Nanofluidics

Nanofluidics is the study of how liquids, ions and molecules move through channels and pores whose width is between a fraction of a nanometer and about a hundred. A nanopore is the shortest case: a channel as long as the membrane is thick. The word was in use by 2005, when Eijkel and van den Berg published a review asking what the field was and what could be expected of it. It grew out of microfluidics and of older work on membranes and on the ion channels of living cells, and has two features that microfluidics lacks. The walls dominate, and the continuum picture of a fluid starts to fail.^[1]

Bocquet and Charlaix opened a 2010 review by asking what justifies the separate name. Their answer is a list of length scales that appear only at the nanometer, among them the Debye length, the slip length at a wall and the size of the molecules themselves, and the couplings between flow and electric field that arise when those scales approach the width of the channel.^[2]

A channel 10 nm across has 100,000 times more wall for each unit of liquid than a channel 1 mm across, since wall area over volume is 4 divided by the diameter. In a millimeter tube the wall is a boundary condition. In a nanopore it is most of the system.

The wall matters first through its charge. At neutral pH the surfaces of silicon nitride and silica are negatively charged, and a cloud of positive ions gathers against them, as described in Electric double layer. The cloud's thickness is the Debye length, which for a salt of two singly charged ions in water is 0.304 nm divided by the square root of the concentration in moles per liter: 0.3 nm in 1 M salt, 3 nm in 10 mM and 9.6 nm in 1 mM. A channel several Debye lengths wide has a neutral center that behaves like the bulk. A channel whose width is comparable to the Debye length is filled with counterions and passes one sign of charge much more easily than the other, which is called selectivity. The same wall charge sets the conductance in dilute solution, where Stein and colleagues found in 2004 that silica channels as thin as 70 nm conducted the same amount whatever the salt concentration or channel height. See Pore conductance for the arithmetic.^[3]

Charged walls also couple the electrical and the mechanical. A voltage along the channel pulls on the counterion cloud, which drags the water with it: electroosmotic flow. A pressure difference pushes the water and carries the cloud with it, which is a current, the streaming current. The two effects are the same coupling run in opposite directions, and a review of 2008 by Schoch, Han and Renaud collects them with ion selectivity, rectification and concentration polarization as the transport phenomena of the field.^[4] In a conical pore with charged walls, one polarity can carry more current than the other, the effect called ion current rectification.

For decades the narrowest channels that could be made had walls rough on the scale of the channel. In 2016 a group at Manchester built slits by van der Waals assembly: flat crystals as top and bottom, separated by spacers of graphene whose number of layers fixed the height, so that the structure was equivalent to removing atomic planes from a bulk crystal and leaving a flat void. Water in channels from one to several dozen atomic planes high moved unexpectedly fast, up to 1 meter per second, which the authors attributed to capillary pressures of about 1,000 bar and large slip lengths.^[5]

The same slits were then used for ions. In 2017 Esfandiar and colleagues found that ions with hydrated diameters larger than the slit can still pass, with reduced mobility, and that anions and cations of equal size behave differently.^[6] In 2019 Gopinadhan and colleagues made slits that hold a single layer of water. Water crossed without resistance, protons crossed, and every other ion was excluded, because its shell of hydration water could not fit.^[7] A potassium ion in bulk water drags a shell about 3.3 Å in radius, which makes the hydrated ion about 6.6 Å across, and a slit one layer of water high is about 3.4 Å tall.^[8]

A paper by groups in Manchester and Paris studied the streaming current in such slits and reported in 2019 that a bias of a fraction of a volt raised the pressure-driven ionic transport by up to 20 times in channels of graphite and of hexagonal boron nitride. The two materials differed markedly, and the authors traced the difference to the friction between the water and the wall.^[9] Its title uses the two words that Molecular Reality Corporation adopted in 2026 for something else, molecules passing through sensors and their data passing to the public. The nanofluidic meaning is described in Molecular streaming (nanofluidics) and the company's in Molecular streaming; each page carries a hatnote to the other.

In 2023 a group working with channels of (sub)nanometer scale reported that the transport of aqueous electrolytes across them has memory, lasting from minutes to hours depending on the channel material and the confinement. The authors used it to demonstrate Hebbian learning, in which a connection strengthens with use, in a fluidic device.^[10]

The ionic current through a solid-state nanopore is a nanofluidic measurement whether or not it is labeled one. The conductance of the pore depends on the walls, the access regions outside the pore carry surface current, and a molecule in the pore is carried by electrophoresis against an electroosmotic flow that runs the other way in a charged pore. Lee and colleagues showed in 2012 that surface conduction inside a pore reshapes the current far outside it, so that its apparent electric size can reach hundreds of nanometers.^[11]

Adjustable channels have a nanofluidic history of their own. In 2007 a group at the University of Michigan stretched plasma-treated silicone rubber until its brittle surface cracked into an array of channels whose width followed the strain; see Mechanical nanopore.^[12]

The company's early work used apertures far too wide to be nanofluidic: the stream that began in 2026 runs on a pore 200 micrometers across, drilled by laser, in which the Debye length of any salt water above 1 mM is a ten-thousandth of the width or less. Journey to Angstromia is the company's name for its plan to go from such pores toward ones a few nanometers across, and the nanofluidic problems arrive on the way. The first is getting water into the hole at all, which Kent Kemmish, the founder, called the biggest of the dumb problems; see Nanopore wetting. A bubble 5 nm in radius is held in place by a Laplace pressure of about 284 atmospheres.

• Microfluidics
• Graphene nanopore
• Surface charge (nanopore)
• Electrophoresis