Equilibrium regained: from nonequilibrium chaos to statistical mechanics

Far-from-equilibrium, spatially extended chaotic systems have generally eluded analytical solution, leading researchers to consider theories based on a statistical rather than a detailed knowledge of the microscopic length scales. Building on the recent discovery of a separation of length scales between macroscopic behavior and microscopic chaos, a simple far-from-equilibrium spatially extended chaotic system has been studied computationally at intermediate, coarse-grained scales. Equilibrium properties such as Gibbs distributions and detailed balance are recovered at these scales, which suggests that the macroscopic behavior of some far-from-equilibrium systems might be understood in terms of equilibrium statistical mechanics.     

Brownian motion of an ellipsoid

We studied the Brownian motion of isolated ellipsoidal particles in water confined to two dimensions and elucidated the effects of coupling between rotational and translational motion. By using digital video microscopy, we quantified the crossover from short-time anisotropic to long-time isotropic diffusion and directly measured probability distributions functions for displacements. We confirmed and interpreted our measurements by using Langevin theory and numerical simulations. Our theory and observations provide insights into fundamental diffusive processes, which are potentially useful for understanding transport in membranes and for understanding the motions of anisotropic macromolecules.     

Myosin V walks by lever action and Brownian motion

Myosin V is a molecular motor that moves cargo along actin filaments. Its two heads, each attached to a long and relatively stiff neck, move alternately forward in a "hand-over-hand" fashion. To observe under a microscope how the necks move, we attached a micrometer-sized rod to one of the necks. The leading neck swings unidirectionally forward, whereas the trailing neck, once lifted, undergoes extensive Brownian rotation in all directions before landing on a site ahead of the leading head. The neck-neck joint is essentially free, and the neck motion supports a mechanism where the active swing of the leading neck biases the random motion of the lifted head to let it eventually land on a forward site.     

Brownian motion of DNA confined within a two-dimensional array

Linear DNA molecules are visualized while undergoing Brownian motion inside media patterned with molecular-sized spatial constraints. The media, prepared by colloidal templating, trap the macromolecules within a two-dimensional array of spherical cavities interconnected by circular holes. Across a broad DNA size range, diffusion does not proceed by the familiar mechanisms of reptation or sieving. Rather, because of their inherent flexibility, DNA molecules strongly localize in cavities and only sporadically "jump" through holes. Jumping closely follows Poisson statistics. By reducing DNA's configurational freedom, the holes act as molecular weight-dependent entropic barriers. Sterically constrained macromolecular diffusion underlies many separation methods and assumes an important role in intracellular and extracellular transport.     

Brownian motion of stiff filaments in a crowded environment

The thermal motion of stiff filaments in a crowded environment is highly constrained and anisotropic; it underlies the behavior of such disparate systems as polymer materials, nanocomposites, and the cell cytoskeleton. Despite decades of theoretical study, the fundamental dynamics of such systems remains a mystery. Using near-infrared video microscopy, we studied the thermal diffusion of individual single-walled carbon nanotubes (SWNTs) confined in porous agarose networks. We found that even a small bending flexibility of SWNTs strongly enhances their motion: The rotational diffusion constant is proportional to the filament-bending compliance and is independent of the network pore size. The interplay between crowding and thermal bending implies that the notion of a filament's stiffness depends on its confinement. Moreover, the mobility of SWNTs and other inclusions can be controlled by tailoring their stiffness.     

New mechanism of nonequilibrium polymer adsorption

Nonequilibrium states of surface composition can be extremely long-lived when polymer chains adsorb competitively. In a model system (polymethylmethacrylate adsorbed from CCl(4) onto oxidized silicon previously saturated with polystyrene), it is shown that a weakly adsorbing polymer was sterically pinned to a surface by a more strongly adsorbing polymer. The dynamical evolution of the surface composition was strongly nonexponential in time and non-Arrhenius in temperature; the phenomenology is analogous to bulk glasses. This interpretation offers a new mechanism to explain why weakly adsorbing chains may bind to surfaces, as well as a direction in which to look for a method to release them.     

Brownian Movement in Color Photomicrography

Crystals, smaller than 1 micron, in Brownian movement have been photographed in color.     

Millikelvin Temperatures Measured with a Noise Thermometer: Brownian motion in electrical circuits has been used to measure temperatures as low as 2 millikelvins

We find that the current-sensitive noise thermometer is a satisfactory sensor of absolute temperature down to 2 mK and possibly below. Its field of application is primarily to magnetically and electrically quiet situations where accuracy is important and high precision is not required. For example the noise thermometer is an ideal instrument for calibrating a secondary thermometer, such as a pressure thermometer, whose calibration can readily and accurately be transferred from one laboratory to another. Further instrumental development for millidegree applications seems unnecessary at present. Rather the major problems in millikelvin noise thermometry that require attention in the immediate future involve establishing thermal contact to the thermometer and reducing its thermal response time.     

Adhesion: molecules and mechanics

There is a difference between adhesion at the molecular level and adhesion in engineering. There is no doubt that molecules of solid materials stick together and can be separated mechanically. The problem is explaining the connection between molecular attractions and mechanical measurements. False ideas such as keying and gluing require critical assessment because they confuse molecules and mechanics. Mechanisms such as adhesive hysteresis, stringing, and clustering deserve evaluation. A rational theory of these phenomena should be based on the theoretical concept of reversible work of adhesion and on the measured quantity of adhesive energy, which includes the extra energy required to restructure the interface as surfaces move.     

Diffusion of nonequilibrium quasi-particles in a cuprate superconductor

We report a transport study of nonequilibrium quasi-particles in a high-transition-temperature cuprate superconductor using the transient grating technique. Low-intensity laser excitation (at a photon energy of 1.5 electron volts) was used to introduce a spatially periodic density of quasi-particles into a high-quality untwinned single crystal of YBa2Cu3O6.5. Probing the evolution of the initial density through space and time yielded the quasi-particle diffusion coefficient and the inelastic and elastic scattering rates. The technique reported here is potentially applicable to precision measurements of quasi-particle dynamics not only in cuprate superconductors but in other electronic systems as well.     

Celestial mechanics experiment

Equipment on Mariner V has yielded values for the masses of Moon and Venus more accurate than any previously reported. Range and Doppler radio tracking data necessary for precise space navigation of the spacecraft from Earth to Venus can also be used to obtain data on the orbits of Earth and Venus.