On the nanoscale (1-100nm), viscosity dominates inertia, and the extremely high degree of thermal noise in the environment makes conventional directed motion all but impossible, because the forces 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 to achieve directed motion, and as such are only viable on the nanoscale.[3]
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 that require movement at the nanoscale, such as protein synthesis and muscular contraction. If this is the case, Brownian motors may have implications for the foundations of life itself.[3]
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 due to its inherent reliance on directed motion.[4][5]
History
20th century
This is a simulation of the Brownian motion of a big particle (dust particle) that collides with a large set of smaller particles (molecules of a gas) which move with different velocities in different random directions.
The Place of the Solitaires
Let the place of the solitaires Be a place of perpetual undulation.
Whether it be in mid-sea On the dark, green water-wheel, Or on the beaches, There must be no cessation Of motion, or of the noise of motion, The renewal of noise And manifold continuation;
And, most, of the motion of thought And its restless iteration,
In the place of the solitaires, Which is to be a place of perpetual undulation.
Wallace Stevens (1879-1955).
The term “Brownian motor” was originally invented by Swiss theoretical physicist Peter Hänggi in 1995.[3] The Brownian motor, like the phenomenon of Brownian motion that underpinned its underlying theory, was also named after 19th century Scottish botanist Robert Brown, who, while looking through a microscope at pollen of the plantClarkia pulchella immersed in water, famously described the random motion of pollen particles in water in 1827. In 1905, almost eighty years later, theoretical physicistAlbert Einstein published a paper where he modeled the motion of the pollen as being moved by individual water molecules,[6] and this was verified experimentally by Jean Perrin in 1908, who was awarded the Nobel Prize in Physics in 1926 "for his work on the discontinuous structure of matter".[7] These developments helped to create the fundamentals of the present theories of the nanoscale world.
Nanoscience has traditionally long remained at the intersection of the physical sciences of physics and chemistry, but more recent developments in research increasingly position it beyond the scope of either of these two traditional fields.[8]
21st century
In 2002, a seminal paper on Brownian motors published in the American Institute of Physics magazine Physics Today, "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' 1919 poem, The Place of the Solitaries to elegantly illustrate, from an abstract perspective, the ceaseless nature of noise.
Inspired by the fascinating mechanism by which proteins move in the face of thermal noise, many physicists are working to understand molecular motors at a mesoscopic scale. An important insight from this work is that, in some cases, thermal noise can assist directed motion by providing a mechanism for overcoming energy barriers. In those cases, one speaks of “Brownian motors.” In this article, we focus on several examples that bring out some prominent underlying physical concepts that have emerged. But first we note that poets, too, have been fascinated by noise; see box 1.
...
In the microscopic world, “There must be no cessation / Of motion, or of the noise of motion” (box 1). Rather than fighting it, Brownian motors take advantage of the ceaseless noise to move particles efficiently and reliably.
—Dean Astumian and Peter Hänggi, "Brownian Motors"
A year after the Astumian-Hänggi paper, David Leigh's organic chemistry group reported the first artificial molecular Brownian motors.[9] In 2007 the same team reported a Maxwell's Demon-inspired molecular information ratchet.[10]
Another important demonstration of nanoengineering and nanotechnology was the building of a practical artificial Brownian motor by IBM in 2018.[11] 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” nanoparticles to produce directed motion. The experiment successfully made the nanoparticles 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 nanoscale.
The Sydney Nanoscience Hub, a AU$150 million purpose-built facility for nanoscale research and education.
Nanoscience research within the Swiss Nanoscience Institute (SNI) is focused on areas of potential benefit to the life sciences, sustainability, and information and communications technologies. The aim is to explore phenomena at a nanoscale and to identify and apply new pioneering principles. This involves researchers immersing themselves in the world of individual atoms and molecules. At this level, the classical disciplines of physics, biology and chemistry merge into one. Interdisciplinary collaboration between different branches of science and institutions is thus a key element of the SNI’s day-to-day work.
—Swiss Nanoscience Institute, The University of Basel Website
Theory
The ratchet model serves as a theoretical underpinning of the Brownian motor.
The thermal noise on the nanoscale is so great that moving in a particular direction is as difficult as “walking in a hurricane” or “swimming in molasses”.[8] The theoretical operation of the Brownian motor can be explained by ratchet theory, wherein strong random thermal fluctuations are allowed to move the particle in the desired direction, while energy is expended to counteract forces that would produce motion 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 is sourced from the molecule adenosine triphosphate (ATP).
