Translation-rotation Coupling Research Articles

Effects of Hydrodynamic Coupling on Electro-Optical Transients

The effects resulting from coupling between translational and rotational diffusion on electro-optical transients were analyzed by Brownian dynamics simulations. Diffusion tensors, including translational-rotational coupling tensors, were derived from bead model simulations. Optical and electrical parameters were assigned according to models and experimental data. Hydrodynamic coupling has a strong impact on the shape of electro-optical transients for objects with a nonsymmetric structure. As expected, hydrodynamic coupling in general does not affect the time constants derived from decay curves. However, special attention is required at singular points, where amplitudes are inverted. Under these conditions, transients are observed with artificially reduced time constants, suggesting compact structures. False conclusions may be avoided, when experimental data are analyzed over a sufficiently wide range of conditions. Because transients induced upon reversal of the field vector are strongly affected by hydrodynamic coupling, dipole types cannot be assigned from these transients simply according to standard rules, when the shape of the objects is nonsymmetric. The time constants obtained from exponential fitting of rise-curves show unexpected dependencies on the field strength, which are mainly due to superposition of individual components with amplitudes of opposite sign that are not resolved upon fitting. Dipole parameters obtained from stationary degrees of orientation via orientation functions may also be strongly affected by hydrodynamic coupling. The high sensitivity of electro-optical transients on details of molecular shape and optical and electrical parameters provides a powerful approach to molecular analysis, but quantitative assignments require special care. When symmetry is lost, which must be expected because of bending in many cases, hydrodynamic coupling effects cannot be neglected.

Nuclear spin conversion of H 2O trapped in solid xenon at 4.2 K: A new assignment of ν2 rovibrational lines

We have measured a nuclear spin conversion time of 100 min for H 2O highly diluted in solid xenon at 4.2 K. Compared with argon matrices, the conversion is very probably enhanced by the high natural abundance of xenon magnetic isotopes. The ν 2 mode absorption spectrum of rare gas isolated H 2O, with polymerization and rotation translation coupling effects, is still debated. In this work, the ν 2 IR absorption spectrum of H 2O in xenon is compared to that observed in neon, argon and krypton matrices. A new assignment of the rovibrational lines is proposed, demonstrated using the nuclear spin conversion studies.

Dynamics of gas-fluidized granular rods

We study a quasi-two-dimensional monolayer of granular rods fluidized by a spatially and temporally homogeneous upflow of air. By tracking the position and orientation of the particles, we characterize the dynamics of the system with sufficient resolution to observe ballistic motion at the shortest time scales. Particle anisotropy gives rise to dynamical anisotropy and superdiffusive dynamics parallel to the rod's long axis, causing the parallel and perpendicular mean-square displacements to become diffusive on different time scales. The distributions of free times and free paths between collisions deviate from exponential behavior, underscoring the nonthermal character of the particle motion. The dynamics show evidence of rotational-translational coupling similar to that of an anisotropic Brownian particle. We model rotational-translational coupling in the single-particle dynamics with a modified Langevin model using nonthermal noise sources. This suggests a phenomenological approach to thinking about collections of self-propelling particles in terms of enhanced memory effects.

Electrostatic-control performance measurement of the inertial sensor with a torsion pendulum

An electrostatic-controlled torsion pendulum was used to simulate the operation of the inertial sensor in flight. The twist motion of the proof mass was monitored by a capacitance transducer and was controlled by an electrostatic actuator. The influences of the parasitic coupling of the capacitance transducer, the magnetic field, and the translation motion coupling were measured. The torque noise of the controlled pendulum came to about 6×10-13 N m Hz-1/2 from 2 mHz to 0.1 Hz, mainly limited by the translation-rotation coupling and the back action of the capacitance sensor, which could be suppressed by extending the gap of the capacitance sensor.

Rotary motors sliding along surfaces

We introduce a family of molecular rotors that may convert light or chemical energy into directed translational motion along surfaces. The dependencies of diffusion coefficient and drift velocity of the rotating molecule on the magnitude of external torque, symmetry of surface potential, and temperature have been investigated. Our simulations show that the rotation-translation coupling could be very effective, and the molecule may move by approximately one surface lattice spacing per complete rotation. We have found that the unidirectionality of the rotary motion is not required to produce efficient directed sliding; this effect can be achieved by applying a time-periodic, or even randomly, oscillating torque which induces alternating molecular reorientations but does not generate complete rotations.

