(526d) Induced-Dipolar and Hydrodynamic Interactions in Multi-Particle Ensembles

Directing the assembly of colloidal particles into ordered 2D and 3D arrays has become an important approach for creating useful structures at micro- and nano-meter length scales. Such structures have a variety of applications in photonic materials, sensors and opto-electronic components. Directing self assembly with AC electric fields offers an economic, reproducible and scalable method for creating large colloidal particle assemblies from a suspension. To be able to control the macro-structures using a bottom-up approach to assembly, a fundamental understanding of the various forces and overall dynamics of the particle ensembles in the suspension is necessary.

This work addresses: (1) fundamental investigations of hydrodynamic interactions between multiple particles close to a no-slip surface and comparisons of experimental data with predictive modeling (Stokesian dynamics simulations) and, (2) investigations of colloidal particle interactions under external AC fields and spontaneous pattern formations in the suspension.

Using blinking holographic optical traps, we investigate hydrodynamic interactions in multi-particle ensembles, influenced by a no-slip surface. We systematically experiment with three- and seven-particle ensembles in different configurations over a range of particle-particle separations and ensemble-wall distances. We observe that with increasing proximity with the surface, the effect of particle-particle hydrodynamic interactions on the short-time self-diffusivities is screened. To understand the correlated motion of particles within the ensembles, we use a Stokeslet description of particles and combine it with the method of images. Analysis of the resultant ensemble eigen-modes reveals that even in dilute suspensions, the effective diffusivities decay as the inverse of the separations. Furthermore, we find quantitative agreement between experimental and Stokesian dynamics simulation results. Analysis of the corresponding residuals of the effective diffusivities confirms that with increasing number of particles in the suspension, the particle-particle interactions over-ride the strong hydrodynamic screening by the wall.

Next, we measure the pair interactions of particles under AC electric fields at varying separations as a function of electric field frequencies and double layer thicknesses. We observe that interactions between particles decrease with increasing frequencies and electrolyte concentrations, a phenomenon consistent with the polarization of the double layer. At short separations, we report for the first time an anomalous doublet rotation of particle pairs measured with optical tweezers. We also show that these anomalous interactions are primarily responsible for the state transitions in colloidal suspensions from an ordered state into a disordered band-like state. The band state consists of large scale particle vortices counter-rotating in the fluid cell. The connection we make between particle interactions and suspension microstructure answers a long-standing debate regarding the mechanism underlying the band structures in experiments employing parallel electrode geometries.