Molecular streaming and its voltage control in {\aa}ngstr\"om scale channels

TL;DR

This paper demonstrates voltage-controlled ionic streaming in angstrom-scale channels, revealing a transistor-like effect that significantly enhances flow mobility, with implications for nanofluidic device design and ionic machinery.

Contribution

It reports the discovery of a voltage-gated ionic streaming effect in molecular-scale channels, supported by a theoretical framework explaining material-dependent frictional interactions.

Findings

Voltage bias enhances ionic streaming mobility up to 20 times.

Material-dependent gating effects observed in graphite and boron nitride channels.

Theoretical model explains the nonlinear flow dynamics at molecular confinement.

Abstract

The field of nanofluidics has shown considerable progress over the past decade thanks to key instrumental advances, leading to the discovery of a number of exotic transport phenomena for fluids and ions under extreme confinement. Recently, van der Waals assembly of 2D materials allowed fabrication of artificial channels with angstr\"om-scale precision. This ultimate confinement to the true molecular scale revealed unforeseen behaviour for both mass and ionic transport. In this work, we explore pressure-driven streaming in such molecular-size slits and report a new electro-hydrodynamic effect under coupled pressure and electric force. It takes the form of a transistor-like response of the pressure induced ionic streaming: an applied bias of a fraction of a volt results in an enhancement of the streaming mobility by up to 20 times. The gating effect is observed with both graphite and boron nitride channels but exhibits marked material-dependent features. Our observations are rationalized by a theoretical framework for the flow dynamics, including the frictional interaction of water, ions and the confining surfaces as a key ingredient. The material dependence of the voltage modulation can be traced back to a contrasting molecular friction on graphene and boron nitride. The highly nonlinear transport under molecular-scale confinement offers new routes to actively control molecular and ion transport and design elementary building blocks for artificial ionic machinery, such as ion pumps. Furthermore, it provides a versatile platform to explore electro-mechanical couplings potentially at play in recently discovered mechanosensitive ionic channels.