Electrorheological fluid

Last updated

Electrorheological (ER) fluids are suspensions of extremely fine non-conducting but electrically active particles (up to 50 micrometres diameter) in an electrically insulating fluid. The apparent viscosity of these fluids changes reversibly by an order of up to 100,000 in response to an electric field. For example, a typical ER fluid can go from the consistency of a liquid to that of a gel, and back, with response times on the order of milliseconds. [1] The effect is sometimes called the Winslow effect after its discoverer, the American inventor Willis Winslow, who obtained a US patent on the effect in 1947 [2] and wrote an article published in 1949. [3]

Contents

The ER effect

The change in apparent viscosity is dependent on the applied electric field, i.e. the potential divided by the distance between the plates. The change is not a simple change in viscosity, hence these fluids are now known as ER fluids, rather than by the older term Electro Viscous fluids. The effect is better described as an electric field dependent shear yield stress. When activated an ER fluid behaves as a Bingham plastic (a type of viscoelastic material), with a yield point which is determined by the electric field strength. After the yield point is reached, the fluid shears as a fluid, i.e. the incremental shear stress is proportional to the rate of shear (in a Newtonian fluid there is no yield point and stress is directly proportional to shear). Hence the resistance to motion of the fluid can be controlled by adjusting the applied electric field.

Composition and theory

ER fluids are a type of smart fluid. A simple ER fluid can be made by mixing cornflour in a light vegetable oil or (better) silicone oil.

There are two main theories to explain the effect: the interfacial tension or 'water bridge' theory, [4] and the electrostatic theory. The water bridge theory assumes a three phase system, the particles contain the third phase which is another liquid (e.g. water) immiscible with the main phase liquid (e.g. oil). With no applied electric field the third phase is strongly attracted to and held within the particles. This means the ER fluid is a suspension of particles, which behaves as a liquid. When an electric field is applied the third phase is driven to one side of the particles by electro osmosis and binds adjacent particles together to form chains. This chain structure means the ER fluid has become a solid. The electrostatic theory assumes just a two phase system, with dielectric particles forming chains aligned with an electric field in an analogous way to how magnetorheological fluid (MR) fluids work. An ER fluid has been constructed with the solid phase made from a conductor coated in an insulator. [5] This ER fluid clearly cannot work by the water bridge model. However, although demonstrating that some ER fluids work by the electrostatic effect, it does not prove that all ER fluids do so. The advantage of having an ER fluid which operates on the electrostatic effect is the elimination of leakage current, i.e. potentially there is no direct current. Of course, since ER devices behave electrically as capacitors, and the main advantage of the ER effect is the speed of response, an alternating current is to be expected.

The particles are electrically active. They can be ferroelectric or, as mentioned above, made from a conducting material coated with an insulator, or electro-osmotically active particles. In the case of ferroelectric or conducting material, the particles would have a high dielectric constant. There may be some confusion here as to the dielectric constant of a conductor, but "if a material with a high dielectric constant is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric" (see main page: Dielectric constant), and since the electric field is zero in an ideal conductor, then in this context the dielectric constant of a conductor is infinite.

Another factor that influences the ER effect is the geometry of the electrodes. The introduction of parallel grooved electrodes showed slight increase in the ER effect but perpendicular[ clarification needed ] grooved electrodes doubled the ER effect. [6] A much larger increase in ER effect can be obtained by coating the electrodes with electrically polarisable materials. This turns the usual disadvantage of dielectrophoresis into a useful effect. It also has the effect of reducing leakage currents in the ER fluid. [7]

The giant electrorheological (GER) fluid was discovered in 2003, [8] and is able to sustain higher yield strengths than many other ER fluids. The GER fluid consists of Urea coated nanoparticles of Barium Titanium Oxalate suspended in silicone oil. The high yield strength is due to the high dielectric constant of the particles, the small size of the particles and the Urea coating. Another advantage of the GER is that the relationship between the electrical field strength and the yield strength is linear after the electric field reaches 1 kV/mm. The GER is a high yield strength, but low electrical field strength and low current density fluid compared to many other ER fluids. The procedure for preparation of the suspension is given in. [8] The major concern is the use of oxalic acid for the preparation of the particles as it is a strong organic acid.

