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

Abstract

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.

Revealing the Existence of Long-Range Liquid–Liquid Interfacial Potential in Phase-Transfer Processes

“…Phase transfer mechanisms have been examined experimentally using several surface selective spectroscopy techniques (e.g., X-ray and neuron reflectivity measurements) that lack the spatial field of view needed to probe the behavior of PTC beyond the geometric interface. − This problem is circumvented by wide-field fluorescence microscopy (WFM) that allows the real-time visualization of the behavior of single molecules/nanoparticles close to the interface. − Optical microscopy based techniques have been extensively used to study the dynamics and motion of particles adsorbed onto liquid–liquid interfaces. − We will use WFM to track the diffusional motion of single nanoparticle-based PTC (i.e., nPTC) close to and far from the liquid–liquid interface. Compared to using single molecules of PTC (e.g., quaternary ammonium salts such as tetraoctylammonium bromide), nPTC display significantly slower diffusivity that allows their motion to be studied using WFM.…”