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A flowFET is a
microfluidic 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 ...
component which allows the rate of flow of
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
in a microfluidic channel to be modulated by the
electrical potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
applied to it. In this way, it behaves as a microfluidic analogue to the
field effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs control ...
, except that in the flowFET the flow of liquid takes the place of the flow of
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
. Indeed, the name of the flowFET is derived from the naming convention of electronic FETs (e.g.
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
,
FINFET A fin field-effect transistor (FinFET) is a multigate device, a MOSFET (metal-oxide-semiconductor field-effect transistor) built on a substrate where the gate is placed on two, three, or four sides of the channel or wrapped around the channel, f ...
etc.).


Mechanism of action

A flowFET relies on the principle of electro-osmotic flow (EOF). In many liquid-solid
interfaces Interface or interfacing may refer to: Academic journals * Interface (journal), ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Lin ...
, there is 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 ...
that develops due to interactions between the two phases. In the case of a microfluidic channel, this results in a charged layer of liquid on the periphery of the fluid column which surrounds the bulk of the liquid. This electric double layer has an associated
potential difference Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
known as the
zeta potential Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for electrokinetic potential in coll ...
. When an appropriately-oriented electrical field is applied to this interfacial double layer (i.e. parallel to the channel and in the plane of the electric double layer), the charged liquid ions experience a motive
Lorentz force In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
. Since this layer sheaths the fluid column, and since this layer moves, the entire column of liquid will begin to move with a speed \nu_. The velocity of the fluid layer " diffuses" into the bulk of the channel from the periphery towards the centre due to viscous coupling. The speed is related to the strength of the electric field E, the magnitude of the zeta potential \zeta, the
permittivity In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ''ε'' ( epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in ...
\epsilon and the
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
\eta of the fluid: \nu_= \zeta E In a FlowFET, the zeta potential between the channel walls and the fluid can be altered by applying an electrical field ''perpendicular'' to the channel walls. This has the effect of altering the motive force experienced by the mobile liquid atoms in the double layer. This change in the zeta-potential can be used to control both the magnitude and direction of the electro-osmotic flow in the microchannel. The controlling voltage need only be in the range of 50 V for a typical microfluidic channel, since this correlates to a gradient of 1.5 MV/cm due to the channel size.


Operational limitations

Variation of the FlowFET dimensions (e.g. insulating layer thickness between the channel wall and gate electrode) due to the manufacturing process can lead to inexact control of the zeta potential. This can be exacerbated in the case of wall contamination, which can alter the channel wall surface's electrical properties adjacent to the gate electrode. This will affect the local flow characteristics, which may be especially important in chemical synthesis systems whose
stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
are directly related to the transport rate of reaction
precursors Precursor or Precursors may refer to: * Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of un ...
and reaction products. There are constraints placed on the fluid that can be manipulated in a FlowFET. Since it relies on EOF, only fluids producing an EOF in response to an applied electric field may be used. While the controlling voltage need only be on the order of 50V, the EOF-producing voltage along the channel axis is larger, on the order of 300V. It is noticed experimentally that
electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...
may occur at the
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
contacts. This water electrolysis can alter the pH in the channel and adversely affect
biological cells The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a membrane, and contains many biomolecules such as proteins, DNA and RNA, as well as many small molecules of nutrients an ...
and
biomolecule A biomolecule or biological molecule is a loosely used term for molecules present in organisms that are essential to one or more typically biological processes, such as cell division, morphogenesis, or development. Biomolecules include large ...
s, while gas bubbles tend to "clog" microfluidic systems. In further analogy with
microelectronic Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture (or microfabrication) of very small electronic designs and components. Usually, but not always, this means micrometre-sc ...
systems, the switching time for a flowFET is
inversely proportional In mathematics, two sequences of numbers, often experimental data, are proportional or directly proportional if their corresponding elements have a constant ratio, which is called the coefficient of proportionality or proportionality constan ...
to its size. Scaling down a flowFET results in a reduction in the amount of time for the flow to equilibrate to a new flow rate following a change in the applied electrical field. It should be noted, however, that the frequency of flowFET is many orders of magnitude slower than with an electronic FET.


Applications

A FlowFET sees potential uses in massively parallel microfluidic manipulation, for example in
DNA microarray A DNA microarray (also commonly known as DNA chip or biochip) is a collection of microscopic DNA spots attached to a solid surface. Scientists use DNA microarrays to measure the expression levels of large numbers of genes simultaneously or to ...
s. Without using a FlowFET, it is necessary to control the rate of EOF by changing the magnitude of the EOF-producing field (i.e. the field parallel to the channel's axis) while leaving the zeta potential unaltered. In this arrangement, however, simultaneous control of EOF in channels connected with each other cannot easily be accomplished. A FlowFET provides a way of controlling microfluidic flow in a way that uses no moving parts. This is in stark contrast to other solutions including pneumatically-actuated peristaltic pumps such as presented by Wu et al. Fewer moving parts allows less opportunity for mechanical breakdown of a microfluidic device. This may be increasingly relevant as large future iterations of large microelectronic fluidic (MEF) arrays continue to increase in size and complexity. The use of bi-directional electronically-controlled flow has interesting options for particle and bubble cleaning operations.


See also

*
Fluidics Fluidics, or fluidic logic, is the use of a fluid to perform analog signal, analog or Digital data, digital operations similar to those performed with electronics. The physical basis of fluidics is pneumatics and hydraulics, based on the theoret ...
*
Microfluidics 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 ...
*
Electro-osmosis Electroosmotic flow (or electro-osmotic flow, often abbreviated EOF; synonymous with electroosmosis or electroendosmosis) is the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or an ...
*
Lab-on-a-chip A lab-on-a-chip (LOC) is a device that integrates one or several laboratory functions on a single integrated circuit (commonly called a "chip") of only millimeters to a few square centimeters to achieve automation and high-throughput screening. ...


References

{{Reflist Fluid dynamics Nanotechnology Biotechnology