Applications

The normal application of ER fluids is in fast acting hydraulic valves [9] and clutches, with the separation between plates being in the order of 1 mm and the applied potential being in the order of 1 kV. In simple terms, when the electric field is applied, an ER hydraulic valve is shut or the plates of an ER clutch are locked together, when the electric field is removed the ER hydraulic valve is open or the clutch plates are disengaged. Other common applications are in ER brakes [10] (think of a brake as a clutch with one side fixed) and shock absorbers [11] (which can be thought of as closed hydraulic systems where the shock is used to try to pump fluid through a valve).

There are many novel uses for these fluids. Potential uses are in accurate abrasive polishing [12] and as haptic controllers and tactile displays. [13]

ER fluid has also been proposed to have potential applications in flexible electronics, with the fluid incorporated in elements such as rollable screens and keypads, in which the viscosity-changing qualities of the fluid allowing the rollable elements to become rigid for use, and flexible to roll and retract for storing when not in use. Motorola filed a patent application for mobile device applications in 2006. [14]

Problems and advantages

A major problem is that ER fluids are suspensions, hence in time they tend to settle out, so advanced ER fluids tackle this problem by means such as matching the densities of the solid and liquid components, or by using nanoparticles, which brings ER fluids into line with the development of magnetorheological fluids. Another problem is that the breakdown voltage of air is ~ 3 kV/mm, which is near the electric field needed for ER devices to operate.

An advantage is that an ER device can control considerably more mechanical power than the electrical power used to control the effect, i.e. it can act as a power amplifier. But the main advantage is the speed of response. There are few other effects able to control such large amounts of mechanical or hydraulic power so rapidly.

Unfortunately, the increase in apparent viscosity experienced by most Electrorheological fluids used in shear or flow modes is relatively limited. The ER fluid changes from a Newtonian liquid to a partially crystalline "semi-hard slush". However, an almost complete liquid to solid phase change can be obtained when the electrorheological fluid additionally experiences compressive stress. [15] This effect has been used to provide electrorheological Braille displays [16] and very effective clutches. [17]

See also

Related Research Articles

<span class="mw-page-title-main">Colloid</span> Mixture of an insoluble substance microscopically dispersed throughout another substance

A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture. A colloid has a dispersed phase and a continuous phase. The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.

<span class="mw-page-title-main">Electro-osmosis</span> Movement of liquid through a conduit due to electric potential

In chemistry, electro-osmotic flow is the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or any other fluid conduit. Because electro-osmotic velocities are independent of conduit size, as long as the electrical double layer is much smaller than the characteristic length scale of the channel, electro-osmotic flow will have little effect. Electro-osmotic flow is most significant when in small channels, and is an essential component in chemical separation techniques, notably capillary electrophoresis. Electro-osmotic flow can occur in natural unfiltered water, as well as buffered solutions.

Electrowetting is the modification of the wetting properties of a surface with an applied electric field.

<span class="mw-page-title-main">Magnetorheological fluid</span>

A magnetorheological fluid is a type of smart fluid in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot is that the fluid's ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications.

<span class="mw-page-title-main">Ferrofluid</span> Special type of liquid which is attracted by poles of a magnet

Ferrofluid is a liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended in a carrier fluid. Each magnetic particle is thoroughly coated with a surfactant to inhibit clumping. Large ferromagnetic particles can be ripped out of the homogeneous colloidal mixture, forming a separate clump of magnetic dust when exposed to strong magnetic fields. The magnetic attraction of tiny nanoparticles is weak enough that the surfactant's Van der Waals force is sufficient to prevent magnetic clumping or agglomeration. Ferrofluids usually do not retain magnetization in the absence of an externally applied field and thus are often classified as "superparamagnets" rather than ferromagnets.

<span class="mw-page-title-main">Smart fluid</span> Fluid whose properties can be changed by applying an electric or magnetic field

A smart fluid is a fluid whose properties can be changed by applying an electric field or a magnetic field.

