Spinning microrods in a rotating electric field with tunable hodograph

Abstract

Hypothesis

In external high-frequency rotational electric fields, the polarization of rod-like colloidal particles experiences a slight temporal delay relative to the field, resulting in a torque that acts upon the particles. This torque depends on the hodograph of the external rotating electric field (the spatial curve traced by the tip of the electric field vector as it changes over time), enabling control over the rotational dynamics of rod-like colloidal particles.

Experiments

The experiments were conducted using synthesized monodisperse silica microrods with average size of $3.29\times 1.12\times 1.12{\mu \text{m}}^3$ dispersed in deionized water, at a mass fraction of 0.2%. The external electric field was generated using an 8-electrode system, and it rotated within the system's plane along an elliptical hodograph at a frequency of 30 kHz. We used an optical microscope with magnification objective of equipped with a CCD-camera (Thorlabs). The experimental data were processed using Fiji software.

Findings

The external high-frequency rotational electric field allows for controlled imposition of three types of rotational dynamics onto rod-like colloidal particles: (i) asynchronous continuous rotation – tunable spinners, (ii) oscillations around a certain direction with sporadic rod flips – rotational jumpers with enhanced directional ordering, and (iii) a regime of “arrested” particle orientation along the principal axes of field anisotropy.