58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Janus particles are particles that have two faces: one polar and the other apolar. Because of their amphiphilicity, it is hypothesized that Janus particles would be very effective in the stabilization of multiphasic fluid mixtures such as emulsions. Inspired by the effect of chemistry and shape on the surfactancy of molecular amphiphiles, we are investigating how the shape and surface wettability of Janus particles affect their behavior at fluid interfaces using both experimental and numerical approaches. In the first year of this grant, we showed that 1) Janus spheres at an air-water interface interact with each other via attractive capillary interactions, 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 spheres.

In the second year, we investigated 1) the lateral interaction of Janus cylinders at an oil-water interface, 2) the behavior of asymmetrically hydrophilic cylinders at the air-water interface, 3) spontaneous transport of spherical particles through three phase fluid boundary between air, water and oil, and 4) thermodynamic stability of Janus dumbbell stabilized emulsions. Below is the summary of our work in the second year:

Geometrically and chemically anisotropic particles at an oil-water interface (Park et al., Soft Matter, 2013, 9, 3383-3388)

Analogous to molecular amphiphiles, the behaviors of amphiphilic particles at fluid interfaces are expected to be significantly influenced by their shape and chemistry. In collaboration with Prof. C.S. Lee at ChungnamNational University,we presented the interfacial behavior of Janus cylinders with chemical and geometrical anisotropy. Janus cylinders with a small aspect ratio adopted an upright configuration, whereas the particles with a large aspect ratio exhibited both the upright and tilted configurations. These observations could be explained by the presence of two minima in the attachment energy profile and are consistent with the theoretical calculations we have presented in the first year of the grant. Unique configurations of these Janus particles have significant influences on their assembly and lateral interactions. Particularly, strong capillary attractions between two tilted Janus cylinders were observed and their scaling behavior of the interaction depended on the lateral alignments of two cylinders.

Double Hydrophilic Janus Cylinders at an Air–Water Interface (Park et al., Langmuir, 2013, 29, 1841-1849)

In this study, we investigated the behavior of asymmetrically hydrophilic Janus cylinders trapped at an air–water interface. We found that these “double hydrophilic” Janus cylinders can adopt either end-on or tilted configurations with respect to the interface. Our numerical calculations showed that the coexistence of these configurations is a result of multiple energy minima present in the attachment energy profile that can be represented as a complex energy landscape. The pair interactions between double hydrophilic Janus cylinders and the non-derterministic assembly behaviors were attributed to hexapolarinterface deformation that is caused by these particles.

Thermodynamically Stable Emulsions Using Janus Dumbbells as Colloid Surfactants (Tuand Lee, Langmuir, 2013, 29, 12679-12687)

For many applications, it is important that emulsions have excellent stability; however, most emulsions tend to destabilize over time due to coarsening and coalescence. Although the stability of emulsions stabilized with homogeneous particles has been shown to be superior to that of surfactant-stabilized emulsions, these Pickering emulsions nevertheless are only kinetically stable. In this theoretical study, we considered thermodynamics of emulsion stabilization using amphiphilic Janus dumbbells, which are nonsphericalparticles made of two partially fused spherical particles of opposite wettability. Our theoretical calculation demonstrated that Janus dumbbells can indeed generate thermodynamically stable Pickering emulsions. In addition, we also found that there exists a total oil–water interfacial area that results in the lowest energy state in the system, which occurs when Janus dumbbells available in the system are completely consumed to fully cover the droplet interfaces. We demonstrated that the geometry of dumbbells as well as the composition of the emulsion mixtures has significant influences on the average size of dumbbell-stabilized emulsions. We also showed that the size ratio of the two spheres of Janus dumbbells can change the average size of emulsions.

Spontaneous Particle Transport through a Triple-Fluid Phase Boundary (Park and Lee, Langmuir, 2013, 29, 9662-9667)

The behavior of particles near or at triple phase boundaries between air, water, and oil, has not been studied extensively. Such investigation is important in understanding the efficacy of oil adsorbent particles in the remediation of oil spills or the interaction of soil with complex mixtures of air, oil, and water in enhanced oil recovery.Here, we investigated the behavior of a single spherical particle in the vicinity of an air–water–oil triple phase boundary. The triple phase boundary was formed by placing a thin oil lens at an air–water interface. We found that the particle spontaneously transport across the triple phase boundary. This movement of particles occurred in two distinct transition stages: a particle initially accelerated upon its adsorption to the air–water–oil triple phase boundary from the air–water interface; subsequently, the particle decelerated after spontaneously detaching from the triple phase boundary. In the first stage, the difference in the particle attachment energy to the three fluid–fluid interfaces was responsible for the observed initial acceleration. Once the particle detached from the air–oil interface and resided solely at the oil–water interface, the particle decelerated due to viscous drag. The shape of oil lens as well as the size of the particles wereshown to have a significant influence on the dynamics of particle transport through the triple-fluid phase boundary. In addition to these original reports, we published a review article summarizing recent developments on the interfacial behavior of Janus particles in the “Emerging Investigator” issue of Soft Matter (Kumar et al., Soft Matter, 2013, 9, 6604-6617). In total, we have published 8 papers based on the DNI grant. The PI has also received numerous awards including AIChE Nanoscale Science and Engineering Young Investigator Award, 3M NontenuredFaculty Award and KIChE Presidential Young Investigator Award.