<span class="mw-page-title-main">Dilatant</span> Material in which viscosity increases with the rate of shear strain

A dilatant material is one in which viscosity increases with the rate of shear strain. Such a shear thickening fluid, also known by the initialism STF, is an example of a non-Newtonian fluid. This behaviour is usually not observed in pure materials, but can occur in suspensions.

Electrohydrodynamics (EHD), also known as electro-fluid-dynamics (EFD) or electrokinetics, is the study of the dynamics of electrically charged fluids. It is the study of the motions of ionized particles or molecules and their interactions with electric fields and the surrounding fluid. The term may be considered to be synonymous with the rather elaborate electrostrictive hydrodynamics. ESHD covers the following types of particle and fluid transport mechanisms: electrophoresis, electrokinesis, dielectrophoresis, electro-osmosis, and electrorotation. In general, the phenomena relate to the direct conversion of electrical energy into kinetic energy, and vice versa.

<span class="mw-page-title-main">Dielectrophoresis</span> Particle motion in a non-uniform electric field due to dipole-field interactions

Dielectrophoresis (DEP) is a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field. This force does not require the particle to be charged. All particles exhibit dielectrophoretic activity in the presence of electric fields. However, the strength of the force depends strongly on the medium and particles' electrical properties, on the particles' shape and size, as well as on the frequency of the electric field. Consequently, fields of a particular frequency can manipulate particles with great selectivity. This has allowed, for example, the separation of cells or the orientation and manipulation of nanoparticles and nanowires. Furthermore, a study of the change in DEP force as a function of frequency can allow the electrical properties of the particle to be elucidated.

<span class="mw-page-title-main">Electroactive polymer</span>

An electroactive polymer (EAP) is a polymer that exhibits a change in size or shape when stimulated by an electric field. The most common applications of this type of material are in actuators and sensors. A typical characteristic property of an EAP is that they will undergo a large amount of deformation while sustaining large forces.

The name electrospray is used for an apparatus that employs electricity to disperse a liquid or for the fine aerosol resulting from this process. High voltage is applied to a liquid supplied through an emitter. Ideally the liquid reaching the emitter tip forms a Taylor cone, which emits a liquid jet through its apex. Varicose waves on the surface of the jet lead to the formation of small and highly charged liquid droplets, which are radially dispersed due to Coulomb repulsion.

Rheometry generically refers to the experimental techniques used to determine the rheological properties of materials, that is the qualitative and quantitative relationships between stresses and strains and their derivatives. The techniques used are experimental. Rheometry investigates materials in relatively simple flows like steady shear flow, small amplitude oscillatory shear, and extensional flow.

Electroacoustic phenomena arise when ultrasound propagates through a fluid containing ions. The associated particle motion generates electric signals because ions have electric charge. This coupling between ultrasound and electric field is called electroacoustic phenomena. The fluid might be a simple Newtonian liquid, or complex heterogeneous dispersion, emulsion or even a porous body. There are several different electroacoustic effects depending on the nature of the fluid.

<span class="mw-page-title-main">Double layer (surface science)</span> Molecular interface between a surface and a fluid

In surface science, a double layer 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 DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge, consists of ions which are adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".

A magnetorheological damper or magnetorheological shock absorber is a damper filled with magnetorheological fluid, which is controlled by a magnetic field, usually using an electromagnet. This allows the damping characteristics of the shock absorber to be continuously controlled by varying the power of the electromagnet. Fluid viscosity increases within the damper as electromagnet intensity increases. This type of shock absorber has several applications, most notably in semi-active vehicle suspensions which may adapt to road conditions, as they are monitored through sensors in the vehicle, and in prosthetic limbs.

Sedimentation potential occurs when dispersed particles move under the influence of either gravity or centrifugation in a medium. This motion disrupts the equilibrium symmetry of the particle's double layer. While the particle moves, the ions in the electric double layer lag behind due to the liquid flow. This causes a slight displacement between the surface charge and the electric charge of the diffuse layer. As a result, the moving particle creates a dipole moment. The sum of all of the dipoles generates an electric field which is called sedimentation potential. It can be measured with an open electrical circuit, which is also called sedimentation current.