Because Brownian motors rely on the random nature of thermal noise to achieve directed motion, they are stochastic in nature, in that they can be analysed statistically but not predicted precisely.[13]
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 protein exist within humans. Two common biomolecular Brownian motors are ATP synthase, a rotary motor, and myosin II, a linear motor.[13] The motor protein ATP synthase produces rotational torque that facilitates the synthesis of ATP from Adenosine diphosphate (ADP) and inorganic phosphate (Pi) through the following overall reaction:
Proteins acting as Brownian motors inside human cells
ATP Synthase
Myosin II
Kinesin
Dynein
Applications
Nanorobotics
The relevance of Brownian motors to the requirement of directed motion in nanorobotics has become increasingly apparent to researchers from both academia and industry.[4][5]
Artificial replication of Brownian motors are informed by and differ from nature, and one specific type is the photomotor, wherein the motor switches states due to pulses of light and generates directed motion. These photomotors, in contrast to their natural counterpartsˇ, are inorganic and possess greater efficiency and average velocity, and are thus better suited to human use than existing alternatives, such as organic protein motors.[14]
Currently, one of the six current "Grand Challenges" of the University of Sydney Nano Institute is to develop nanorobotics for health, a key aspect of which is a “nanoscale parts foundry” that can produce nanoscale Brownian motors for “active transport around the body”. The Institute predicts that among the implications of this research is a "paradigm shift" in healthcare "away from the "break-fix" model to a focus on prevention and early intervention," such as in the case with heart disease:[15]
The molecular-level changes in early heart disease occur on the nanoscale. To detect these changes, we are building nanoscale robots, smaller than cells, that will navigate the body. This will enable us to see inside even the narrowest blood vessels, to detect the fatty deposits (atherosclerotic plaque) that signal the start of arterial blockage and allow treatment before the disease progresses.
...
The impact of this project will be extensive. It will improve health outcomes for all Australians with heart disease and reduce healthcare costs. It has potential to benefit other health challenges, including cancer, dementia and other neurodegenerative diseases. It will provide a world-class collaborative environment to train the next generation of Australian researchers, driving innovation and development of new industries and jobs in Australia.
If I could miniaturise myself inside the body... I could detect early, treatable damage in your coronary arteries when you are 25 years old and thus avoid your premature death.
1 2 Astumian, RD (2007-10-07). "Design Principles for Brownian Molecular Machines: How to Swim in Molasses and Walk in a Hurricane". Physical Chemistry Chemical Physics. 9 (37): 5067–83. Bibcode:2007PCCP....9.5067A. doi:10.1039/b708995c. PMID17878982.
↑ Rozenbaum, Viktor M.; Dekhtyar, Marina L.; Lin, Sheng Hsien; Trakhtenberg, Leonid I. (2016-08-12). "Photoinduced diffusion molecular transport". The Journal of Chemical Physics. 145 (6): 064110. Bibcode:2016JChPh.145f4110R. doi:10.1063/1.4960622. ISSN0021-9606.
Freund, J. A.; Pöschel, T. (2000). "Stochastic processes in physics, chemistry, and biology". Lecture Notes in Physics, vol. 557. Berlin: Springer. ISBN978-3-540-41074-4.
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, first analysed in 1912 as a thought experiment by Polish physicist Marian Smoluchowski. It was popularised by American Nobel laureate physicist Richard Feynman in a physics lecture at the California Institute of Technology on May 11, 1962, during his Messenger Lectures series The Character of Physical Law in Cornell University in 1964 and in his text The Feynman Lectures on Physics as an illustration of the laws of thermodynamics. The simple machine, consisting of a tiny paddle wheel and a ratchet, appears to be an example of a Maxwell's demon, able to extract mechanical work from random fluctuations (heat) in a system at thermal equilibrium, in violation of the second law of thermodynamics. Detailed analysis by Feynman and others showed why it cannot actually do this.
Stochastic resonance (SR) is a phenomenon in which a signal that is normally too weak to be detected by a sensor, can be boosted by adding white noise to the signal, which contains a wide spectrum of frequencies. The frequencies in the white noise corresponding to the original signal's frequencies will resonate with each other, amplifying the original signal while not amplifying the rest of the white noise – thereby increasing the signal-to-noise ratio, which makes the original signal more prominent. Further, the added white noise can be enough to be detectable by the sensor, which can then filter it out to effectively detect the original, previously undetectable signal.
Nanoid robotics, or for short, nanorobotics or nanobotics, is an emerging technology field creating machines or robots whose components are at or near the scale of a nanometer. More specifically, nanorobotics refers to the nanotechnology engineering discipline of designing and building nanorobots with devices ranging in size from 0.1 to 10 micrometres and constructed of nanoscale or molecular components. The terms nanobot, nanoid, nanite, nanomachine and nanomite have also been used to describe such devices currently under research and development.
A nanomotor is a molecular or nanoscale device capable of converting energy into movement. It can typically generate forces on the order of piconewtons.
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 switches and motors. Naturally occurring or biological molecular machines are responsible for vital living processes such as DNA replication and ATP synthesis. Kinesins and ribosomes are examples of molecular machines, and they often take the form of multi-protein complexes. For the last several decades, scientists have attempted, with varying degrees of success, to miniaturize machines found in the macroscopic world. The first example of an artificial molecular machine (AMM) was reported in 1994, featuring a rotaxane with a ring and two different possible binding sites.
