One atom thick angstrom-scale capillaries: Water flow*

It has been an ultimate but seemingly distant goal of nanofluidics to controllably fabricate capillaries with dimensions approaching the size of small ions and water molecules. But surface roughness makes it challenging to produce capillaries with precisely controlled dimensions. We have developed a method for fabrication of narrow and smooth angstrom (Å) scale capillaries through van der Waals assembly of 2D-materials, with atomically flat sheets at the top and bottom separated by spacers made of 2D-crystals with a precisely controlled number of layers. These capillaries can be envisaged as if individual atomic planes are removed from a bulk layered crystal leaving behind flat voids of a chosen height.
Water transport through the channels, ranging in height from one to several dozen atomic planes, is characterized by unexpectedly fast flow (up to 1 metre per second) that we attribute to high capillary pressures of about 1000 bar and large slip lengths [1]. For channels that accommodate only a few layers of water, the flow exhibits a marked enhancement that we associate with an increased structural order. In this talk, I will also discuss about how ions reconfigure their hydration shell, becoming essentially “squashed” while flowing through these capillaries[2]. Water and ionic flows are coupled in such confinement, and the transport, driven by pressure and applied electric field, reveals a transistor-like electrohydrodynamic effect [3]. I will conclude the talk by showing our recent results where only one layer of water is confined in these channels, essentially leading to two-dimensional water [4]. Our results lay the basis for exploration of such Å-size channels in nanofluidics, molecular separation and other nanotechnologies.

[1] B. Radha et al., Nature 538, 222 (2016).
[2] A. Esfandiar et al., Science 358, 511 (2017).
[3] T. Mouterde et al., Nature 567, 87 (2019).
[4] K. Gopinadhan et al., Science 363, 145 (2019).