Brownian motors are

nanoscale Nanotechnology is the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing propertie ...

molecular machine Molecular machines are a class of molecules typically described as an assembly of a discrete number of molecular components intended to produce mechanical movements in response to specific stimuli, mimicking macromolecular devices such as switch ...

s that use

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

s to generate directed motion in space. The theory behind Brownian motors relies on the phenomenon of

Brownian motion Brownian motion is the random motion of particles suspended in a medium (a liquid or a gas). The traditional mathematical formulation of Brownian motion is that of the Wiener process, which is often called Brownian motion, even in mathematical ...

, random motion of

particle In the physical sciences, a particle (or corpuscle in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

s suspended in a

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

or a

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

) resulting from their

collision In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great for ...

with the fast-moving

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s in the fluid. On the

(1–100 nm),

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

dominates

inertia Inertia is the natural tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes the velocity to change. It is one of the fundamental principles in classical physics, and described by Isaac Newto ...

, and the extremely high degree of

thermal noise A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...

in the environment makes conventional directed motion all but impossible, because the

force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

s impelling these motors in the desired direction are minuscule when compared to the random forces exerted by the environment. Brownian motors operate specifically to utilise this high level of

random noise In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In particular, noise is inherent in physics and central to thermod ...

to achieve directed motion, and as such are only viable on the

. The concept of Brownian motors is a recent one, having only been coined in 1995 by Peter Hänggi, but the existence of such motors in nature may have existed for a very long time and help to explain crucial

cellular processes The cell is the basic structural and functional unit of all life, forms of life. Every cell consists of cytoplasm enclosed within a Cell membrane, membrane; many cells contain organelles, each with a specific function. The term comes from the ...

that require movement at the

, such as

protein synthesis Protein biosynthesis, or protein synthesis, is a core biological process, occurring inside cells, balancing the loss of cellular proteins (via degradation or export) through the production of new proteins. Proteins perform a number of critica ...

and

muscular contraction Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...

. If this is the case, Brownian motors may have implications for the foundations of

life Life, also known as biota, refers to matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes. It is defined descriptively by the capacity for homeostasis, Structure#Biological, organisation, met ...

itself. In more recent times, humans have attempted to apply this knowledge of natural Brownian motors to solve human problems. The applications of Brownian motors are most obvious in

nanorobotics Nanoid robotics, or for short, nanorobotics or nanobotics, is an emerging technology field creating machines or robots, which are called nanorobots or simply nanobots, whose components are at or near the scale of a nanometer (10−9 meters). ...

due to its inherent reliance on directed motion.

History

20th century

The term "Brownian motor" was originally invented by Swiss theoretical physicist Peter Hänggi in 1995. The Brownian motor, like the phenomenon of Brownian motion that underpinned its underlying theory, was also named after 19th century Scottish botanist

Robert Brown Robert Brown may refer to: Robert Brown (born 1965), British Director, Animator and author Entertainers and artists * Washboard Sam or Robert Brown (1910–1966), American musician and singer * Robert W. Brown (1917–2009), American printmaker ...

, who, while looking through a

microscope A microscope () is a laboratory equipment, laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic ...

pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...

of the

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

'' Clarkia pulchella'' immersed in water, famously described the random motion of pollen particles in water in 1827. In 1905, almost eighty years later,

theoretical physicist Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain, and predict natural phenomena. This is in contrast to experimental physics, which uses experi ...

Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...

published a paper where he modeled the motion of the pollen as being moved by individual water molecules, and this was verified experimentally by

Jean Perrin Jean Baptiste Perrin (; 30 September 1870 – 17 April 1942) was a French atomic physicist who, in his studies of the Brownian motion of minute particles suspended in liquids (sedimentation equilibrium), verified Albert Einstein's explanation o ...

in 1908, who was awarded the

Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...

in 1926 "for his work on the discontinuous structure of matter". These developments helped to create the fundamentals of the present theories of the

world.

Nanoscience Nanotechnology is the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing propertie ...

has traditionally long remained at the intersection of the physical sciences of

physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

and

chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, but more recent developments in research increasingly position it beyond the scope of either of these two traditional fields.

21st century

In 2002, a seminal paper on Brownian motors published in the

American Institute of Physics The American Institute of Physics (AIP) promotes science and the profession of physics, publishes physics journals, and produces publications for scientific and engineering societies. The AIP is made up of various member societies. Its corpora ...

magazine ''

Physics Today ''Physics Today'' is the membership magazine of the American Institute of Physics. First published in May 1948, it is issued on a monthly schedule, and is provided to the members of ten physics societies, including the American Physical Society. ...

'', "Brownian motors", by Dean Astumian and Peter Hänggi. There, they proposed the then novel concept of Brownian motors and posited that "thermal motion combined with input energy gives rise to a channeling of chance that can be used to exercise control over microscopic systems". Astumian and Hänggi provide in their paper a copy of

Wallace Stevens Wallace Stevens (October 2, 1879 – August 2, 1955) was an American modernist poet. He was born in Reading, Pennsylvania, educated at Harvard and then New York Law School, and spent most of his life working as an executive for an insurance compa ...

