique configurations that were not observed in the study described above. In the case of amphiphilic ellipsoids, their orientation as a function of particle geometry and wettability can be divided into three regimes: upright orientation, tilted orientation, and coexistence of upright and tilted orientations due to the presence of secondary energy minima under appropriate conditions. In general, the secondary energy minimum is present when the aspect ratio of the ellipsoids becomes high. As for amphiphilic dumbbells, in addition to upright and tilted orientations, they can adopt intermediate orientations, especially when the size of the two spheres is significantly different, which leads to the detachment of one of the two spheres from the oil-water interface.

In summary, our results on the lateral interactions between Janus spheres at fluid interfaces provide critical insights on the importance of controlling the Janus boundary undulations in controlling the interfacial behavior of Janus particles. Our results based on numerical calculations provide guidelines for designing nonspherical amphiphilic particles with suitable geometry and wettability to tailor their properties as solid surfactants for emulsion stabilization and fluid interface modification.