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

TL;DR

This work addresses the molecular-scale mechanism of nanoscale AC-EHD flow by performing MD simulations of a gold-NaCl aqueous system under very high-frequency AC up to $100~\mathrm{GHz}$. The authors demonstrate localized heating near electrodes driven by periodic water-dipole reorientation, producing a steep $z$-direction temperature gradient, with heating largely independent of ionic species. They show that net directional flow emerges only in asymmetric electrode geometries, with a strongest flow around $v_x \approx 0.78~\mathrm{m\,s^{-1}}$, and identify buoyancy, electrothermal, and Maxwell-stress/electrostriction mechanisms as collectively responsible for the observed AC-EHD flow. The study provides design insights for nanoscale fluid manipulation and suggests future work using a polarizable water model to quantify forces and extend predictive capability for nanofluidic applications.

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 explained that the heat generation is due to the periodic change in the alignment of water molecules under AC potential and that 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 forces acting on the fluid were identified such as buoyancy-driven convection due to temperature gradient, electrothermal effects influenced by the temperature-dependent properties of water, and Maxwell stress due to the non-uniform electric field. Moreover, the asymmetric electrode structure created an imbalance in these forces 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.

Figures (11)

• Figure 1: Schematic diagram of a typical simulation setup.
• Figure 2: The applied potential difference and the corresponding surface charge density as a function of time for 4.0 V DC, 1 GHz, 10 GHz, and 100 GHz AC voltages.
• Figure 3: (a) Temperature of the fluid as a function of simulation time for different frequencies. (b) Heat removed by the thermostat as a function of simulation time for different frequencies.
• Figure 4: (a) Temperature profile along the $z$ direction (top to bottom). The average temperature is 409.61 K. (b) Temperature heat map in the $x$-$z$ plane.
• Figure 5: (a) Temperature of the fluid as a function of simulation time for different thermostat settings. (b) The heat removed by the thermostat as a function of simulation time for different thermostat settings.