Use of the Rotation Vector in Brownian Dynamics Simulation of Transient Electro‐Optical Properties

AbstractWe have recently developed a new singularity‐free algorithm for Brownian dynamics simulation of free rotational diffusion. The algorithm is rigorously derived from kinetic theory and makes use of the Cartesian components of the rotation vector as the generalized coordinates describing angular orientation. Here, we report on the application of this new algorithm in Brownian dynamics simulations of transient electro‐optical properties. This work serves two main purposes. Firstly, it demonstrates the integrity of the new algorithm for BD‐simulations of the most common transient electro‐optic experiments. Secondly, it provides new insight into the performance of the new algorithm compared to algorithms that make use of the Euler angles. We study the transient electrically induced birefringence in dilute solutions of rigid particles with anisotropic polarization tensor in response to external electric field pulses. The use of both one single electric pulse and two electric pulses with opposite polarity are being analyzed. We document that the new singularity‐free algorithm performs flawlessly. We find that, for these types of systems, the new singularity‐free algorithm, in general, outperforms similar algorithms based on the Euler angles. In a wider perspective, the most important aspect of this work is that it serves as an important reference for future development of efficient BD‐algorithms for studies of more complex systems. These systems include polymers consisting of rigid segments with single‐segment translational–rotational coupling, segment–segment fluid‐dynamic interactions and holonomic constraints.magnified image

Inherent Characteristics of 3K-II Spur Planetary Gear Trains

A translational-rotational coupling model of 3K-Ⅱ spur planetary gear trains is developed to investigate the natural frequencies and corresponding vibration modes. The model admits three planar degrees of freedom for each of the sun, rings, carrier and planets and includes some key factors affecting planetary gears vibration such as component bearing stiffness, time-varying mesh stiffness and gyroscopic effect. By deriving the displacement relationships between gears and carrier, the governing differential equations are obtained. The solution of associated eigenvalue problem leads to natural frequencies and vibration modes of the transmission. The vibration modes are classified into three types, i.e., rotational modes, translational modes, and planetary gear modes based on their unique properties.

Pumping of Confined Water in Carbon Nanotubes by Rotation-Translation Coupling

Bidirectional single file water transport in a carbon nanotube is known to occur in "bursts" in short nanotubes. Here we show that in long carbon nanotubes, when the orientation of the water molecules is maintained along one direction, a net water transport along that direction can be attained due to coupling between rotational and translational motions. The rotations of the water molecules are correlated more with the translation of the neighboring water molecule with the acceptor oxygen than the neighbor with the donor hydrogen. This mechanism can be used to pump water through nanotubes.

Long-time self-diffusion for Brownian Gaussian-core particles

Using extensive Brownian dynamics computer simulations, the long-time self-diffusion coefficient is calculated for Gaussian-core particles as a function of the number density. Both spherical and rod-like particles interacting via Gaussian segments are considered. For increasing concentration we find that the translational self-diffusion behaves non-monotonically reflecting the structural reentrance effect in the equilibrium phase diagram. Both in the limits of zero and infinite concentration, it approaches its short-time value. The microscopic Medina-Noyola theory qualitatively accounts for the translational long-time diffusion. The long-time orientational diffusion coefficient for Gaussian rods, on the other hand, remains very close to its short-time counterpart for any density. Some implications of the weak translation–rotation coupling for ultrasoft rods are discussed.

Analysis of a heterodyne-detected transient-grating experiment on a molecular supercooled liquid. II. Application tom-toluidine

This paper reports the first detailed analysis of a transient grating (TG) experiment on a supercooled molecular liquid, m-toluidine, from 330K (1.75Tg) to 190K (1.01Tg) based on the theoretical model presented in Paper I of this series. This method allows one to give a precise description, over a wide dynamical range, of the different physical phenomena giving rise to the signals. Disentangling the isotropic and the anisotropic parts of the TG response, a careful fitting analysis yields detailed information on the rotation-translation coupling function. We also extract the structural relaxation times related to the "longitudinal" viscosity over almost 10 decades in time and the corresponding stretching coefficient. The value of some other parameters and information on their thermal behavior is also reported.

