Two-dimensional colloidal particle assembly in ionic surfactant solutions under an oscillatory electric field

Assembling colloidal particle under oscillatory electric field (OEF) has become an emerging bottom-up technique to synthesize advanced materials and fabricate micro/nano devices. Surfactants have been often used in colloid dispersions for improving their stability. However, the precise roles of surfactants in particle assembly processes under OEF are unknown. Here, we systematically study the effects of ionic surfactants on the colloidal particle assembling process subject to a fixed OEF, by using an anionic surfactant (sodium dodecyl sulfate, SDS) and a cationic surfactant (cetyltrimethylammonium bromide, CTAB). In the concentration range below the critical micelle concentration (CMC), the particles tend to form three different aggregate structures, including randomly close-packed (RCP), hexagonally closed-packed (HCP) crystal, and randomly nonclose-packed (RNCP) structures. In the concentration range at CMC and above, the particles cannot assemble into any aggregate structures under the given OEF. With increasing the surfactant concentration, the average interparticle separation distance within the aggregate can be regulated in a wide range. We qualitatively interpret the observed variation of particle assembling behaviors in different surfactant solutions based on the particle/particle interaction energy, implying that the repulsive EDL interaction is significantly enhanced with increasing the ionic surfactant concentration and hinders the particles approaching close with high energy barrier. These results suggest that the particle aggregate structures and interparticle separation distance of the 2D colloidal assembly can be manipulated by controlling the ionic surfactant concentration according to the requirements of practical applications.