<span class="mw-page-title-main">Capillary electrochromatography</span> Method of separating components of a mixture via electro-osmosis

In chemical analysis, capillary electrochromatography (CEC) is a chromatographic technique in which the mobile phase is driven through the chromatographic bed by electro-osmosis. Capillary electrochromatography is a combination of two analytical techniques, high-performance liquid chromatography and capillary electrophoresis. Capillary electrophoresis aims to separate analytes on the basis of their mass-to-charge ratio by passing a high voltage across ends of a capillary tube, which is filled with the analyte. High-performance liquid chromatography separates analytes by passing them, under high pressure, through a column filled with stationary phase. The interactions between the analytes and the stationary phase and mobile phase lead to the separation of the analytes. In capillary electrochromatography capillaries, packed with HPLC stationary phase, are subjected to a high voltage. Separation is achieved by electrophoretic migration of solutes and differential partitioning.

In rheology, the Farris Effect describes the decrease of the viscosity of a suspension upon increasing the dispersity of the solid additive, at constant volume fraction of the solid additive. That is, that a broader particle size distribution yields a lower viscosity than a narrow particle size distribution, for the same concentration of particles. The phenomenon is names after Richard J. Farris, who modeled the effect. The effect is relevant whenever suspensions are flowing, particularly for suspensions with high loading fractions. Examples include hydraulic fracturing fluids, metal injection molding feedstocks, cosmetics, and various geological processes including sedimentation and lava flows.

A flowFET is a microfluidic component which allows the rate of flow of liquid in a microfluidic channel to be modulated by the electrical potential applied to it. In this way, it behaves as a microfluidic analogue to the field effect transistor, except that in the flowFET the flow of liquid takes the place of the flow of electric current. Indeed, the name of the flowFET is derived from the naming convention of electronic FETs.

Electrical capacitance volume tomography (ECVT) is a non-invasive 3D imaging technology applied primarily to multiphase flows. It was first introduced by W. Warsito, Q. Marashdeh, and L.-S. Fan as an extension of the conventional electrical capacitance tomography (ECT). In conventional ECT, sensor plates are distributed around a surface of interest. Measured capacitance between plate combinations is used to reconstruct 2D images (tomograms) of material distribution. In ECT, the fringing field from the edges of the plates is viewed as a source of distortion to the final reconstructed image and is thus mitigated by guard electrodes. ECVT exploits this fringing field and expands it through 3D sensor designs that deliberately establish an electric field variation in all three dimensions. The image reconstruction algorithms are similar in nature to ECT; nevertheless, the reconstruction problem in ECVT is more complicated. The sensitivity matrix of an ECVT sensor is more ill-conditioned and the overall reconstruction problem is more ill-posed compared to ECT. The ECVT approach to sensor design allows direct 3D imaging of the outrounded geometry. This is different than 3D-ECT that relies on stacking images from individual ECT sensors. 3D-ECT can also be accomplished by stacking frames from a sequence of time intervals of ECT measurements. Because the ECT sensor plates are required to have lengths on the order of the domain cross-section, 3D-ECT does not provide the required resolution in the axial dimension. ECVT solves this problem by going directly to the image reconstruction and avoiding the stacking approach. This is accomplished by using a sensor that is inherently three-dimensional.

References

  1. ↑ Khanicheh, Azadeh; Mintzopoulos, Dionyssios (June 2008). et al. "Evaluation of Electrorheological Fluid Dampers for Applications at 3-T MRI Environment" (PDF). IEEE/ASME Transactions on Mechatronics. 3. 13 (3): 286–294. doi:10.1109/TMECH.2008.924043. S2CID   14188698. Archived from the original (PDF) on 2014-07-22. Retrieved 2016-10-12.
  2. ↑ U.S. Patent 2,417,850 : Winslow, W. M.: 'Method and means for translating electrical impulses into mechanical force', 25 March 1947
  3. ↑ Winslow, Willis M. (1949). "Induced fibration of suspensions". J. Appl. Phys. 20 (12): 1137–1140. Bibcode:1949JAP....20.1137W. doi:10.1063/1.1698285.
  4. ↑ Stangroom, JE (1983). "Electrorheological Fluids". Physics in Technology. 14 (6): 290–296. Bibcode:1983PhTec..14..290S. doi:10.1088/0305-4624/14/6/305.
  5. ↑ Tam, W Y; Yi, G H; Wen, W; Ma, H; Sheng, P (April 1997). "New Electrorheological Fluid: Theory and Experiment" (PDF). Phys. Rev. Lett. 78 (15): 2987–2990. Bibcode:1997PhRvL..78.2987T. doi:10.1103/PhysRevLett.78.2987.
  6. ↑ Georgiades, G; Oyadiji, SO (2003). "Effects of electrode geometry on the performance of electrorheological fluid valves". Journal of Intelligent Material Systems and Structures. 14 (2): 105–111. doi:10.1177/1045389X03014002006. S2CID   110195091.
  7. ↑ Monkman, G. J. (1991). "Addition of solid structures to electrorheological fluids". Journal of Rheology. Society of Rheology. 35 (7): 1385–1392. Bibcode:1991JRheo..35.1385M. doi:10.1122/1.550237. ISSN   0148-6055.
  8. 1 2 Wen, W; Huang, X; Yang, S; Lu, K; Sheng, P (November 2003). "The giant electrorheological effect in suspensions of nanoparticles". Nature Materials. 2 (11): 727–730. Bibcode:2003NatMa...2..727W. doi:10.1038/nmat993. PMID   14528296. S2CID   6416226.
  9. ↑ Simmonds, AJ (July 1991). "Electro-rheological valves in a hydraulic circuit". IEE Proceedings D - Control Theory and Applications. 138 (4): 400–404. doi:10.1049/ip-d.1991.0054.
  10. ↑ Seed, M; Hobson, GS; Tozer, RC; Simmonds, AJ (September 1986). "Voltage-controlled Electrorheological brake". Proc. IASTED Int. Symp. Measurement, Sig. Proc. and Control. Taormina, Italy: ACTA Press. pp. Paper No. 105–092–1.
  11. ↑ Stanway, R; Sproston, JL; El-Wahed, AK (August 1996). "Applications of electro-rheological fluids in vibration control: a survey". Smart Mater. Struct. 5 (4): 464–482. Bibcode:1996SMaS....5..464S. doi:10.1088/0964-1726/5/4/011. S2CID   250745595.
  12. ↑ KIM W. B.; LEE S. J.; KIM Y. J.; LEE E. S. (2003). "The electromechanical principle of electrorheological fluid-assisted polishing". International Journal of Machine Tools & Manufacture. Kidlington, UK: Elsevier. 43 (1): 81–88. doi:10.1016/S0890-6955(02)00143-8.
  13. ↑ Liu, Y; Davidson, R; Taylor, P (2005). Flatau, Alison B (ed.). "Investigation of the touch sensitivity of ER fluid based tactile display". Proceedings of SPIE. Smart Structures and Materials 2005: Smart Structures and Integrated Systems. 5764: 92–99. Bibcode:2005SPIE.5764...92L. doi:10.1117/12.598713. S2CID   109965266.
  14. ↑ "Foldable/ rollable phone from Motorola". unwiredview.com. 25 January 2008.
  15. ↑ Monkman, G J (1995-03-14). "The electrorheological effect under compressive stress". Journal of Physics D: Applied Physics. IOP Publishing. 28 (3): 588–593. Bibcode:1995JPhD...28..588M. doi:10.1088/0022-3727/28/3/022. ISSN   0022-3727. S2CID   250762153.
  16. ↑ Monkman, G. J. (1992). "An Electrorheological Tactile Display". Presence: Teleoperators and Virtual Environments. MIT Press - Journals. 1 (2): 219–228. doi:10.1162/pres.1992.1.2.219. ISSN   1054-7460. S2CID   32555319.
  17. ↑ Monkman, G.J. (1997). "Exploitation of compressive stress in electrorheological coupling". Mechatronics. Elsevier BV. 7 (1): 27–36. doi:10.1016/s0957-4158(96)00037-2. ISSN   0957-4158.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.