Lattice dynamics of sodium superoxide

Abstract An extended three body force shell model (ETSM) has been applied for the prediction of lattice dynamics of orientationally disordered NaO2, by incorporating the effect of translational–rotational coupling between the translational modes and the orientation of superoxide O−2 molecules. The phonon dispersion curves (PDCs) computed from ETSM is generally, in good agreement with their available experimental data measured with the inelastic neutron scattering. The unusual feature is the upward curvature seen in transverse acoustic branch is more soft near the zone centre. We have also reported the cohesive, thermal and elastic properties of NaO2.

UV laser desorption of nitric oxide from semiconducting C60/Cu(111)

The desorption of NO from a well-characterized, epitaxially grown semiconducting C60 surface is reported. Two different channels are identified in the laser desorption. Both channels yield a comparably high desorption cross section of σ1=7.0×10-17 cm2 and σ2=5.5×10-17 cm2 for the first and second channel, respectively. The laser desorbed NO molecules are detected with rovibrational state selectivity by (1+1) REMPI in the \(A^2\Sigma^+ \to{X}^2 \Pi\,\gamma\)-bands. In the first channel the desorbing molecules are highly excited with an average kinetic energy of 〈Ekin〉=174 meV. The rotational population distribution can be fitted by a rotational temperature of Trot=800 K. A rotational–translational coupling is observed, with velocities ranging from 1000 m/s for low to 1300 m/s for high rotational states. The vibrationally excited population is estimated to be less than 1% of the ground state. The second channel yields less excited molecules and an almost Boltzmann distributed rotational population with a temperature of Trot=280 K. The apparent velocity distribution derived from the pump-probe delay yields molecules much too slow to be explained by even a thermal desorption. This desorption is probably caused by a long-lived electronic excitation in the substrate for which a lifetime of τ≈160 μs is estimated.

Direct Observation of Hydrodynamic Rotation-Translation Coupling between Two Colloidal Spheres

By combining optical tweezers with polarization microscopy, the hydrodynamic coupling between position and orientation fluctuations in a pair of colloidal spheres has been measured. Imaging of birefringent particles under crossed polarizers allows for the simultaneous determination of the positions and orientations of both particles. The temporal cross-correlation function between random displacements of one particle and orientation fluctuations of its neighbor allows for the quantification of the hydrodynamic rotation-translation coupling between the spheres. Our results are in good agreement with predictions for the hydrodynamic mobility tensors calculated in the creeping-flow limit of the Navier-Stokes equation.

Analysis of the transverse and the longitudinal pseudodiffusion of CO2 in sub- and supercritical states: A molecular-dynamics analysis

We have performed molecular-dynamics simulations of CO(2) system along the gas-liquid coexistence curve and on the isochore 94.22 cm(3) mol(-1) (which corresponds to the critical isochore). The calculation has been carried out in order to analyze the diffusion of CO(2) and particularly to figure out how the diffusion coefficient may be decomposed along the molecular axes. This makes it possible to analyze the anisotropy of the diffusion along these axes and to shed light on the microscopic changes which accompany such behavior. This anisotropy is traced back to the effect of the translation-rotation coupling (TRC) along the molecular axes. Along the liquid-gas coexistence curve, the pseudolongitudinal diffusion is found to be more rapid than the transverse one. The opposite trend is found along the isochore 94.22 cm(3) mol(-1). The role of the local structure was explored by calculating intermediate scattering function and the autocorrelation functions for the forces acting along the molecular axes. It is shown that the strength of the TRC effect is correlated to the difference between the relaxation times of the local structure, that of the reorientation along the molecular axes, and that of the translational motion. The analysis of the correlation time and the average mean square force along the longitudinal and transverse directions confirms the anisotropy of the local environment that determines the translational dynamics of a molecule.

Brownian molecular motors driven by rotation-translation coupling

We investigated three models of Brownian motors which convert rotational diffusion into directed translational motion by switching on and off a potential. In the first model a spatially asymmetric potential generates directed translational motion by rectifying rotational diffusion. It behaves much like a conventional flashing ratchet. The second model utilizes both rotational diffusion and drift to generate translational motion without spatial asymmetry in the potential. This second model can be driven by a combination of a Brownian motor mechanism (diffusion driven) or by powerstroke (drift driven) depending on the chosen parameters. In the third model, elements of both the Brownian motor and powerstroke mechanisms are combined by switching between three distinct states. Relevance of the model to biological motor proteins is discussed.

