Multimodal Oscillatory Driving Forces and Precise Manipulation of Particle Motion

The goal of this project is to answer the question: how do “multimodal” oscillatory forces affect the motion of particles and other objects? Most prior research has focused on “unimodal” forces, which can be characterized by a single frequency (e.g., 1 Hz). In contrast, a multimodal driving force involves the superposition of multiple frequencies (e.g., 1 Hz and 2 Hz).

Although multimodal oscillatory forces are more complicated, an object subject to a multimodal force may not move anywhere on average because the average of the force itself is zero. Recent experimental work, however, has revealed that this interpretation is not necessarily correct. Specifically, application of a dual-mode oscillatory electric field induces net motion of particles preferentially towards one electrode, but only if the two frequency modes are the ratio of odd and even numbers (e.g., 3 Hz and 2 Hz).

Further preliminary experiments revealed that net motion is also observed for solid objects placed on a flat surface made to vibrate laterally with dual frequency modes, again only if the modes are the ratio of odd and even numbers. These similar observations in two very distinct systems suggest that certain types of multimodal forces can be exploited to precisely manipulate the motion of objects using oscillatory forces.

A series of experiments and modeling studies will be conducted on three prototype systems to uncover the mechanism that governs the net motion in response to a multimodal field. Results from the project will be useful in a variety of technological applications, including particle manipulation for lab-on-a-chip devices, electrostatic dehydrators, and particle separations for granular materials.

Based on the preliminary observations, this project tests the hypothesis that the net particle electrophoresis and the net vibratory drift both stem from symmetry breaking in the nonlinear driving terms of the respective governing momentum balances, provided the oscillatory driving force is “non-antiperiodic.” High-speed video and particle imaging velocimetry will be used to precisely track the motion of objects in three different experimental systems: particle electrophoresis, charged droplet levitation, and vibrated solid objects. Specific experiments will be aimed at directly testing the hypotheses generated from the proposed mechanism, by systematically varying the ratio of imposed modes, the magnitude of the nonlinear resistances, and the overall amplitude of the driving force.

The experimental observations will be tested against corresponding numerical models specific to each system, providing groundbreaking insight on the behavior of nonlinear systems with multimodal fields. Because nonlinear effects are ubiquitous in nature, if corroborated the mechanism proposed here will provide fundamental and broadly applicable impact on how multimodal driving forces can be used to manipulate particle motion.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.