Behavior Of Janus Particles Research Articles

Janus Particles at Fluid Interfaces: Stability and Interfacial Rheology.

The use of the Janus motif in colloidal particles, i.e., anisotropic surface properties on opposite faces, has gained significant attention in the bottom-up assembly of novel functional structures, design of active nanomotors, biological sensing and imaging, and polymer blend compatibilization. This review is focused on the behavior of Janus particles in interfacial systems, such as particle-stabilized (i.e., Pickering) emulsions and foams, where stabilization is achieved through the binding of particles to fluid interfaces. In many such applications, the interface could be subjected to deformations, producing compression and shear stresses. Besides the physicochemical properties of the particle, their behavior under flow will also impact the performance of the resulting system. This review article provides a synopsis of interfacial stability and rheology in particle-laden interfaces to highlight the role of the Janus motif, and how particle anisotropy affects interfacial mechanics.

Orientation of Janus particles under thermal fields: The role of internal mass anisotropy

Janus particles (JPs) are a special kind of colloids that incorporate two hemispheres with distinct physical properties. These particles feature a complex phase behavior, and they can be propelled with light by heating them anisotropically when one of the hemispheres is metallic. It has been shown that JPs can be oriented by a homogeneous thermal field. We show using multiscale simulations and theory that the internal mass gradient of the JPs can enhance and even reverse the relative orientation of the particle with the thermal field. This effect is due to a coupling of the internal anisotropy of the particle with the heat flux. Our results help rationalize previous experimental observations and open a route to control the behavior of JPs by exploiting the synergy of particle-fluid interactions and particle internal mass composition.

Effect of Orientation and Wetting Properties on the Behavior of Janus Particles at the Air-Water Interface.

The adhesion force and contact angle of gold-capped silica Janus particles and plain silica particles at an air-water interface are studied via colloidal atomic force microscopy. Particles are attached to cantilevers at various orientations, and wetting properties of the gold surface are varied through modification with dodecanethiol. Thiol modification increases the hydrophobicity of the gold surface, thereby increasing the difference between the contact angles of the gold hemisphere and the silica hemisphere and, thus, increasing the degree of amphiphilicity of the Janus particle. Subsequently, the colloidal probe is pushed into a stationary bubble from the water phase followed by retraction back into the water phase. Adhesion force is found to be higher for Janus particles than isotropic silica particles, regardless of orientation of the anisotropic hemisphere. Particles with their polar half oriented toward the water and apolar half facing the air show an increase in adhesion force and contact angle as the degree of amphiphilicity of the particles increases. For particles of the reverse orientation, no significant difference is observed as wetting properties change. Both adhesion force and contact angle display an inverse relationship with a cap angle for particles with a higher degree of amphiphilicity. These results are of importance for using Janus particles to stabilize interfaces as well as for understanding the equilibrium height of Janus particles at the interface, which will impact capillary interactions and thus self-assembly.

The dynamics of self-propelled Janus microspheres near obstacles with different geometries

Self-propelled particles exhibit interesting behavior when approaching boundaries or obstacles, which has been drawn a lot of attention due to its potential applications in areas of cargo delivery, sensing and environmental remediation. However, our understanding on the mechanism of how they interact with boundaries or obstacles is still limited. Here, using video particle-tracking microscopy, we experimentally studied the dynamics of self-propelled Janus microsphere driven by H<sub>2</sub>O<sub>2</sub> near obstacles. The Janus particles used are sulfuric polystyrene (PS) microspheres (hydrodynamic diameter is 3.2 μm) with only half surface being sputter-coated with a five-micron-thick platinum layer. Two different types of obstacles are used. One is cylindrical post and the other is PS microsphere. To understand the size effect of obstacles, cylindrical posts with three different diameters (3 μm, 10 μm and 20 μm), and PS microspheres with four different diameters (1.0 μm, 1.8 μm, 2.4 μm and 7.2 μm) are tested, respectively. The results show that when obstacles are larger than a critical size, the self-propelled Janus microspheres will be captured and orbit around them. The retention time and the orbiting speed of the Janus particles increase with the concentration of H<sub>2</sub>O<sub>2</sub>, as well as with the diameter of obstacles no matter whether cylindrical posts or PS microspheres are used as obstacles. However, we found that under the same concentration of H<sub>2</sub>O<sub>2</sub>, compared with the case of PS microspheres as obstacles, when Janus particles orbit around cylindrical posts, the retention time is larger and the average speed is smaller. These results indicate that the self-propelled behavior of Janus particles near obstacles is closely dependent on the geometrical properties of obstacles. Our results of Janus spheres are different from earlier work on Au-Pt Janus rods [Takagi D, Palacci J, Braunschweig A B, Shelley M J, Zhang J 2014 <i>Soft Matter</i> <b>10</b> 1784]. By comparing the speed of Janus particles before and after they are captured by spherical obstacles, for our case, the speed of Janus spheres is reduced, while for the case of Au-Pt rods, the speed of Au-Pt rods doesn’t change much. Such discrepancies may originate from different driven mechanisms in these two systems (electropheoresis mechanism for Au-Pt micro-rods and diffusiophoresis mechanism for PS-Pt Janus microspheres), which are then resulted in different flow fields and different distributions of catalytic solutions. But to test this hypothesis, further work is needed. Our study provides us a better understanding on the dynamic behavior of self-propelled particles near obstacles, which will be helpful for applications in, for example, designing micro-structures to guide the motion of self-propelled particles.

Influence of particle surface roughness on the behaviour of Janus particles at interfaces

We demonstrate that it is of critical importance to prepare Janus particles with a high degree of control over the surface roughness. To elucidate information regarding the behaviour of Janus particles at an interface, we have used a gel-trapping technique coupled with metal evaporation to prepare Janus particles. Since one face is covered in gold, direct observation of the different faces at air–water and water–oil interfaces can be carried out. For particles prepared by this gel-trapping technique, the interface between hemispheres is rough and this leads to the observed orientation being random. We show that for these particles the surface roughness can dominate over the surface chemistry with regards to particle orientation. This has important implications for many of the synthetic routes towards Janus particles suggesting that, for optimum performance, it is necessary to prepare particles where the surface roughness is kept to a minimum.