Tikalon Blog by Dev Gualtieri

Brownian Motion Energy Harvesting

January 15, 2024

Human visual acuity is quite remarkable. We can"resolve objects as small as 0.1 millimeters (100 micrometers) such as a human hair whose general width (a hair's breadth) is about the same, and as wide as 181 micrometers. For long distances, the eye's angular resolution of about 1 arc-minute gives us the ability to resolve objects 30 centimeters apart at a distance of 1 kilometer. Since the semi-major axis of the Moon's orbit around the Earth is about 385,000 kilometers, we can resolve objects there spaced about 115 kilometers apart, which is about three quarters the distance from New York City to Philadelphia.

Leeuwenhoek Crater, located on the far side of the Moon in the Moon's southern hemisphere.

Antonie van Leeuwenhoek (1632-1723) was a Dutch microscopist, microbiologist, and the "Father of Microbiology." He began his microbiology research several decades after the invention of the microscope. He created his microscope lenses by melting the ends of soda lime glass fibers into ball lenses.

Wikimedia Commons image based on a NASA Lunar Orbiter 2 image.

Most of science advances through improvement in scientific instrumentation. Microscopes and optical telescopes have given us the ability to see things in greater detail than can be seen with the unaided eye. The invention of the microscope is credited to German-Dutch spectacle-maker, Hans Lipperhey (c.1570-1619). Lipperhey is also credited with the 1608 invention of the refracting telescope. A refracting telescope, now known as the Galilean telescope, was perfected the following year by Galileo Galilei (1564-1642). Galileo was the first to use a telescope for astronomical observations that were detailed in his 1610 publication, Sidereus Nuncius, and he also designed and constructed an improved microscope shortly after 1624.

Nobel Physics Laureate, Ernest Rutherford (1871-1937), once said that "all science is either physics or stamp collecting." Indeed, the early years of microscopy were just stamp collecting, the discovery and classification of microscale organisms. The first physics by microscopy was the observation of Brownian motion, the random motion of particles suspended in a fluid, by botanist, Robert Brown (1773-1858). In 1827, Brown observed the random motion in water of pollen grains of about 10 micrometers in size.

A simulation of Brownian motion in two dimensions

A thousand step simulation of Brownian motion in two dimensions.

(A Wikimedia Commons image, modified, by PAR. Click for larger image.)

Brownian motion was explained by Albert Einstein in 1905 in one of his four Annus mirabilis (miracle year) papers in the scientific journal, Annalen der Physik. Einstein had the insight to consider Brownian particles, which are much larger than water molecules, to be just another type of molecule in a solution for which ordinary statistical mechanics can be applied. This theory of Brownian motion was an affirmation of the existence of atoms and molecules. French physicist, Jean Perrin (1870-1942), was awarded the 1926 Nobel Prize in Physics for his experimental confirmation of this (for his work on the discontinuous structure of matter...) in 1908. Details of the history of Brownian motion can be found in refs. 1-4.[1-4]

In 1900, physicist and subsequent Nobel Physics Laureate, Gabriel Lippmann (1845-1921), proposed a thought experiment in thermodynamics that's now known as the Brownian ratchet (see figure). This is a device to harvest the energy from the motion of Brownian particles that appears to be a perpetual motion machine, thereby violating the second law of thermodynamics. An analysis in the 1960s by Richard Feynman (1918-1988) showed that work will be done only if the temperature T₁ > T₂, reducing the device to a typical heat engine that obeys all laws of thermodynamics.

diagram of the Brownian ratchet. The size and construction of the device are assumed to be adjusted such that the paddles will be moved by impact with the gas molecules. The paddle wheels would rotate the shaft in one direction only because of the ratchet mechanism. Work is done in lifting the mass m against gravity, but only if T₁>T₂. (Created by the author using Inkscape. Also available at Wikimedia Commons. Click for larger image.)

A recent open access paper in APL Materials by a research team from East Eight Energy Co., Ltd. (Shanghai, China) and Nankai University (Tianjin, China) reports on a piezoelectric energy harvester of Brownian motion.[5-6] This device builds from previous research that demonstrated the Brownian motion of tethered nanowires in liquid.[7-8] There exists considerable energy in thermal motion. As an example, the average kinetic energy of thermal motion per mole of molecules in an ideal gas at room temperature is 3.7 kilojoules.[5] The researchers call their device a molecular thermal motion harvester, or MTMH.[5]

The device uses arrays of Zinc oxide (ZnO) nanowires, selected because they're easy to create, and ZnO is a piezoelectric material.[6] The device has top and bottom interpenetrating arrays of such nanowires, the top array being gold coated (see figure). The space between the arrays is filled with a non-conductive liquid, the molecules of which cause the nanowires to undergo worm-like bending, flexing, or wriggling.[5]

Molecular motion energy harvester using ZnO nanowires

Molecular motion energy harvester using Zinc oxide (ZnO) nanowires.

This diagram shows just two of many nanowires of the device. High purity n-octane fills the space between the nanowires

The thermal motion of the molecules of the liquid causes the nanowires to bend, thereby producing an electrical charge via the piezoelectric effect.

(Created using Inkscape.)

The liquid medium for the device was electronic-grade, "four-nines" purity (99.99%), n-octane, with a purity of greater than 99.99 percent.[6] The n-octane has a suitable boiling point, low dielectric constant, low toxicity, and low viscosity.[5] Its high purity was essential to reduce conductivity from ions, which would discharge the ZnO nanowires.[5] In the device, the gold-coated nanowires were used as the negative electrode, and the uncoated piezoelectric ZnO nanowires as the positive electrode.[6] The 2 centimeter by 2 centimeter device was sealed with epoxy.[6] The easy flexibility of the nanowires was essential to device operation.

At room temperature, the device had an output voltage of 2.28 millivolts, and an output current of 2.47 nanoamperes, with these values increasing with the liquid temperature.[5-6] Parameters influencing device performance are friction from interfacial tension, viscous drag force from the surrounding fluid, and temperature.[5] The demonstrated device produces a mere 5 picowatts; however, better versions of such an energy harvester would be useful in low power Internet-of-things (IOT) devices, especially since mechanical vibration will also generate power in such a device.

Brownian Motion Energy Harvesting

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