N G van Kampen 1981 Amsterdam: North-Holland xiv + 419 pp price Dfl 180 This is a book which, at a lower price, could be expected to become an essential part of the library of every physical scientist concerned with problems involving fluctuations and stochastic processes, as well as those who just enjoy a beautifully written book. It provides an extensive graduate-level introduction which is clear, cautious, interesting and readable.

https://iopscience.iop.org/article/10.1088/0031-9112/34/4/040/pdf

Stochastic Processes in Physics and Chemistry

We show that an electric field can drive single-stranded RNA and DNA molecules through a 2.6-nm diameter ion channel in a lipid bilayer membrane. Because the channel diameter can accommodate only a single strand of RNA or DNA, each polymer traverses the membrane as an extended chain that partially blocks the channel. The passage of each molecule is detected as a transient decrease of ionic current whose duration is proportional to polymer length. Channel blockades can therefore be used to measure polynucleotide length. With further improvements, the method could in principle provide direct, high-speed detection of the sequence of bases in single molecules of DNA or RNA.

/pdf/characterization-of-individual-polynucleotide-molecules-b5ji6o583u.pdf

Characterization of individual polynucleotide molecules using a membrane channel

Differently from passive Brownian particles, active particles, also known as self-propelled Brownian particles or microswimmers and nanoswimmers, are capable of taking up energy from their environment and converting it into directed motion. Because of this constant flow of energy, their behavior can be explained and understood only within the framework of nonequilibrium physics. In the biological realm, many cells perform directed motion, for example, as a way to browse for nutrients or to avoid toxins. Inspired by these motile microorganisms, researchers have been developing artificial particles that feature similar swimming behaviors based on different mechanisms. These man-made micromachines and nanomachines hold a great potential as autonomous agents for health care, sustainability, and security applications. With a focus on the basic physical features of the interactions of self-propelled Brownian particles with a crowded and complex environment, this comprehensive review will provide a guided tour through its basic principles, the development of artificial self-propelling microparticles and nanoparticles, and their application to the study of nonequilibrium phenomena, as well as the open challenges that the field is currently facing.

/pdf/active-particles-in-complex-and-crowded-environments-4pyirdi1aj.pdf

Active Particles in Complex and Crowded Environments

In systems possessing spatial or dynamical symmetry breaking, Brownian motion combined with unbiased external input signals, deterministic and random alike, can assist directed motion of particles at submicron scales. In such cases, one speaks of ``Brownian motors.'' In this review the constructive role of Brownian motion is exemplified for various physical and technological setups, which are inspired by the cellular molecular machinery: the working principles and characteristics of stylized devices are discussed to show how fluctuations, either thermal or extrinsic, can be used to control diffusive particle transport. Recent experimental demonstrations of this concept are surveyed with particular attention to transport in artificial, i.e., nonbiological, nanopores, lithographic tracks, and optical traps, where single-particle currents were first measured. Much emphasis is given to two- and three-dimensional devices containing many interacting particles of one or more species; for this class of artificial motors, noise rectification results also from the interplay of particle Brownian motion and geometric constraints. Recently, selective control and optimization of the transport of interacting colloidal particles and magnetic vortices have been successfully achieved, thus leading to the new generation of microfluidic and superconducting devices presented here. The field has recently been enriched with impressive experimental achievements in building artificial Brownian motor devices that even operate within the quantum domain by harvesting quantum Brownian motion. Sundry akin topics include activities aimed at noise-assisted shuttling other degrees of freedom such as charge, spin, or even heat and the assembly of chemical synthetic molecular motors. This review ends with a perspective for future pathways and potential new applications.

/pdf/artificial-brownian-motors-controlling-transport-on-the-1553poaezw.pdf

Artificial Brownian motors: Controlling transport on the nanoscale

We measure current blockade and time distributions for single-stranded DNA polymers during voltage-driven translocations through a single alpha-hemolysin pore. We use these data to determine the velocity of the polymers in the pore. Our measurements imply that, while polymers longer than the pore are translocated at a constant speed, the velocity of shorter polymers increases with decreasing length. This velocity is nonlinear with the applied field. Based on this data, we estimate the effective diffusion coefficient and the energy penalty for extending a molecule into the pore.

https://dash.harvard.edu/bitstream/handle/1/3109361/VoltageDrivenDNA.pdf?sequence=2

Voltage-driven DNA translocations through a nanopore.

