Colloidal molecules assembled from binary spheres under an AC electric field.

Colloidal particles are envisioned as analogues of atoms and molecules, however they often lack the complexities present in their counterparts. In this work, we report the assembly of colloidal molecules from a binary mixture of polystyrene spheres (1, 1.6, 2, and 4 µm) under an alternating current electric field. The rich family of assembled oligomers typically consists of a large sphere that is closely surrounded by a number of smaller petal particles, driven by the dipolar attraction between large and small particles. In deionized water, the number of satellite particles, i.e., the coordination number increases with the increasing size ratio of the constituent particles. For a given size ratio, the coordination number decreases with the increasing frequency of the applied field. These trends have also been correctly captured by computing the electric energy of different oligomers based on induced dipolar and double-layer interactions. By suspending the particles in polyvinylpyrrolidone aqueous solution, we can further tune the bond length of the oligomers independent of their coordination numbers. The addition of polyvinylpyrrolidone also allows us to lock the assembled colloidal molecules so that they remain intact after the electric field is turned off. Our method provides a robust way to produce a family of colloidal molecules with well-defined geometry and high yield.

Electrolyte-Dependent Aggregation of Colloidal Particles near Electrodes in Oscillatory Electric Fields.

Colloidal particles adjacent to electrodes have been observed to exhibit drastically different aggregation behavior depending on the identity of the suspending electrolyte. For example, particles suspended in potassium chloride aggregate laterally near the electrode upon application of a low-frequency (∼100 Hz) oscillatory electric field, but the same particles suspended in potassium hydroxide are instead observed to separate. Previous work has interpreted the particle aggregation or separation in terms of various types of electrically induced fluid flow around the particle, but the details remain poorly understood. Here we present experimental evidence that the aggregation rate is highly correlated to both the particle zeta potential and the electric field amplitude, both of which depend on the electrolyte type. Measurement of the aggregation rate in 26 unique electrolyte-particle combinations demonstrates that the aggregation rate decreases with increasing zeta potential magnitude (i.e., particles with a large zeta potential tended to separate regardless of sign). Likewise, direct measurements of the oscillatory electric field in different electrolytes revealed that the aggregation rate was negatively correlated with solution conductivity and thus positively correlated with the field strength. We tested the experimentally measured aggregation rates against a previously developed point dipole model and found that the model fails to capture the observed electrolyte dependence. The results point to the need for more detailed modeling to capture the effect of electrolyte on the zeta potential and solution conductivity to predict fluid flow around colloids near electrodes.