Study of the Molecular Level Mechanism of Nanoscale Alternating Current Electrohydrodynamic Flow

TL;DR

This paper uses molecular dynamics simulations to explore the molecular mechanisms of nanoscale AC electrohydrodynamic flow, revealing heat generation, temperature gradients, and directional flow in asymmetric electrode configurations at high frequencies.

Contribution

It uncovers a novel nanoscale AC-EHD flow mechanism operating at high frequencies, independent of ionic concentration, and analyzes the effects of electrode asymmetry on flow directionality.

Findings

Localized heat generation near electrodes increases with frequency.

Asymmetric electrodes produce a net directional flow.

Flow is driven by temperature gradients and electric field effects.

Abstract

This study investigates the molecular-level mechanism of Alternating Current Electrohydrodynamic (AC-EHD) flow in nanopores under high-frequency conditions, using molecular dynamics simulations. A gold-NaCl system with symmetric and asymmetric electrode configurations is used to analyze the flow patterns under high-frequency AC potentials. Our findings reveal localized heat generation near the electrode leading to a steep temperature gradient. An order parameter analysis indicates that the heat generation is due to the periodic change in the alignment of water molecules under AC potentials. At these high frequencies the influence of Na$^+$ and Cl$^-$ ions are negligible. The heat generation and temperature gradient are found to increase with the applied AC frequency. Three different electrode configurations were studied by varying the size and distance between the electrodes. A net directional flow develops in the asymmetric electrode structures. A possible mechanism for this is proposed by analyzing the flow patterns using velocity and temperature profiles, order parameters, streamline plots and mean square displacements. Different effects on the fluid were identified including those associated with temperature gradients, temperature-dependent fluid properties, and non-uniform electric fields. The asymmetric electrode structure created an imbalance in these effects and generated a net directional flow. These findings suggest the existence of a form of nanoscale AC-EHD flow that operates in a frequency regime above that of conventional electroosmotic and electrothermal mechanisms and that, unlike these mechanisms, occurs independently of ionic concentration. Thereby this work provides insights for optimizing AC-EHD flow in nanoscale systems where precise fluid manipulation is critical.