Interpretation of Electrostatic Self-Potential Measurements Using Interface-Trapped Microspheres with Surface Heterogeneity

Abstract

Electrostatic self-potentials of individual particles trapped at an oil–water interface were determined, and the effects of surface chemical nonuniformity on heterogeneous self-potentials and equilibrium microstructures were investigated. Direct measurement of the pair interactions and the self-potentials of polystyrene microspheres were performed using optical laser tweezers. The individual particles had different self-potentials even when they possessed the same surface functionalities. Atomic force microscopy measurements elucidated the relationship between nonuniform surface charge distribution and heterogeneity and magnitude of self-potentials. Monte Carlo simulations demonstrated that self-potential heterogeneity led to the formation of more melted microstructures that showed excellent consistency with experiments.

Quantification of polystyrene microsphere attachment probability at the oil‒water interface using a microfluidic platform

“…A flow cell was constructed with inner and outer cylinders, as previously reported [ 24 , 25 , 49 ]. Briefly, the inner hollow cylinder was a Teflon ring (9 mm in height and 12 mm in diameter) attached to the inside of an aluminum ring (8 mm in height and 14 mm in diameter).…”

“…When a particle-laden fluid interface is formed, two types of interparticle forces, the electrostatic dipolar force, and the local interface deformation-induced capillary force, become unusually strong and compete with each other, resulting in unique assembly microstructures [ [17] , [18] , [19] , [20] , [21] ]. Generally, charged spherical particles under low electrolyte conditions are governed by the electrostatic repulsive force, whereas nonspherical particles under low electrolyte conditions or spherical particles under high electrolyte conditions are dominated by the attractive capillary force [ [22] , [23] , [24] , [25] , [26] , [27] , [28] , [29] , [30] , [31] , [32] , [33] ]. Furthermore, the surface free energy associated with the attachment of particles to fluid–fluid interfaces is influenced by several factors, such as the chemical nature, size, roughness, and wettability of particles [ [34] , [35] , [36] ].…”

“…We evaluated three types of polystyrene (PS) particles with different surface functionalities and wettabilities, namely carboxyl PS (CPS), sulfonate PS (SPS), and amidine PS (APS). Interparticle interactions at the oil–water interface were directly measured using optical laser tweezers [ 24 , 46 , 47 ], and the three-phase contact angles of the particles at the interface were determined using the gel trapping method [ 48 ].…”

Retardation of Capillary Force between Janus Particles at the Oil–Water Interface

“…We believe that the retardation of the capillary force between the interface-trapped particles should be universal regardless of their shape and surface properties. In addition to the factors of nondeterministic undulation of the three-phase contact line, − non-uniform surface properties, , and kinetic interface-attachment behaviors, the azimuthal rotation-induced capillary retardation is another significant factor that aggravates the interaction heterogeneity. Given that there are various applications of interfacially dispersed particle systems, such as enhanced interface stabilization, phase-transfer catalysis, biomedical formulations, and assembly into complex structures and functional materials, this work provides an important methodology for better understanding the complexity of these systems, which may arise from randomness caused by the multiple factors of interaction heterogeneity.…”

“…We measured the pair interaction force using both direct [active (Figure e)] − and indirect [passive (Figure f)] ,, methods using optical laser tweezers. The measured force profiles were analyzed in terms of the scaling exponent of the force with interparticle separation.…”