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This invention covers a device and a method of measuring properties of uniform fluids and of complex fluids with greater accuracy, speed, and precision. Human body fluids are an example of complex fluids and are called by such names as: aqueous humour, vitreous humour, bile, blood, blood serum, breast milk, cerebrospinal fluid, cerumen (earwax), endolymph and perilymph, female ejaculate, gastric juice, mucus, peritoneal fluid, pleural fluid, saliva, sebum (skin oil), semen, sweat, tears, vaginal secretion, vomit and urine. This invention has the potential to provide significant improvement in the speed, accuracy, precision and cost of determining physical and mechanical properties of fluids. Such properties may include viscosity, shear rate, clotting time, stiffness, velocity, drag energy, temperature, storage modulus (elastic modulus −G′), loss modulus (viscous modulus G″), dielectric constant, resistivity, and percent of the constituents that make up the complex fluid.
This invention uses micro-electromechanical (MEMS) manufacturing techniques in its preferred embodiment. MEMS manufacturing offers competitive advantages over other devices because of its low cost, small size and low power requirements. This suspended parallel plate design offers superior accuracy, sensitivity, and stability.
This invention uses a novel way of measuring physical properties of fluids, a suspended plate that moves in contact with the fluid. This invention does not require levers as discussed in U.S. Pat. No. 6,467,761. In the preferred embodiment, the plate is suspended by a plurality of springs. These springs allow for harmonic motion. An actuator moves the suspended plate. The actuator motion causes the suspended plate to move. The dynamic equation becomes:
m{umlaut over (x)}+c{umlaut over (x)}+kx=F sin(ωt+φ) Eq. [1]
In equation 1 above: m=mass, c=total damping, k=total stiffness, {umlaut over (x)}=acceleration of plate, {umlaut over (x)}=velocity of the plate, x=amplitude of the motion of the plate, F is the force from the actuator (driving force), ω=the frequency of the driving force, t=time and finally φ=the phase shift between the driving force and the natural frequency.
The natural frequency (fn) of the system is shown in equation 2 below:
From the above two equations the properties of the fluids are determined. It is well known in the art how to solve equations 1 and 2. These equations become the bases for determining the properties of the fluids.
This invention allows a suspended plate to move parallel to a surface without crosstalk or the Abbe effect. The Abbe effect is a well known error in precision electronics caused by sensor placement resulting in displacement errors. The suspended plate moves parallel to a surface. The plate may be over the fluid and in contact with fluid, under the fluid and in contact with the fluid or in the fluid. The plate is suspended by springs and an actuator moves the plate. The fluid is in contact with the plate. The actuator can be a thermo-actuator, electric motor or piezoelectric actuator. This invention also includes a method of determining the properties of the fluid.
For viscosity properties, the suspended plate is moved by the actuator using a sinusoidal, square wave or other known wave form. The viscosity is determined by the power that is needed to move the plate. This is novel because other MEMs viscometers use cantilevers, cone and plate, or time to empty a known volume. Other viscometers require a sensor on the device. This design requires only the measurement of the power to determine the energy going to the actuator. If the viscosity of the fluid increases the power required to translate the plate back and forth also increases. Similarly if the viscosity of the fluid in contact with the plate decreases the power to move the plate also decreases. It is this correlation between power and viscosity that enables the device to determine the viscosity of the fluid. The power to move known fluids with known viscosities is first determined thereby rendering a viscosity versus power database. Displacement sensors may also be used. The sensor may be of any suitable design, for example a capacitor or optical type. For optical type, the phase change from the forcing frequency and the actual frequency will give you the drag energy. This drag energy is proportional to viscosity. For displacement sensors, the coefficient of damping (C) in equation 1 is proportional to viscosity and can be determined once the amplitude of the motion is known.
For shear rate properties, the suspended plate is moved by the actuator using a sinusoidal, square wave or other known wave form where the plate in contact with the fluid is over, under or in the fluid. The shear rate is proportional to the rate of increase or decrease in the power that is needed to move the plate. The device is powered at different frequencies and the different viscosities are again determined. For example if the complex fluid is shear thinning, the power to move the plate will decrease as the frequency of the plate is increased.
For temperature readings, the suspended plate is moved by the actuator again using a sinusoidal, square wave or other known wave form where the fluid in contact with the plate is over, under or in the fluid. The fluid's viscous properties are known as a function of temperature. Since viscosity is a function of temperature one skilled in the art can establish a correlation between power to move the suspended plate and temperature. For example, if the temperature is increased, the viscosity will decrease and the power to move the plate is also decreased. If the temperature is decreased the viscosity will increase and the power to move the plate also increases.
For clotting time of human blood or other complex fluids, the suspended plate in contact with the fluid is moved by the actuator again using a sinusoidal, square wave or other wave form. As the blood clots the viscosity of the blood increases with time. As a result the power to move the plate and drag energy also increases. The change in power is measured; thereby the clotting rate of the blood can be determined. The phase angle between the driving frequency and frequency of the moving plate can also give you the rate of change in drag energy which is proportional to clotting rate. Drag energy is the amount of energy consumed by the suspended plate due to contact with the fluid during movement. Finally displacement sensors can be used as well. For example, the displacement of the suspended plate will be less as the viscosity of the blood increases. The rate of reduction in displacement is a function of the clotting time rate.
For flow velocity readings, the suspended plate in contact with a flowing fluid is moved by the actuator using a sinusoidal, square wave form or other known wave form. The drag energy increases as the power increases to move the suspended plate if the velocity of the fluid is in the opposite direction of the direction of movement of the suspended plate. Therefore, the change in energy required to move the suspended plate is a function of the velocity of the fluid. For example if the fluid is not moving, the power to move the suspended plate in one direction is the same as the power to move the suspended plate in the opposite direction. If the fluid is moving from left to right, the power to move the suspended plate from left to right is less than the power to move the suspended plate from right to left. It is this difference in the power consumed by the actuator that is proportional to the velocity of the fluid.