' 1919 poem " The Place of the Solitaires" to elegantly illustrate, from an abstract perspective, the ceaseless nature of noise. A year after the Astumian-Hänggi paper, David Leigh's organic chemistry group reported the first artificial molecular Brownian motors. In 2007 the same team reported a

Maxwell's demon Maxwell's demon is a thought experiment that appears to disprove the second law of thermodynamics. It was proposed by the physicist James Clerk Maxwell in 1867. In his first letter, Maxwell referred to the entity as a "finite being" or a "being ...

-inspired molecular information ratchet.Leigh's Group's illustrated explanation of the "ratchet"
Another important demonstration of

nanoengineering Nanoengineering is the practice of engineering on the nanoscale. It derives its name from the nanometre, a unit of measurement equalling one billionth of a meter. Nanoengineering is largely a synonym for nanotechnology, but emphasizes the engine ...

and

nanotechnology Nanotechnology is the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing propertie ...

was the building of a practical artificial Brownian motor by

IBM International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...

in 2018. Specifically, an energy landscape was created by accurately shaping a nanofluidic slit, and alternate potentials and an oscillating electric field were then used to "rock"

nanoparticle A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At ...

s to produce directed motion. The experiment successfully made the

s move along a track in the shape of the outline of the IBM logo and serves as an important milestone in the practical use of Brownian motors and other elements at the

Additionally, various institutions around the world, such as the

University of Sydney The University of Sydney (USYD) is a public university, public research university in Sydney, Australia. Founded in 1850, it is the oldest university in both Australia and Oceania. One of Australia's six sandstone universities, it was one of the ...

Nano Institute, headquartered at the Sydney Nanoscience Hub (SNH), and the Swiss Nanoscience Institute (SNI) at the

University of Basel The University of Basel (Latin: ''Universitas Basiliensis''; German: ''Universität Basel'') is a public research university in Basel, Switzerland. Founded on 4 April 1460, it is Switzerland's oldest university and among the world's oldest univ ...

, are examples of the research activity emerging in the field of nanoscience. Brownian motors remain a central concept in both the understanding of natural

molecular motor Molecular motors are natural (biological) or artificial molecular machines that are the essential agents of movement in living organisms. In general terms, a motor is a device that consumes energy in one form and converts it into motion or mech ...

s and the construction of useful nanoscale machines that involve directed motion.

Theory

The

on the

is so great that moving in a particular direction is as difficult as "walking in a

hurricane A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its ...

" or "swimming in

molasses Molasses () is a viscous byproduct, principally obtained from the refining of sugarcane or sugar beet juice into sugar. Molasses varies in the amount of sugar, the method of extraction, and the age of the plant. Sugarcane molasses is usuall ...

". The theoretical operation of the Brownian motor can be explained by ratchet theory, wherein strong random

thermal fluctuations In statistical mechanics, thermal fluctuations are random deviations of an atomic system from its average state, that occur in a system at equilibrium.In statistical mechanics they are often simply referred to as fluctuations. All thermal fluctu ...

are allowed to move the particle in the desired direction, while

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

is expended to counteract

s that would produce

motion In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...

in the opposite direction. This motion can be both linear and rotational. In the biological sense and in the extent to which this phenomenon appears in nature, this exists as

chemical energy Chemical energy is the energy of chemical substances that is released when the substances undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (20 ...

is sourced from the molecule

adenosine triphosphate Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...

(ATP). The

Brownian ratchet In the philosophy of thermal and statistical physics, the Brownian ratchet or Feynman–Smoluchowski ratchet is an apparent perpetual motion machine of the second kind (converting thermal energy into mechanical work), first analysed in 1912 as ...

is an apparent

perpetual motion machine Perpetual motion is the motion of bodies that continues forever in an unperturbed system. A perpetual motion machine is a hypothetical machine that can do work indefinitely without an external energy source. This kind of machine is impossible ...

that appears to violate the

second law of thermodynamics The second law of thermodynamics is a physical law based on Universal (metaphysics), universal empirical observation concerning heat and Energy transformation, energy interconversions. A simple statement of the law is that heat always flows spont ...

, but was later debunked upon more detailed analysis by

Richard Feynman Richard Phillips Feynman (; May 11, 1918 – February 15, 1988) was an American theoretical physicist. He is best known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of t ...

and other

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

s. The difference between real Brownian motors and fictional

s is that only in Brownian motors is there an input of

in order to provide the necessary

to hold the motor in place to counteract the

that try to move the motor in the opposite direction. Because Brownian motors rely on the random nature of

to achieve directed motion, they are

stochastic Stochastic (; ) is the property of being well-described by a random probability distribution. ''Stochasticity'' and ''randomness'' are technically distinct concepts: the former refers to a modeling approach, while the latter describes phenomena; i ...

in nature, in that they can be analysed statistically but not predicted precisely.

Examples in nature

biology Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, History of life, origin, evolution, and ...

