Electric Field‐Directed Convective Assembly of Ellipsoidal Colloidal Particles to Create Optically and Mechanically Anisotropic Thin Films

Abstract

A method of simultaneous field‐ and flow‐directed assembly of anisotropic titania (TiO2) nanoparticle films from a colloidal suspension is presented. Titania particles are oriented by an alternating (ac) electric field as they simultaneously advect towards a drying front due to evaporation of the solvent. At high field frequencies (ν > ∼25 kHz) and field strengths (E > 300 V cm−1), the particles orient with their major axis along the field direction. As the front recedes, a uniform film with thicknesses of 1–10 µm is deposited on the substrate. The films exhibit a large birefringence (Δn ≈ 0.15) and high packing fraction (ϕ = 0.75 ± 0.08), due to the orientation of the particles. When the frequency is lowered, the particle orientation undergoes a parallel–random–perpendicular transition with respect to the field direction. The orientation dependence on field frequency and strength is explained by the polarizability of ellipsoidal particles using an interfacial polarization model. Particle orientation in the films also leads to anisotropic mechanical properties, which are manifested in their cracking patterns. In all, it is demonstrated that the field‐directed assembly of anisotropic particles provides a powerful means for tailoring nanoparticle film properties in situ during the deposition process.

Elucidating the Mechanisms of Two Unique Phenomena Governed by Particle-Particle Interaction under DEP: Tumbling Motion of Pearl Chains and Alignment of Ellipsoidal Particles

“…Both the electrostatic potential energy and torque predict an orientation of perpendicular alignment. These modeling results agree with both theoretical prediction and the experimental results by Mittal and Furst [ 18 , 19 ] where the titanium oxide ellipsoidal particles align perpendicularly to the electric field at low frequency due to high surface conductance. At high frequency, the particles undergo transition from pDEP to nDEP and start to align parallel to the electric field.…”

Role of particle shape anisotropy on crack formation in drying of colloidal suspension

“…From SEM micrograph it is clear that, ellipsoidal particles are arranged in a closed packed structure with major axis of the particle parallel to the crack direction. A similar ordered arrangement of ellipsoidal particles in particulate films fabricated via convective and vertical deposition have been reported 9 11 . We have recently shown that the ordering and hence the particle arrangement in the film is dictated by particle surface charge 11 .…”

Magnetic Field Aligned Assembly of Nonmagnetic Composite Dumbbells in Nanoparticle-Based Aqueous Ferrofluid

“…Figure shows snapshots of the dumbbells observed 1 min after initiation of 1 MHz electric field applications at 25, 50, and 100 V/mm (also see Figure S5). The applications at field strengths of 50 and 100 V/mm could form field-oriented chain structures similar to those observed in Figure b. It seemed that dumbbell chains observed at 100 V/mm were stiffer than those at 50 V/mm (compare Movie 2 (la5b00737_si_003.avi) for 50 V/mm and Movie 3 (la5b00737_si_004.avi) for 100 V/mm).…”