For the dielectric property of the fluid, a capacitor or set of capacitors are used. The capacitor has a plurality of flat plates. A flat plate is located ether above or below the suspended plate. Any conductive material is deposited onto the surface of the suspended plate and onto the surface of a stationary surface or plate above or below the suspended plate. The stationary surface or plate will be separated from the moving suspended plate by the fluid at a distance d. The capacitance is a function of the dielectric constant of the fluid, the distance d, and the plate's average area A. The suspended plate does not have to be moved. In the preferred embodiment, the moving plate has one conductive surface deposited on the moving suspended plate and a conductive material deposited on the stationary plate or surface. The stationary plate or surface can have two or more conductive surfaces. For example, the capacitance of a parallel-plate capacitor constructed of two parallel plates both of area A separated by a distance d is approximately equal to the following:
Methods of determining capacitance are well known in the art. The dielectric property of the fluid can be determined once the capacitance is measured.
For resistivity properties of the fluid, a structure similar to that of the capacitor, or set of capacitors discussed above are again used. An electric field inside the device is created in the fluid between the conductive plates. A potential between the plates is used to determine resistivity. The potential difference will cause a current to flow. The electrical resistivity (ρ) is defined as the ratio of the electric field to the current it creates:
where
ρ is the resistivity of the conductor material (measured in ohm·meter, Ω·m),
E is the magnitude of the electric field (in volts per meter, V·m−1),
J is the magnitude of the current density (in amperes per square meter, A·m−2).
Conductivity of the fluid is the inverse of resistivity and can be calculated.
For storage modulus (elastic modulus −G′), loss modulus (viscous modulus G″), and total stiffness, the suspended plate in contact with the liquid is moved by the actuator using a sinusoidal, square wave or other known wave form. The displacement of the suspended plate is determined. Equation 1 is used to determine the fluid properties. At least three difference driving frequencies are used to determine the unknown variables. With the driving frequencies and displacement of the plate known, the displacement, velocity and acceleration can be determined as a function of time. One skilled in the art of controls knows how to determine the stiffness, damping ratio, storage modulus, and loss modulus from equation 1 above.
For percent of the constituents that compose the complex fluid, the suspended plate in contact with the fluid is moved by the actuator using a sinusoidal, square wave or known wave form. The displacement of the suspended plate is determined and equation 1 is used to find total stiffness. The stiffness of each constituent that makes up the complex material is then calculated by first determining the displacement at numerous frequencies. In equation 1 the stiffness is the total stiffness of the device plus the stiffness of the fluid. With complex fluids the stiffness of the fluid is the sum of all the stiffness's of the constituents that makeup the complex fluid.
where ktotal=is the total stiffness and kN is the stiffness of each constitutent. As an example, for a complex fluid with three constituents that make up the complex fluid, at least five different driving frequencies are used. Since the driving frequencies are known, the displacement, velocity and acceleration are determined as a function of time from equation 1. One skilled in the art of controls knows how to determine the total stiffness and the damping ratio. Equations 1 and 5 are used to determine the percent of each constituent because the stiffness of each constituent is known and the total stiffness is also known. One skilled in the art knows how to solve for the unknowns in equations 1 and 5.
For mass sensing, an absorbent is deposited onto the moving suspended plate. The moving suspended plate is moved by the actuator using a sinusoidal, square wave or known wave form. The driving frequency is known and the power to move the suspended plate is determined. As mass collects on the absorbent, the energy to move the suspended plate is increased. The energy increase is proportional to the mass collected by the absorbent. Changes in natural frequency (equation 2) can also be used to determine the change in mass collected by the absorbent. The current required to drive the device decreases at resonance. A current sensor can be used to determine the shift in natural frequency which is proportional to the mass collected by the device.
FIG. 1—Depicts a parallel plate MEMS device.
FIG. 2—Depicts a vertical plate device
FIGS. 3A and 3B—Depicts the Spring Design for Resonance
FIG. 4—Depicts the power measurements
FIGS. 5A and 5B—Depicts capacitance measurements
FIG. 6—Depicts input and output variables for the processor
FIG. 6A—Depicts the method of determining the natural frequency
FIG. 6B—Depicts the method of determining the drag energy
FIG. 6C—Depicts the method of determining the viscosity
FIG. 6D—Depicts the method of determining the clotting time
FIG. 6E—Depicts the method of determining the mass absorbed
FIG. 6F—Depicts the method of determining the shear rate of the fluid
FIG. 6G—Depicts the method of determining the velocity, acceleration and displacement of the plate
FIG. 6H—Depicts the method of determining the stiffness, storage modulus and loss modulus
FIG. 6I—Depicts the method of using an optical sensor to determine the velocity, acceleration and displacement
FIG. 7—Depicts blood clotting versus time
FIG. 8—Depicts natural frequency shift when mass is collected on the plate
FIG. 9—Depicts shear thinning versus frequency
FIG. 10—Depicts phase shift versus mass collected on plate
As depicted in
The phase shift 1030 between the actual frequency 1020 of the suspended plate 10 and the frequency 1030 supplied to the actuator 60 is caused by drag energy or viscosity change of the fluid.
These inventions were developed under a cooperative research and development agreement (CRADA-CN-11-0019) between a small business and a Federal Research Laboratory. The Federal Laboratory is US Department of Commerce's National Institute of Standards and Technology (NIST). NIST developed a positioning stage that uses a series of levers and was granted a U.S. Pat. No. 6,467,761 B1. This patent application is subject to the CREATE ACT of 2004.