In physics, an entropic force acting in a system is an emergent phenomenon resulting from the entire system's statistical tendency to increase its entropy, rather than from a particular underlying force on the atomic scale.
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 mechanical work; for example, many protein-based molecular motors harness the chemical free energy released by the hydrolysis of ATP in order to perform mechanical work. In terms of energetic efficiency, this type of motor can be superior to currently available man-made motors. One important difference between molecular motors and macroscopic motors is that molecular motors operate in the thermal bath, an environment in which the fluctuations due to thermal noise are significant.
Magnetic resonance force microscopy (MRFM) is an imaging technique that acquires magnetic resonance images (MRI) at nanometer scales, and possibly at atomic scales in the future. MRFM is potentially able to observe protein structures which cannot be seen using X-ray crystallography and protein nuclear magnetic resonance spectroscopy. Detection of the magnetic spin of a single electron has been demonstrated using this technique. The sensitivity of a current MRFM microscope is 10 billion times greater than a medical MRI used in hospitals.
Molecular biophysics is a rapidly evolving interdisciplinary area of research that combines concepts in physics, chemistry, engineering, mathematics and biology. It seeks to understand biomolecular systems and explain biological function in terms of molecular structure, structural organization, and dynamic behaviour at various levels of complexity. This discipline covers topics such as the measurement of molecular forces, molecular associations, allosteric interactions, Brownian motion, and cable theory. Additional areas of study can be found on Outline of Biophysics. The discipline has required development of specialized equipment and procedures capable of imaging and manipulating minute living structures, as well as novel experimental approaches.
The nanoscopic scale usually refers to structures with a length scale applicable to nanotechnology, usually cited as 1–100 nanometers (nm). A nanometer is a billionth of a meter. The nanoscopic scale is a lower bound to the mesoscopic scale for most solids.
Molecular propeller is a molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers: it has several molecular-scale blades attached at a certain pitch angle around the circumference of a shaft, aligned along the rotational axis.
The following outline is provided as an overview of and topical guide to nanotechnology:
Christoph Gerber is a titular professor at the Department of Physics, University of Basel, Switzerland.
Single-particle tracking (SPT) is the observation of the motion of individual particles within a medium. The coordinates time series, which can be either in two dimensions (x, y) or in three dimensions (x, y, z), is referred to as a trajectory. The trajectory is typically analyzed using statistical methods to extract information about the underlying dynamics of the particle. These dynamics can reveal information about the type of transport being observed (e.g., thermal or active), the medium where the particle is moving, and interactions with other particles. In the case of random motion, trajectory analysis can be used to measure the diffusion coefficient.
Peter Hänggi is a theoretical physicist from Switzerland, Professor of Theoretical Physics at the University of Augsburg. He is best known for his original works on Brownian motion and the Brownian motor concept, stochastic resonance and dissipative systems. Other topics include, driven quantum tunneling, such as the discovery of coherent destruction of tunneling (CDT), phononics, relativistic statistical mechanics and the foundations of classical and quantum thermodynamics.
Wet nanotechnology involves working up to large masses from small ones.
Force Spectrum Microscopy (FSM) is an application of active microrheology developed to measure aggregate random forces in the cytoplasm. Large, inert flow tracers are injected into live cells and become lodged inside the cytoskeletal mesh, wherein it is oscillated by repercussions from active motor proteins. The magnitude of these random forces can be inferred from the frequency of oscillation of tracer particles. Tracking the fluctuations of tracer particles using optical microscopy can isolate the contribution of active random forces to intracellular molecular transport from that of Brownian motion.
A DNA walker is a class of nucleic acid nanomachines where a nucleic acid "walker" is able to move along a nucleic acid "track". The concept of a DNA walker was first defined and named by John H. Reif in 2003. A nonautonomous DNA walker requires external changes for each step, whereas an autonomous DNA walker progresses without any external changes. Various nonautonomous DNA walkers were developed, for example Shin controlled the motion of DNA walker by using 'control strands' which needed to be manually added in a specific order according to the template's sequence in order to get the desired path of motion. In 2004 the first autonomous DNA walker, which did not require external changes for each step, was experimentally demonstrated by the Reif group.
Stochastic thermodynamics is an emergent field of research in statistical mechanics that uses stochastic variables to better understand the non-equilibrium dynamics present in many microscopic systems such as colloidal particles, biopolymers, enzymes, and molecular motors.
A Molecular demon or biological molecular machine is a biological macromolecule that resembles and seems to have the same properties as Maxwell's demon. These macromolecules gather information in order to recognize their substrate or ligand within a myriad of other molecules floating in the intracellular or extracellular plasm. This molecular recognition represents an information gain which is equivalent to an energy gain or decrease in entropy. When the demon is reset i.e. when the ligand is released, the information is erased, energy is dissipated and entropy increases obeying the second law of thermodynamics. The difference between biological molecular demons and the thought experiment of Maxwell's demon is the latter's apparent violation of the second law.
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