Electrostatic interactions of poly (methyl methacrylate) colloids: deposition patterns of evaporating non-aqueous colloidal droplets

We generate controllable micrometer-range electrostatic interactions in a suspension by using a charge control additive: an anionic surfactant, dioctyl sodium sulfosuccinate (AOT), and an organic salt, tetradodecylammonium tetrakis (3,5-bis (trifluoromethyl)phenyl)borate (TDAT) in non-aqueous solvents. Both systems function via different mechanisms of altering the electrostatic interaction between poly (methyl methacrylate) (PMMA) colloids. For the AOT system, the particle surface charge is modified by the adsorption of charged and neutral AOT micelles on the surface, hence affecting the interactions between particles. The measured scaled surface potential is independent on the AOT concentration, so that the AOT-PMMA system approximates the constant surface potential (CSP) limit. For the electrolytic TDAT non-aqueous system, the screening length of the solution was altered due to the presence of free ions in the solution. This is confirmed by the conductivity and theoretical Debye length λ_D values. However, from the force measurement using the blinking optical tweezers (BOTs), it was revealed that the measured screening length κ⁻¹, and the particle effective charge Z_eff shows a non-monotonic dependence on the TDAT concentration. The deviation between these values revealed that at high TDAT concentrations, the classical DLVO theory-Debye-Hückel limit is no longer valid for the system. We relate the formation of clusters and aggregates formed in the bulk system with the deposition patterns of a colloidal droplet on a hydrophobically coated glass substrate. The drying of droplets containing monodisperse PMMA particles was studied by confocal microscopy.