Electric birefringence and dichroism of DNA fragments in aqueous solution

A major task in this work is to study the mechanisms of orientation of DNA fragments in dilute solution in presence of an electric field. The DNA fragments are modeled as charged and polarizable bent rods. Considering the participation of electrophoresis and electric moments in the orientation of particles, we first obtain the Fokker–Planck equation for the system in the local coordinate system and finally arrive to the Smoluchowski equations for the steady state in the laboratory system. We here demonstrate that the solution of this equation, the orientational distribution function of molecules, is independent from the location of the origin of the molecule coordinate system. This property is based on the fact that the drift vector in the Fokker–Planck equation is also invariant to this change. This characteristic enables us to state that when the bent rods move by electrophoresis, wherever the bent rod coordinate system is located, the orientation of the ensemble always results in a translation–rotation coupling of particles with the origin of the molecule coordinate system in their center of charge. We analyze the orientation characteristics of the particles due to an electric moment m that appears when the origin of the coordinate system is located in a different point from its center of charge and we compare it with the electric dipole moment μ , originated by the charge distribution with null net electric charge.

Translation-rotation coupling and heat transfer in orientationally-disordered phase of CCl4

Abstract The isochoric thermal conductivity of an orientationally-disordered phase of CCl4 is analysed within a model in which heat is transferred by phonons and above the phonon mobility edge by ”diffusive” modes migrating randomly from site to site. The mobility edge ω0 is found from the condition that the phonon mean-free path cannot become smaller than half the phonon wavelength. The contributions of phonon-phonon, one-, and two-phonon scattering to the total thermal resistance of solid CCl4 are calcualted under the assumption that the different scattering mechanisms contribute additively. An increase in the isochoric thermal conductivity with temperature is explained by suppression of phonon scattering at rotational excitations due to a decrease in correlation in the rotation of neighbouring molecules.

Cross-orientational motion in liquid dichloromethane and trichloromethane (chloroform) by molecular dynamics simulation

Using molecular dynamics the l = 1 orientational auto- and cross-correlation functions of one of the C–H bond directions of neat dichloromethane at 273 K and that of chloroform between 301 and 438 K in its neat state and in solution with tetrachloromethane and liquefied methane (at 183 K) have been simulated. The simulations are confined to a model of singly rotating, individual molecules in the absence of induced absorption and translation-rotation coupling. It is found throughout that the orientational cross-correlation functions are equivalent to the auto-correlation functions, therefore bearing the same information content. On the basis of a simple thermodynamic argument, this phenomenon is extended to be generally valid to orientational correlation functions from vibration–rotation spectra of mobile liquids. Consequences of the equivalence are, first, that dilution does not break the symmetry between orientational auto- and cross-correlation functions of the solute species and, second, that the simulated auto-correlation functions of total and individual dipole moments of undressed dipoles in the system are indistinguishable.

Low-temperature infrared spectroscopy ofH2in crystallineC60

Diffuse reflectance infrared spectroscopy is used to measure the quantum dynamics of molecular hydrogen trapped within a C60 lattice at temperatures as low as 10 K. Crystal field effects in conjunction with rotational translational coupling lead to a rich spectrum with multiply split peaks that are more than an order of magnitude sharper than at room temperature. The induced redshifts in the vibrational-rotational mode frequencies are explained using a simple model in which the state dependence of the H2 polarizability leads to changes in the C60-H2 interaction potential.

Laser desorption of NO from a thick C 60 film

The desorption of NO molecules from a thick C 60 film is reported. A thermal desorption spectrum indicates two adsorption sites with binding energies of E b = 0.30 eV and 0.55 eV. For laser desorption the fullerene surface is exposed to NO and excited by 7 ns UV laser pulses. Desorbing NO molecules are recorded state selectively as well as time resolved. The time-of-flight measurement indicates three different desorption pathways. A fast channel shows rovibronic temperatures of T rot( v″ = 0) = 370 K, T rot( v″ = 1) = 390 K and T vib = 610 K as well as strong rotational–translational coupling. The desorption yield for the fast channel increases linearly with pulse energy with a desorption cross section of σ = (5.1 ± 0.9) × 10 −17 cm 2. Dominating the signal for small J″ values is a slow channel with low rotational and translational temperatures of about 110 K. We assign this peak to a laser-induced thermal desorption. For large pump–probe delays the data deviate from the Maxwellian flux distribution and a third channel appears with extremely late arrival times.