Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2000-12-13
2004-10-19
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S450000, C204S451000
Reexamination Certificate
active
06805783
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to devices for controlling the movement of electrolyte solutions. More particularly, the present invention pertains to electro-osmotic pumps. The present invention is particularly, but not exclusively, useful for an electro-osmotic pump that creates a charged surface with a ferroelectric material.
BACKGROUND OF THE INVENTION
The electro-osmotic effect can be employed to pump or otherwise control the movement of electrolyte solutions. Devices utilizing the electro-osmotic effect are particularly applicable in micro-fluidics where the manipulation of small amounts of electrolyte solution is required to perform chemical or biochemical reactions. These micro-fluidic processes are often carried out on “biochips” or “bioarrays” and have found increasing usage because only small quantities of reactants and enzymes are needed to conduct each analysis.
To explain the electro-osmotic effect, consider an electric field applied to an electrolyte solution. The electrolyte solution generally contains positive ions, negative ions and a medium for the ions such as water. When the electric field is applied to the electrolyte solution, the positive ions receive a force in the direction of the electric field. Likewise, the negative ions receive a force that is equal in magnitude to the force received by the positive ions, but applied in a direction opposite to the force on the positive ions. The result is that the solution as a whole receives a net force of zero, and the electrolyte solution as a whole does not flow. To summarize, the mere placing of an electrolyte solution in an electric field does not produce an electro-osmotic effect.
Now consider the introduction of a charged surface into the electrolyte solution. Depending on whether the charged surface is acidic (negatively charged) or caustic (positively charged), the charged surface will attract either positive or negative ions from the solution. When an electric field is applied parallel to the charged surface, the resulting electric force acting on the ions that are bound to the charged surface is generally transmitted to the charged surface. On the other hand, the oppositely charged ions (those ions not attracted to the charged surface) are free to move under the influence of the electric field. This electro-osmotic effect causes the solution to receive a net force from the applied electric field, which in turn causes the solution to flow. The direction of flow depends on the polarity of the charged surface, as well as the direction of the applied electric field.
It is known that the electro-osmotic effect can be used to effectuate a simple fluid pump. For example, a pair of electrodes (driving electrodes) can be inserted into the lumen of a tube for contact with an electrolyte solution to create an electric field along the length of the tube. In this arrangement, the inner wall of the tube can be coated with an acidic or caustic material that attracts positive or negative ions from the solution. A voltage source can then be activated to create a potential difference between the electrodes. In response, the solution will flow along the length of the tube. Note that for the simple pump described in this example, the solution will not flow in response to an alternating current (AC) applied to the electrodes, because the time-averaged force on the ions that are not attracted to the tube wall will be zero.
Electrophoresis is often used to separate charged macromolecules (by migration) from a stagnant or non-flowing solution. In these electrophoresis operations, the electro-osmotic effect is undesirable because it causes the solution to flow. To avoid the electro-osmotic effect, the vessel wall can be coated with a passivating material such as Teflon® which does not interact with either the positive ions or the negative ions.
The present invention recognizes that a ferroelectric material can be used to create the charged surface that is required to produce the electro-osmotic effect. By ‘temporarily’ applying an electric field to the ferroelectric material, the ferroelectric material can be ‘permanently’ polarized allowing creation of a charged surface for a device featuring an electro-osmotic effect. Subsequently, the ferroelectric material can be depolarized to create a passivated surface and thereby eliminate any electro-osmotic effect within the device. Depending on the application, the ferroelectric surface can be polarized to produce a surface that either attracts positive ions or negative ions. Further, the magnitude of polarization and thus the total charge placed on the ferroelectric surface can be varied during the operation of the device.
Importantly, as detailed further below, the use of a ferroelectric material allows an electro-osmotic effect to be created when an AC current is applied to the driving electrodes. When an AC current is used, the driving electrodes are not necessarily required to be in direct contact with the electrolyte solution. Rather, a dielectric material can be interposed between the driving electrode and the solution. This is particularly advantageous in situations where direct contact between the electrodes and the solution may be detrimental due to electrochemical reactions at the surface of the electrodes.
Ferroelectric materials differ from ordinary dielectric materials. In an ordinary dielectric material, the electric displacement, D, is generally proportional to the electric field, E. The ratio of the electric displacement and the electric field being the dielectric constant &egr;. Since the relationship between the electric displacement, D, and the electric field, E, is linear, an ordinary dielectric material does not retain an electric displacement after removal of an electric field.
The ferroelectric material is analogous to the more familiar ferromagnetic materials such as ferromagnetic iron except the magnetic field and the magnetic induction are replaced by the electric field, E, and the electric displacement, D. The relationship between the electric field, E and the electric displacement, D, of the ferroelectric material is depicted in FIG.
1
. In
FIG. 1
, point “a” shows the ferroelectric material in the non-polarized state with E=0 and D=0. When a positive electric field is applied and increased, the relationship between D and E follows the curve from point “a” to point “b” where a maximum displacement D
MAX
occurs. A subsequent decrease in the electric field, E, causes the displacement, D, to decrease along the curve between points “b” and “c”. At point “c,” the electric field, E, is zero but the displacement, D, is finite. This is the ‘poled’ state and the value of D at point “c” is known as the remnant polarization. A subsequent reversal of the electric field causes the remnant polarization to vanish (moving along the curve from point “c” to point “d” in FIG.
1
). The relationship between E and D then follows a typical hysteresis curve, passing through points “e,” “f” and “g” as shown in FIG.
1
.
The remnant polarization of a ferroelectric material can be removed by a method similar to the depolarization of a magnet. Specifically, when an alternating electric field of decreasing amplitude is applied, the area enclosed by the hysteresis curve becomes smaller and smaller as the amplitude of the alternating electric field is decreased. Eventually, the remnant polarization decreases to zero and the ferroelectric material returns to its original unpolarized state (point “a” in FIG.
1
).
A wide range of the ferroelectric materials are available, including the metal-titanates such as barium-titanate, metal-tantalates, metal-niobates and metal-tungustates. Ferroelectric materials are known that have a maximum displacement of several tenths of Coulomb per square meter. When these ferroelectric materials are polarized, a surface charge of about 10% of available surface lattice sites can result.
When a ferroelectric material is used as the tube material in the simple fluid pump example described above, the electro-
Mutschler Brian L.
Nguyen Nam
Nydegger & Associates
Toyo Technologies Inc.
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