, much of what we understand to be

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

-based

s may also in fact be Brownian motors. These molecular motors facilitate critical

living organisms An organism is any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been pro ...

and, indeed, are fundamental to

itself. Researchers have made significant advances in terms of examining these organic processes to gain insight into their inner workings. For example, molecular Brownian motors in the form of several different types of

exist within humans. Two common

biomolecular A biomolecule or biological molecule is loosely defined as a molecule produced by a living organism and essential to one or more typically biological processes. Biomolecules include large macromolecules such as proteins, carbohydrates, lipi ...

Brownian motors are

ATP synthase ATP synthase is an enzyme that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). ATP synthase is a molecular machine. The overall reaction catalyzed ...

, a rotary motor, and

myosin II Myosins () are a family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-ba ...

, a linear motor. The motor protein

produces rotational

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

that facilitates the synthesis of ATP from

Adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbon ...

(ADP) and inorganic

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

(P_i) through the following overall reaction: ADP + P_i + 3H⁺_out ⇌ ATP + H₂O + 3H⁺_in In contrast, the

produced by

is linear and is a basis for the process of

muscle contraction Muscle contraction is the activation of Tension (physics), tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in musc ...

. Similar

motor protein Motor proteins are a class of molecular motors that can move along the cytoskeleton of cells. They do this by converting chemical energy into mechanical work by the hydrolysis of ATP. Cellular functions Motor proteins are the driving force b ...

s include

kinesin A kinesin is a protein complex belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule (MT) filaments and are powered by the hydrolysis of adenosine triphosphate (ATP) (thus kinesins are ATPases, a typ ...

and

dynein Dyneins are a family of cytoskeletal motor proteins (though they are actually protein complexes) that move along microtubules in cells. They convert the chemical energy stored in ATP to mechanical work. Dynein transports various cellular cargo ...

, which all convert chemical energy into mechanical work by the

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of ATP. Many

s within

human cells The list of human cell types provides an enumeration and description of the various specialized cells found within the human body, highlighting their distinct functions, characteristics, and contributions to overall physiological processes. Cell ...

act as Brownian motors by producing directed motion on the

, and some common proteins of this type are illustrated by the following

computer-generated images Computer-generated imagery (CGI) is a specific-technology or application of computer graphics for creating or improving images in art, printed media, simulators, videos and video games. These images are either static (i.e. still images) or d ...

. File:Atp synthase.PNG, ATP Synthase File:Myosine.gif, Myosin II File:064-Kinesin-3kin-composite.png, Kinesin File:Cytoplasmic-dynein-binding-run-length-and-velocity-are-guided-by-long-range-electrostatic-srep31523-s1.ogv, Dynein

Applications

Nanorobotics

The relevance of Brownian motors to the requirement of directed motion in

has become increasingly apparent to researchers from both academia and industry. Artificial replications of Brownian motors are informed by and differ from nature, and one specific type is the photomotor, wherein the motor switches

states State most commonly refers to: * State (polity), a centralized political organization that regulates law and society within a territory **Sovereign state, a sovereign polity in international law, commonly referred to as a country **Nation state, a ...

due to pulses of

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

and generates directed motion. These photomotors, in contrast to their natural counterpartsˇ, are

inorganic An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''. Inor ...

and possess greater

efficiency Efficiency is the often measurable ability to avoid making mistakes or wasting materials, energy, efforts, money, and time while performing a task. In a more general sense, it is the ability to do things well, successfully, and without waste. ...

and average

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

, and are thus better suited to human use than existing alternatives, such as organic protein motors. Currently, one of the six current "Grand Challenges" of the

Nano Institute is to develop nanorobotics for

health Health has a variety of definitions, which have been used for different purposes over time. In general, it refers to physical and emotional well-being, especially that associated with normal functioning of the human body, absent of disease, p ...

, a key aspect of which is a "

parts

foundry A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals pr ...

" that can produce

Brownian motors for "

active transport In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellula ...

around the body". The institute predicts that among the implications of this research is a "paradigm shift" in

healthcare Health care, or healthcare, is the improvement or maintenance of health via the preventive healthcare, prevention, diagnosis, therapy, treatment, wikt:amelioration, amelioration or cure of disease, illness, injury, and other disability, physic ...

"away from the " break-fix" model to a focus on

prevention Prevention may refer to: Health and medicine * Preventive healthcare, measures to prevent diseases or injuries rather than curing them or treating their symptoms General safety * Crime prevention, the attempt to reduce deter crime and crimin ...

and early intervention," such as in the case with

heart disease Cardiovascular disease (CVD) is any disease involving the heart or blood vessels. CVDs constitute a class of diseases that includes: coronary artery diseases (e.g. angina pectoris, angina, myocardial infarction, heart attack), heart failure, ...

: Professor Paul Bannon, an adult cardiothoracic surgeon of international standing and leading

medical research Medical research (or biomedical research), also known as health research, refers to the process of using scientific methods with the aim to produce knowledge about human diseases, the prevention and treatment of illness, and the promotion of ...

er, summarises the benefits of nanorobotics in health.

Notes

External links

* * * * * *
Brownian motor on arxiv.org
* {{Fractals Nanotechnology Thermodynamics