Nanofluidic circuitry is a
nanotechnology
Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal o ...
aiming for control of
fluid
In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
s in
nanometer
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
scale. Due to the effect of an
electrical double layer
A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The D ...
within the fluid channel, the behavior of
nanofluid
A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, ca ...
is observed to be significantly different compared with its
microfluid
Microfluidics refers to the behavior, precise control, and manipulation of fluids that are geometrically constrained to a small scale (typically sub-millimeter) at which surface forces dominate volumetric forces. It is a multidisciplinary field tha ...
ic counterparts. Its typical characteristic dimensions fall within the range of 1–100 nm. At least one dimension of the structure is in
nanoscopic scale
The nanoscopic scale (or nanoscale) usually refers to structures with a length scale applicable to nanotechnology, usually cited as 1–100 nanometers (nm). A nanometer is a billionth of a meter. The nanoscopic scale is (roughly speaking) a lo ...
. Phenomena of fluids in nano-scale structure are discovered to be of different properties in
electrochemistry and
fluid dynamics
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) an ...
.
Background
With the development of microfabrication and nanotechnology, the study of microfluidics and nanofluidics is drawing more attention. Research on microfluidic found its advantages in DNA analysis, lab-on-a-chip, and micro-TAS. Devices in a microfluidic system include channels, valves, mixers, and pumps. Integration of these microfluidic devices enables sorting, transporting, and mixing of substances within fluids. However, the failure of moving parts in these systems is usually the critical issue and the main drawback. Mechanisms to control flow without using mechanical parts are always desired for reliability and lifetime.
In 1997, Wei,
Bard
In Celtic cultures, a bard is a professional story teller, verse-maker, music composer, oral historian and genealogist, employed by a patron (such as a monarch or chieftain) to commemorate one or more of the patron's ancestors and to praise t ...
and Feldberg discovered that ion rectification occurs at the tip of a nano-sized pipe. They observed that the surface charge at the wall of a nano-pipet induced a non-neutral electrical potential within the orifice. The electrical potential then modifies the concentration of ion species, resulting in an asymmetric current-voltage characteristic for the current through the pipet.
Transport of ions in the electrolyte can be adjusted by tuning the pH value in a dilute ionic solution, or by introducing an external electrical potential to change the surface charge density of the wall. As an analogy to semiconductor devices, the mechanism to control charge carrier transport in electronic devices was established in the area of nanofluidics. In nanofluidics, the active control of ion transport is realized using nano-scale channels or pores.
Research efforts on micro-scaled fluidic systems started to focus on the rectifying phenomena, which can be seen only in nano-scaled systems. In 2006, Professor Majumdar and Professor Yang in University of California, Berkeley built the first "nanofluidic" transistor. The transistor can be turn on or off by an external electrical signal, allowing the control of ionic fluids in a nano-scaled channel. Their work implies a possibility to create a nanofluidic circuitry with logic functions.
The main researchers in the area of nanofluidic devices include Arun Majumdar and Peidong Yang in University of California - Berkeley, Harold Craighead and Brian Kirbyat Cornell University, Juan Santiago at Stanford University, Albert van den Berg in University of Twente,
Zuzanna Siwy in University of California - Irvine, and Mark Shannon in University of Illinois - Urbana-Champaign.
Basic principles
For electrolyte solution in a channel with a macro- or micro-scaled radius, surface charges at the wall attract counterions and repel co-ions due to electrostatic force. Therefore, an electrical double layer exists between the wall of channel and the solution. The dimension of the electrical double layer is determined by the Debye length in this system, which is typically much smaller than the channel radius. Most of the solution in the channel is electrically neutral due to the shielding effect of the electrical double layer.
In a nanochannel, however, the solution is charged when the dimension of channel radius is smaller than the
Debye length. Therefore, it is possible to manipulate the flow of ions inside the nanochannel by introducing surface charges on the wall or by applying an external electrical potential.
Ionic concentration of solution has an important effect on the ion transport. Because a higher concentration leads to a shorter Debye length for the electrical double layer at the channel wall. Its rectifying effect decreases with increasing ionic concentration. On the other hand, ion rectification can be improved by having a dilute solution.
Ion transport
To analyze the transport of ions in the channel, behaviors of system in electrochemistry as well as fluid mechanics need to be considered. The Poisson–Nernst–Planck (PNP) equations are utilized to describe ionic current flowing through a channel, and the Navier–Stokes (NS) equations are used to represent the fluid dynamics in the channel.
The PNP equations consist of the
Poisson equation
Poisson's equation is an elliptic partial differential equation of broad utility in theoretical physics. For example, the solution to Poisson's equation is the potential field caused by a given electric charge or mass density distribution; with t ...
:
and the
Nernst–Planck equations, which gives the particle flux of ion species
due to a concentration gradient and electric potential gradient:
where
is the electrostatic potential,
is the unit charge of electron,
is the permittivity in vacuum, and
is the dielectric constant of solution;
,
and
are the diffusivity, the number density of ions, and the valence of ion species
.
The solution in steady-state satisfies the continuity equation. To describe fluid velocity field in the channel, using
Navier–Stokes equations: