Engineering

A ferroelectric-fluid motor may challenge magnetic-motor designs



It took 100 years to come to fruition, but this recently created fluid enables non-magnetic electrostatic-based motors.

Say the word “motor” to most engineers and they almost always associate it with magnetics of some type, whether the term is explicitly stated or not. Of course, not all electrically powered motors use magnetism: piezoelectric motors use that well-known principle and the voltage-driven elongation of a crystal, and there are also tiny MEMS-based motors.

Still, magnetics is front of mind for most cases, with motors that take electric current and power and transform it into a magnetic force that drives the rotor. (Come to think of it, there are also pneumatic and hydraulic motors often used by mechanical engineers, often with some electronic control – but that’s another story for another day.)

That’s why a recent development led by a team at the Institute of Science Tokyo may change our thinking about non-magnetic motors. They have developed a rotary motor based on electrostatic forces rather than the usual electromagnetic ones. Using a ferroelectric fluid, motors that previously required fairly high voltages on the order of 1000 V can operate at much lower voltages. (Not familiar with ferroelectric fluids, or have them confused with ferrofluids? See the sidebar at the end.)

A critical factor in their approach is that the force generated by electricity is not limited to attraction along the direction of the applied voltage. Instead, there is also a force that acts perpendicular to that direction, creating a sideways-pushing force. In conventional materials, this sideways force is extremely weak and has long been considered too small to utilize and, as a result, has attracted little attention.

However, the team demonstrated that the sideways electrostatic force can, under the right conditions, become surprisingly strong. They placed the fluid between two electrodes separated by only a few millimeters and applied a voltage. The result was striking: the liquid was pushed sideways and moved nearly 10 centimeters even against gravity. When the same experiment was performed with conventional liquids, this motion did not occur; the effect appeared only with the ferroelectric fluid.

Another interesting finding was how the force increased. In ordinary materials, increasing the voltage does not easily lead to a large increase in force. In contrast, with the ferroelectric fluid, even a small increase in voltage led to a proportional increase in force. Through detailed deep-physics analysis, the team found that the electric field causes the molecules in the liquid to align in an ordered way, generating the sideways-pushing force.

Using ferroelectric nematic liquid crystals, they were able to show that transverse electrostatic force (TEF) can elevate the fluid between electrodes with a gap of 2.5 millimeters (mm) up to more than 80 mm at only 28 V/mm, corresponding to a stress greater than 1000 newtons/m2 (Figure 1).


Figure 1 Direct observation of the TEF produced interesting results. (a) Experimental setup where TEF is balanced with the gravitational force. Voltage-dependent fluid heights at a 10mm gap under applied voltages of 0 V (b), 50 V (c), 100 V (d), and 150 V (e). Note that the zero-point height is 1 cm on the ruler. (f) Case of a 2.5mm gap at DC 80 V. (g) Height of fluid, H, plotted as a function of the applied voltage V for 1.0 (green), 2.5 (pink), 5.0 (yellow), and 10mm (purple). The data for silicone oil and nematic liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) at 80 V at a 2.5mm gap are shown as a gray bullet. (h) Generated stress, σ⊥, plotted as a function of the applied electric field E. (The inset is a magnification at a 1.0mm gap.) (Image source: Springer Nature)

This discovery led to a next logical next step: if this force can push, could it also be used to create rotation? The answer is yes, and they developed a prototype motor that does not use magnets, a metal rotor, or rare-earth metals (Figure 2).


Figure 2 A prototype plastic ferroelectric motor successfully implemented rotation (Left: angled view; Right: bottom view with the lower electrode removed to reveal the resin rotor) (Image source: Institute of Science Tokyo)

The structure can also be simpler and lighter. Because the rotating part can be made of resin rather than metal, devices can be made lighter and respond more quickly. The absence of magnetic materials also means it can be used where having these materials in proximity to other system parts can induce magnetic noise or distortions.

Additional tests were done to assess both stability and scalability for the TEF-based approach (Figure 3).


Figure 3 Testing of TEF scalability and stability further confirmed concept viability. (a) Lifting a resin weight (circled in white) by TEF generated in a ferroelectric fluid, before a DC voltage was applied (upper) and after lifting for 22mm upon application of DC 20 V (lower). The mass was 5 mg, constrained between the electrodes. The interelectrode  distance was fixed at 1.0mm. (b) Experimental setup for testing TEF scalability and stability. A tensile-testing machine holds a 3D-printed resin piece whose base area is 100mm2. The resin piece is immersed in the ferroelectric fluid reservoir to a depth of 1 mm; the aluminum parallel-plate electrodes of 56 mm length were fixed at a distance of 2.5 mm. The temperature was controlled by heaters with thermocouples inserted into the reservoir. The temperature of the ferroelectric fluid was directly measured by a thermocouple immersed in the fluid. (c) Time-course measurement of TEF under 60 V DC using the tensile-testing machine experiment setup shown in (b). Generated TEF, F⊥, was plotted as a function of time t. The force represented by the red line is the five-point moving average before and after the raw data shown in gray. (Image source: Springer Nature)

Of course, the true endpoint of this project was not just to explore these fluids but make something that at least initially appears potentially useful, such as an electric motor. To do this, they built a motor roughly analogous to a DC stepper motor by following three basic design rules (Figure 4):

  1. The fundamental components should consist of three pairs of stators and two rotors.
  2. The width of the rotor should be longer than that of the stator but shorter than the sum of the width of the stator and the space between stators.
  3. The relationship between the total number of stators and rotors should be an integer multiple of 3:2.


Figure 4 The ferroelectric motor’s novel design followed three basic rules. (a) (Upper) Case of the width of a mover being shorter than that of the electrodes. (Lower) The case of the width of a mover being longer. (b) A fundamental configuration of electrodes for continuous movement. The number in the upper-left corner of each panel corresponds to the phases of the pulse shown at the bottom. (c) CAD designs and photographs of the rotor (bottom inset) and the stator (top inset). The position of the upper stator in the CAD designs is not the actual position for clarity. The outer diameter, each pole’s length, and thickness of the rotor are 18 mm, 4 mm, and 1 mm, respectively. The stator consists of three-layered electrodes (U, V, and W poles) with two insulating layers. The outer diameter, each pole’s length, and total thickness of the rotor are 28 mm, 4 mm, and 2.1 mm, respectively. The length of the overlapping part of the poles of the rotor and the stator is about 3.5 mm. The gap between the upper and lower stators is 2 mm. (d) External view of the assembled motor. (Image source: Springer Nature)

Their prototype ferroelectric motor consists of a 3-phase 24-pole stator with an 11.25° electrode angle and a 3.75° space angle between stators and a single-phase 16-pole rotor with a 12.5° electrode angle and a 10° space angle. It was driven with a 3-phase, 60-V square wave with a duty ratio of 33%.

Most electrostatic motors studied thus far required a high applied electric field of several 10–100 MV/m; in contrast, the ferroelectric motor rotated with an electric field as weak as 0.03 MV/m (here, 60 V/2 mm), which means that a driving voltage as low as one-thousandth of that figure could be used.

The most important advantage of this ferroelectric motor over ordinary electromagnetic motors is that it does not require a voltage to be applied to the rotor, which means that the rotor can be made of resin instead of metal, making it lightweight and reducing inertia. In addition, because the rotor does not need to be energized, mechanisms such as carbon brushes and slip rings are unnecessary.

The work is fully detailed in their paper “Huge transverse Maxwell stress in ferroelectric fluids and prototyping of new ferroelectric motors” published in Nature’s Communication Engineering (why there – I can’t say). It includes the associated deep-physics analysis as well as links to some “action” videos.

Do you think there’s a future for these electrostatic motors, or will real-world considerations hinder their advance? Or will they be like ferrofluids, and find uses far removed from their design objectives?

The history and development of ferroelectric fluids is a 100-year journey from an early 20th-century theoretical prediction culminating in 21st-century reality. In 1916, physicist Max Born predicted that if a fluid’s molecules possess a strong enough electric dipole, they would naturally form a spontaneously polar (ferroelectric) fluid that can withstand thermal fluctuations.

The concept of a polar, electric-field-responsive liquid was finally transformed into reality in 2017 when scientists experimentally produced the first stable ferroelectric nematic liquid crystals. They successfully synthesizing and identifies the elusive ferroelectric nematic phase in a highly polar rod-shaped molecular material (known in chemistry as RM734). This experimentally proved the existence of 3D fluids with stable, switchable macroscopic electric polarization. (See Science Advances, Development of ferroelectric nematic fluids with giant-ε dielectricity and nonlinear optical properties (2021).)

Ferroelectric fluids are easily confused with ferrofluids, but they are very different. Ferrofluids are colloidal suspensions of magnetic nanoparticles (like iron oxide) in a carrier oil and they respond to magnetic fields. They were developed by NASA’s Steve Papell in 1963 as a liquid rocket fuel that could be drawn toward a fuel pump in a weightless environment by applying a magnetic field. Although that application didn’t work out, these ferrofluids found other uses such as liquid seals around spinning drive shafts and in loudspeakers to remove heat from the voice coil while also passively damping the movement of the cone. In contrast, ferroelectric fluids (FNLCs) are pure or mixture-based 3D molecular fluids. They have spontaneous electric polarity and respond natively to electric fields.

—Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors and signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing, and he also developed significant mechanical-engineering insight while designing control electronics for large materials-testing systems.

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