Single-particle tracking of the formation of a pseudoequilibrium state prior to charged microgel cluster formation at interfaces

The correlation between micron-sized particles and their self-assembly at fluid interfaces is important in several applications, including the stabilization of Pickering emulsions and creation of colloidosomes. In this study, through real-time visualization of the diffusion of microgel particles at the air-water interface of an aqueous pendant drop, the formation of a pseudoequilibrium state is observed prior to cluster formation. It is shown here that at the microscopic level, a pendant drop surface has nonuniform principal curvatures and exhibits positive deviatoric curvature (+c) gradients. The +c gradients confer superdiffusive motion to single ionic microgel particles and are responsible for bringing particles that are initially far apart to common sites on the interface with high curvatures. Prior to two-particle cluster formation, the balance between pairwise repulsion, capillary attraction and +c-induced energy that pushes the pair of particles to a high curvature creates a pseudoequilibrium state where the interparticle distance remains relatively invariant for a long period of time. This observation is also noted during higher-order cluster formation. Thereafter, a sufficiently strong long-range attraction potential is activated to facilitate cluster formation. Real-time tracking of the evolution of cluster formation provides useful insights into the interplay between various interactions experienced by ionic microgels. Emulsions: Keeping track of stabilizing particlesTracking the motion of microparticles in a liquid in real time can lead to a better understanding of how they self-assemble into clusters. Some liquids do not mix when combined; instead, droplets of one liquid become suspended in the other, forming an emulsion. Solid particles can be added to stabilize the emulsion and prevent the liquids separating. Wuguo Bi from Harbin Engineering University, China, and Edwin Yeow from Nanyang Technological University, Singapore, investigated these so-called Pickering emulsions by real-time motion tracking of microgel particles at the air-water interface of a droplet. Using a technique called wide-field fluorescence microscopy with a camera that recorded images at a speed of 29 frames per second, the researchers were able to see the microparticles reach a pseudo-equilibrium state before forming a cluster. Nonuniform principal curvatures of air-water interfaces confer superdiffusive motion to single charge microgels attached onto the interface, and are responsible to bring microgel particles to common sites for cluster formation. The balance between various forces prior to cluster formation gives rise to a pseudoequilibrium state.