We construct a new statistical physical model of polymer translocation through a pore in a membrane treated as the diffusion process across a free energy barrier. We determine the translocation time in terms of chain flexibility yielding an entropic barrier, as well as in terms of the driving mechanisms such as transmembrane chemical potential difference and Brownian ratchets. It turns out that, while the chemical potential differences induce pronounced effects on translocation due to the long-chain nature of the polymer, the ratchets suppress this effect and chain flexibility.

Polymer Translocation through a Pore in a Membrane.

We study the translocation of a flexible polymer through a pore in a membrane induced by its adsorption on the trans side of the membrane. When temperature T is higher than Tc, the adsorption–desorption transition temperature, the attractive interaction between polymer and membrane plays little role in affecting polymer conformation, leading to translocation time that scales as τ∼L3, where L is the polymer contour length. When T<Tc, however, the translocation time undergoes a sharp crossover to τ∼L2 for sufficiently long polymers, following the second-order conformational (adsorption) transition. The translocation time is found to exhibit the crossover around T=Tc′, which is lower than Tc for polymers shorter than a critical length (N<Nc).

Polymer translocation induced by adsorption

Translocation of a polymer out of a curved surface or membrane is studied via the mean first passage time approach. Membrane curvature gives rise to a constraint on polymer conformation, which effectively drives the polymer to the outside of confinement, where the available volume of the polymer conformational fluctuation is larger. Considering a polymer release out of a spherical vesicle, the polymer translocation time $\ensuremath{\tau}$ is changed to the scaling behavior $\ensuremath{\tau}\ensuremath{\sim}{L}^{2}$ for $Rl{R}_{G},$ from $\ensuremath{\tau}\ensuremath{\sim}{L}^{3}$ for $R\ensuremath{\gg}{R}_{G},$ where $L$ is the polymer contour length, and $R$ and ${R}_{G}$ are the vesicle radius and polymer radius of gyration, respectively. Also, the polymer capture into a spherical bud is studied, and a possible apparatus for easy capture is suggested.

Polymer release out of a spherical vesicle through a pore

Owing to the growth of demand for highly integrated electronic devices, high heat dissipation of thermal management materials is essential. Epoxy composites have been prepared with vertically aligned (VA) three-dimensional (3D)-structured SiC sheet scaffolds. The required VA-SiC sheet scaffolds were prepared by a novel approach starting with a graphene oxide (GO) scaffold. The VA-GO scaffolds were reduced to VA-graphene scaffolds in an argon environment, and the latter were subsequently transformed into VA-SiC sheet scaffolds by a template-assisted chemical vapor deposition method. Epoxy resin was filled in the empty spaces of the 3D scaffold of SiC sheets to prepare the composite mass. The material so prepared shows anisotropic thermal property with ultrahigh through-plane conductivity of 14.32 W·m-1·K-1 at a SiC sheet content of 3.71 vol %. A thermal percolation is observed at 1.78 vol % SiC filler. The SiC sheet scaffold of covalently interconnected SiC nanoparticles plays a vital role in the formation of the thermal conductive network to significantly enhance the thermal conductivity of epoxy composites. The application of the VA-SiC/epoxy composite as an efficient thermal dissipating material has also been presented. The VA-SiC/epoxy composites have a strong potential for preparing heat-dissipating components in integrated microelectronics.

High Thermal Conductivity Enhancement of Polymer Composites with Vertically Aligned Silicon Carbide Sheet Scaffolds

We study the dynamics of a flexible polymer under a Kramers potential using the multidimensional barrier crossing theory. The activation free energy of the polymer evaluated by a lattice statistics yields the crossing rate as a function of chain length, temperature, and potential parameters. It is found that a long chain has an enhanced crossing rate due to the extended conformation of low free energy it has at the barrier, while a short chain has the crossing dynamics of a Brownian particle. A chain of intermediate length can exhibit a minimum in the crossing rate as chain length varies. For any range of chain lengths and potential parameters, the crossing rate is larger than that of a polymer in the globular state, due to the internal degrees of freedom of the unfolded flexible chain. Our results can be applied to macromolecular separation and DNA electrophoresis.

Pyeong Jun Park

Papers

Polymer Translocation through a Pore in a Membrane.

Polymer translocation induced by adsorption

Polymer release out of a spherical vesicle through a pore

High Thermal Conductivity Enhancement of Polymer Composites with Vertically Aligned Silicon Carbide Sheet Scaffolds

Dynamics of a polymer surmounting a potential barrier: The Kramers problem for polymers